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765 lines
27 KiB
765 lines
27 KiB
RAID arrays |
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=========== |
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Boot time assembly of RAID arrays |
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--------------------------------- |
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Tools that manage md devices can be found at |
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https://www.kernel.org/pub/linux/utils/raid/ |
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You can boot with your md device with the following kernel command |
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lines: |
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for old raid arrays without persistent superblocks:: |
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md=<md device no.>,<raid level>,<chunk size factor>,<fault level>,dev0,dev1,...,devn |
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for raid arrays with persistent superblocks:: |
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md=<md device no.>,dev0,dev1,...,devn |
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or, to assemble a partitionable array:: |
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md=d<md device no.>,dev0,dev1,...,devn |
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``md device no.`` |
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+++++++++++++++++ |
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The number of the md device |
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================= ========= |
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``md device no.`` device |
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================= ========= |
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0 md0 |
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1 md1 |
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2 md2 |
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3 md3 |
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4 md4 |
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================= ========= |
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``raid level`` |
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++++++++++++++ |
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level of the RAID array |
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=============== ============= |
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``raid level`` level |
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=============== ============= |
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-1 linear mode |
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0 striped mode |
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=============== ============= |
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other modes are only supported with persistent super blocks |
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``chunk size factor`` |
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+++++++++++++++++++++ |
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(raid-0 and raid-1 only) |
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Set the chunk size as 4k << n. |
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``fault level`` |
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+++++++++++++++ |
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Totally ignored |
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``dev0`` to ``devn`` |
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++++++++++++++++++++ |
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e.g. ``/dev/hda1``, ``/dev/hdc1``, ``/dev/sda1``, ``/dev/sdb1`` |
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A possible loadlin line (Harald Hoyer <[email protected]>) looks like this:: |
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e:\loadlin\loadlin e:\zimage root=/dev/md0 md=0,0,4,0,/dev/hdb2,/dev/hdc3 ro |
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Boot time autodetection of RAID arrays |
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-------------------------------------- |
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When md is compiled into the kernel (not as module), partitions of |
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type 0xfd are scanned and automatically assembled into RAID arrays. |
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This autodetection may be suppressed with the kernel parameter |
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``raid=noautodetect``. As of kernel 2.6.9, only drives with a type 0 |
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superblock can be autodetected and run at boot time. |
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The kernel parameter ``raid=partitionable`` (or ``raid=part``) means |
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that all auto-detected arrays are assembled as partitionable. |
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Boot time assembly of degraded/dirty arrays |
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------------------------------------------- |
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If a raid5 or raid6 array is both dirty and degraded, it could have |
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undetectable data corruption. This is because the fact that it is |
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``dirty`` means that the parity cannot be trusted, and the fact that it |
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is degraded means that some datablocks are missing and cannot reliably |
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be reconstructed (due to no parity). |
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For this reason, md will normally refuse to start such an array. This |
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requires the sysadmin to take action to explicitly start the array |
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despite possible corruption. This is normally done with:: |
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mdadm --assemble --force .... |
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This option is not really available if the array has the root |
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filesystem on it. In order to support this booting from such an |
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array, md supports a module parameter ``start_dirty_degraded`` which, |
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when set to 1, bypassed the checks and will allows dirty degraded |
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arrays to be started. |
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So, to boot with a root filesystem of a dirty degraded raid 5 or 6, use:: |
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md-mod.start_dirty_degraded=1 |
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Superblock formats |
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------------------ |
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The md driver can support a variety of different superblock formats. |
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Currently, it supports superblock formats ``0.90.0`` and the ``md-1`` format |
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introduced in the 2.5 development series. |
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The kernel will autodetect which format superblock is being used. |
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Superblock format ``0`` is treated differently to others for legacy |
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reasons - it is the original superblock format. |
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General Rules - apply for all superblock formats |
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------------------------------------------------ |
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An array is ``created`` by writing appropriate superblocks to all |
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devices. |
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It is ``assembled`` by associating each of these devices with an |
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particular md virtual device. Once it is completely assembled, it can |
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be accessed. |
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An array should be created by a user-space tool. This will write |
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superblocks to all devices. It will usually mark the array as |
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``unclean``, or with some devices missing so that the kernel md driver |
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can create appropriate redundancy (copying in raid 1, parity |
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calculation in raid 4/5). |
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When an array is assembled, it is first initialized with the |
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SET_ARRAY_INFO ioctl. This contains, in particular, a major and minor |
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version number. The major version number selects which superblock |
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format is to be used. The minor number might be used to tune handling |
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of the format, such as suggesting where on each device to look for the |
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superblock. |
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Then each device is added using the ADD_NEW_DISK ioctl. This |
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provides, in particular, a major and minor number identifying the |
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device to add. |
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The array is started with the RUN_ARRAY ioctl. |
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Once started, new devices can be added. They should have an |
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appropriate superblock written to them, and then be passed in with |
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ADD_NEW_DISK. |
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Devices that have failed or are not yet active can be detached from an |
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array using HOT_REMOVE_DISK. |
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Specific Rules that apply to format-0 super block arrays, and arrays with no superblock (non-persistent) |
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-------------------------------------------------------------------------------------------------------- |
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An array can be ``created`` by describing the array (level, chunksize |
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etc) in a SET_ARRAY_INFO ioctl. This must have ``major_version==0`` and |
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``raid_disks != 0``. |
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Then uninitialized devices can be added with ADD_NEW_DISK. The |
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structure passed to ADD_NEW_DISK must specify the state of the device |
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and its role in the array. |
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Once started with RUN_ARRAY, uninitialized spares can be added with |
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HOT_ADD_DISK. |
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MD devices in sysfs |
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------------------- |
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md devices appear in sysfs (``/sys``) as regular block devices, |
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e.g.:: |
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/sys/block/md0 |
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Each ``md`` device will contain a subdirectory called ``md`` which |
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contains further md-specific information about the device. |
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All md devices contain: |
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level |
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a text file indicating the ``raid level``. e.g. raid0, raid1, |
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raid5, linear, multipath, faulty. |
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If no raid level has been set yet (array is still being |
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assembled), the value will reflect whatever has been written |
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to it, which may be a name like the above, or may be a number |
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such as ``0``, ``5``, etc. |
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raid_disks |
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a text file with a simple number indicating the number of devices |
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in a fully functional array. If this is not yet known, the file |
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will be empty. If an array is being resized this will contain |
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the new number of devices. |
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Some raid levels allow this value to be set while the array is |
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active. This will reconfigure the array. Otherwise it can only |
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be set while assembling an array. |
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A change to this attribute will not be permitted if it would |
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reduce the size of the array. To reduce the number of drives |
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in an e.g. raid5, the array size must first be reduced by |
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setting the ``array_size`` attribute. |
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chunk_size |
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This is the size in bytes for ``chunks`` and is only relevant to |
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raid levels that involve striping (0,4,5,6,10). The address space |
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of the array is conceptually divided into chunks and consecutive |
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chunks are striped onto neighbouring devices. |
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The size should be at least PAGE_SIZE (4k) and should be a power |
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of 2. This can only be set while assembling an array |
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layout |
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The ``layout`` for the array for the particular level. This is |
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simply a number that is interpreted differently by different |
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levels. It can be written while assembling an array. |
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array_size |
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This can be used to artificially constrain the available space in |
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the array to be less than is actually available on the combined |
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devices. Writing a number (in Kilobytes) which is less than |
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the available size will set the size. Any reconfiguration of the |
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array (e.g. adding devices) will not cause the size to change. |
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Writing the word ``default`` will cause the effective size of the |
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array to be whatever size is actually available based on |
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``level``, ``chunk_size`` and ``component_size``. |
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This can be used to reduce the size of the array before reducing |
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the number of devices in a raid4/5/6, or to support external |
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metadata formats which mandate such clipping. |
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reshape_position |
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This is either ``none`` or a sector number within the devices of |
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the array where ``reshape`` is up to. If this is set, the three |
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attributes mentioned above (raid_disks, chunk_size, layout) can |
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potentially have 2 values, an old and a new value. If these |
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values differ, reading the attribute returns:: |
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new (old) |
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and writing will effect the ``new`` value, leaving the ``old`` |
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unchanged. |
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component_size |
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For arrays with data redundancy (i.e. not raid0, linear, faulty, |
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multipath), all components must be the same size - or at least |
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there must a size that they all provide space for. This is a key |
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part or the geometry of the array. It is measured in sectors |
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and can be read from here. Writing to this value may resize |
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the array if the personality supports it (raid1, raid5, raid6), |
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and if the component drives are large enough. |
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metadata_version |
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This indicates the format that is being used to record metadata |
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about the array. It can be 0.90 (traditional format), 1.0, 1.1, |
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1.2 (newer format in varying locations) or ``none`` indicating that |
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the kernel isn't managing metadata at all. |
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Alternately it can be ``external:`` followed by a string which |
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is set by user-space. This indicates that metadata is managed |
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by a user-space program. Any device failure or other event that |
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requires a metadata update will cause array activity to be |
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suspended until the event is acknowledged. |
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resync_start |
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The point at which resync should start. If no resync is needed, |
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this will be a very large number (or ``none`` since 2.6.30-rc1). At |
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array creation it will default to 0, though starting the array as |
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``clean`` will set it much larger. |
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new_dev |
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This file can be written but not read. The value written should |
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be a block device number as major:minor. e.g. 8:0 |
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This will cause that device to be attached to the array, if it is |
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available. It will then appear at md/dev-XXX (depending on the |
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name of the device) and further configuration is then possible. |
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safe_mode_delay |
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When an md array has seen no write requests for a certain period |
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of time, it will be marked as ``clean``. When another write |
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request arrives, the array is marked as ``dirty`` before the write |
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commences. This is known as ``safe_mode``. |
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The ``certain period`` is controlled by this file which stores the |
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period as a number of seconds. The default is 200msec (0.200). |
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Writing a value of 0 disables safemode. |
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array_state |
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This file contains a single word which describes the current |
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state of the array. In many cases, the state can be set by |
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writing the word for the desired state, however some states |
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cannot be explicitly set, and some transitions are not allowed. |
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Select/poll works on this file. All changes except between |
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Active_idle and active (which can be frequent and are not |
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very interesting) are notified. active->active_idle is |
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reported if the metadata is externally managed. |
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clear |
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No devices, no size, no level |
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Writing is equivalent to STOP_ARRAY ioctl |
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inactive |
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May have some settings, but array is not active |
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all IO results in error |
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When written, doesn't tear down array, but just stops it |
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suspended (not supported yet) |
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All IO requests will block. The array can be reconfigured. |
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Writing this, if accepted, will block until array is quiessent |
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readonly |
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no resync can happen. no superblocks get written. |
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Write requests fail |
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read-auto |
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like readonly, but behaves like ``clean`` on a write request. |
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clean |
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no pending writes, but otherwise active. |
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When written to inactive array, starts without resync |
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If a write request arrives then |
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if metadata is known, mark ``dirty`` and switch to ``active``. |
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if not known, block and switch to write-pending |
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If written to an active array that has pending writes, then fails. |
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active |
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fully active: IO and resync can be happening. |
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When written to inactive array, starts with resync |
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write-pending |
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clean, but writes are blocked waiting for ``active`` to be written. |
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active-idle |
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like active, but no writes have been seen for a while (safe_mode_delay). |
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bitmap/location |
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This indicates where the write-intent bitmap for the array is |
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stored. |
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It can be one of ``none``, ``file`` or ``[+-]N``. |
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``file`` may later be extended to ``file:/file/name`` |
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``[+-]N`` means that many sectors from the start of the metadata. |
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This is replicated on all devices. For arrays with externally |
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managed metadata, the offset is from the beginning of the |
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device. |
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bitmap/chunksize |
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The size, in bytes, of the chunk which will be represented by a |
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single bit. For RAID456, it is a portion of an individual |
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device. For RAID10, it is a portion of the array. For RAID1, it |
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is both (they come to the same thing). |
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bitmap/time_base |
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The time, in seconds, between looking for bits in the bitmap to |
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be cleared. In the current implementation, a bit will be cleared |
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between 2 and 3 times ``time_base`` after all the covered blocks |
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are known to be in-sync. |
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bitmap/backlog |
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When write-mostly devices are active in a RAID1, write requests |
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to those devices proceed in the background - the filesystem (or |
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other user of the device) does not have to wait for them. |
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``backlog`` sets a limit on the number of concurrent background |
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writes. If there are more than this, new writes will by |
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synchronous. |
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bitmap/metadata |
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This can be either ``internal`` or ``external``. |
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``internal`` |
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is the default and means the metadata for the bitmap |
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is stored in the first 256 bytes of the allocated space and is |
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managed by the md module. |
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``external`` |
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means that bitmap metadata is managed externally to |
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the kernel (i.e. by some userspace program) |
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bitmap/can_clear |
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This is either ``true`` or ``false``. If ``true``, then bits in the |
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bitmap will be cleared when the corresponding blocks are thought |
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to be in-sync. If ``false``, bits will never be cleared. |
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This is automatically set to ``false`` if a write happens on a |
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degraded array, or if the array becomes degraded during a write. |
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When metadata is managed externally, it should be set to true |
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once the array becomes non-degraded, and this fact has been |
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recorded in the metadata. |
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consistency_policy |
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This indicates how the array maintains consistency in case of unexpected |
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shutdown. It can be: |
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none |
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Array has no redundancy information, e.g. raid0, linear. |
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resync |
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Full resync is performed and all redundancy is regenerated when the |
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array is started after unclean shutdown. |
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bitmap |
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Resync assisted by a write-intent bitmap. |
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journal |
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For raid4/5/6, journal device is used to log transactions and replay |
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after unclean shutdown. |
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ppl |
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For raid5 only, Partial Parity Log is used to close the write hole and |
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eliminate resync. |
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The accepted values when writing to this file are ``ppl`` and ``resync``, |
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used to enable and disable PPL. |
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uuid |
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This indicates the UUID of the array in the following format: |
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xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx |
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As component devices are added to an md array, they appear in the ``md`` |
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directory as new directories named:: |
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dev-XXX |
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where ``XXX`` is a name that the kernel knows for the device, e.g. hdb1. |
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Each directory contains: |
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block |
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a symlink to the block device in /sys/block, e.g.:: |
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/sys/block/md0/md/dev-hdb1/block -> ../../../../block/hdb/hdb1 |
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super |
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A file containing an image of the superblock read from, or |
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written to, that device. |
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state |
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A file recording the current state of the device in the array |
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which can be a comma separated list of: |
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faulty |
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device has been kicked from active use due to |
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a detected fault, or it has unacknowledged bad |
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blocks |
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in_sync |
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device is a fully in-sync member of the array |
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writemostly |
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device will only be subject to read |
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requests if there are no other options. |
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This applies only to raid1 arrays. |
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blocked |
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device has failed, and the failure hasn't been |
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acknowledged yet by the metadata handler. |
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Writes that would write to this device if |
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it were not faulty are blocked. |
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spare |
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device is working, but not a full member. |
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This includes spares that are in the process |
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of being recovered to |
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write_error |
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device has ever seen a write error. |
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want_replacement |
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device is (mostly) working but probably |
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should be replaced, either due to errors or |
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due to user request. |
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replacement |
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device is a replacement for another active |
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device with same raid_disk. |
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This list may grow in future. |
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This can be written to. |
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Writing ``faulty`` simulates a failure on the device. |
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Writing ``remove`` removes the device from the array. |
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Writing ``writemostly`` sets the writemostly flag. |
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Writing ``-writemostly`` clears the writemostly flag. |
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Writing ``blocked`` sets the ``blocked`` flag. |
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Writing ``-blocked`` clears the ``blocked`` flags and allows writes |
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to complete and possibly simulates an error. |
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Writing ``in_sync`` sets the in_sync flag. |
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Writing ``write_error`` sets writeerrorseen flag. |
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Writing ``-write_error`` clears writeerrorseen flag. |
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Writing ``want_replacement`` is allowed at any time except to a |
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replacement device or a spare. It sets the flag. |
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Writing ``-want_replacement`` is allowed at any time. It clears |
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the flag. |
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Writing ``replacement`` or ``-replacement`` is only allowed before |
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starting the array. It sets or clears the flag. |
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This file responds to select/poll. Any change to ``faulty`` |
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or ``blocked`` causes an event. |
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errors |
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An approximate count of read errors that have been detected on |
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this device but have not caused the device to be evicted from |
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the array (either because they were corrected or because they |
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happened while the array was read-only). When using version-1 |
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metadata, this value persists across restarts of the array. |
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|
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This value can be written while assembling an array thus |
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providing an ongoing count for arrays with metadata managed by |
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userspace. |
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slot |
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This gives the role that the device has in the array. It will |
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either be ``none`` if the device is not active in the array |
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(i.e. is a spare or has failed) or an integer less than the |
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``raid_disks`` number for the array indicating which position |
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it currently fills. This can only be set while assembling an |
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array. A device for which this is set is assumed to be working. |
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offset |
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This gives the location in the device (in sectors from the |
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start) where data from the array will be stored. Any part of |
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the device before this offset is not touched, unless it is |
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used for storing metadata (Formats 1.1 and 1.2). |
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size |
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The amount of the device, after the offset, that can be used |
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for storage of data. This will normally be the same as the |
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component_size. This can be written while assembling an |
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array. If a value less than the current component_size is |
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written, it will be rejected. |
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recovery_start |
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When the device is not ``in_sync``, this records the number of |
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sectors from the start of the device which are known to be |
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correct. This is normally zero, but during a recovery |
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operation it will steadily increase, and if the recovery is |
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interrupted, restoring this value can cause recovery to |
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avoid repeating the earlier blocks. With v1.x metadata, this |
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value is saved and restored automatically. |
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|
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This can be set whenever the device is not an active member of |
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the array, either before the array is activated, or before |
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the ``slot`` is set. |
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|
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Setting this to ``none`` is equivalent to setting ``in_sync``. |
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Setting to any other value also clears the ``in_sync`` flag. |
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bad_blocks |
|
This gives the list of all known bad blocks in the form of |
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start address and length (in sectors respectively). If output |
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is too big to fit in a page, it will be truncated. Writing |
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``sector length`` to this file adds new acknowledged (i.e. |
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recorded to disk safely) bad blocks. |
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|
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unacknowledged_bad_blocks |
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This gives the list of known-but-not-yet-saved-to-disk bad |
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blocks in the same form of ``bad_blocks``. If output is too big |
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to fit in a page, it will be truncated. Writing to this file |
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adds bad blocks without acknowledging them. This is largely |
|
for testing. |
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ppl_sector, ppl_size |
|
Location and size (in sectors) of the space used for Partial Parity Log |
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on this device. |
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An active md device will also contain an entry for each active device |
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in the array. These are named:: |
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rdNN |
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where ``NN`` is the position in the array, starting from 0. |
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So for a 3 drive array there will be rd0, rd1, rd2. |
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These are symbolic links to the appropriate ``dev-XXX`` entry. |
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Thus, for example:: |
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cat /sys/block/md*/md/rd*/state |
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will show ``in_sync`` on every line. |
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Active md devices for levels that support data redundancy (1,4,5,6,10) |
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also have |
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sync_action |
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a text file that can be used to monitor and control the rebuild |
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process. It contains one word which can be one of: |
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resync |
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redundancy is being recalculated after unclean |
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shutdown or creation |
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recover |
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a hot spare is being built to replace a |
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failed/missing device |
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idle |
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nothing is happening |
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check |
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A full check of redundancy was requested and is |
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happening. This reads all blocks and checks |
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them. A repair may also happen for some raid |
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levels. |
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repair |
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A full check and repair is happening. This is |
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similar to ``resync``, but was requested by the |
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user, and the write-intent bitmap is NOT used to |
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optimise the process. |
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This file is writable, and each of the strings that could be |
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read are meaningful for writing. |
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``idle`` will stop an active resync/recovery etc. There is no |
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guarantee that another resync/recovery may not be automatically |
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started again, though some event will be needed to trigger |
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this. |
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``resync`` or ``recovery`` can be used to restart the |
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corresponding operation if it was stopped with ``idle``. |
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``check`` and ``repair`` will start the appropriate process |
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providing the current state is ``idle``. |
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This file responds to select/poll. Any important change in the value |
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triggers a poll event. Sometimes the value will briefly be |
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``recover`` if a recovery seems to be needed, but cannot be |
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achieved. In that case, the transition to ``recover`` isn't |
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notified, but the transition away is. |
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degraded |
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This contains a count of the number of devices by which the |
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arrays is degraded. So an optimal array will show ``0``. A |
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single failed/missing drive will show ``1``, etc. |
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This file responds to select/poll, any increase or decrease |
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in the count of missing devices will trigger an event. |
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mismatch_count |
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When performing ``check`` and ``repair``, and possibly when |
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performing ``resync``, md will count the number of errors that are |
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found. The count in ``mismatch_cnt`` is the number of sectors |
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that were re-written, or (for ``check``) would have been |
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re-written. As most raid levels work in units of pages rather |
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than sectors, this may be larger than the number of actual errors |
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by a factor of the number of sectors in a page. |
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bitmap_set_bits |
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If the array has a write-intent bitmap, then writing to this |
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attribute can set bits in the bitmap, indicating that a resync |
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would need to check the corresponding blocks. Either individual |
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numbers or start-end pairs can be written. Multiple numbers |
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can be separated by a space. |
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Note that the numbers are ``bit`` numbers, not ``block`` numbers. |
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They should be scaled by the bitmap_chunksize. |
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sync_speed_min, sync_speed_max |
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This are similar to ``/proc/sys/dev/raid/speed_limit_{min,max}`` |
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however they only apply to the particular array. |
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If no value has been written to these, or if the word ``system`` |
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is written, then the system-wide value is used. If a value, |
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in kibibytes-per-second is written, then it is used. |
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When the files are read, they show the currently active value |
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followed by ``(local)`` or ``(system)`` depending on whether it is |
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a locally set or system-wide value. |
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sync_completed |
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This shows the number of sectors that have been completed of |
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whatever the current sync_action is, followed by the number of |
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sectors in total that could need to be processed. The two |
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numbers are separated by a ``/`` thus effectively showing one |
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value, a fraction of the process that is complete. |
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A ``select`` on this attribute will return when resync completes, |
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when it reaches the current sync_max (below) and possibly at |
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other times. |
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sync_speed |
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This shows the current actual speed, in K/sec, of the current |
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sync_action. It is averaged over the last 30 seconds. |
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suspend_lo, suspend_hi |
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The two values, given as numbers of sectors, indicate a range |
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within the array where IO will be blocked. This is currently |
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only supported for raid4/5/6. |
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sync_min, sync_max |
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The two values, given as numbers of sectors, indicate a range |
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within the array where ``check``/``repair`` will operate. Must be |
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a multiple of chunk_size. When it reaches ``sync_max`` it will |
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pause, rather than complete. |
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You can use ``select`` or ``poll`` on ``sync_completed`` to wait for |
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that number to reach sync_max. Then you can either increase |
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``sync_max``, or can write ``idle`` to ``sync_action``. |
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The value of ``max`` for ``sync_max`` effectively disables the limit. |
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When a resync is active, the value can only ever be increased, |
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never decreased. |
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The value of ``0`` is the minimum for ``sync_min``. |
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Each active md device may also have attributes specific to the |
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personality module that manages it. |
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These are specific to the implementation of the module and could |
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change substantially if the implementation changes. |
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These currently include: |
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stripe_cache_size (currently raid5 only) |
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number of entries in the stripe cache. This is writable, but |
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there are upper and lower limits (32768, 17). Default is 256. |
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strip_cache_active (currently raid5 only) |
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number of active entries in the stripe cache |
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preread_bypass_threshold (currently raid5 only) |
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number of times a stripe requiring preread will be bypassed by |
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a stripe that does not require preread. For fairness defaults |
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to 1. Setting this to 0 disables bypass accounting and |
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requires preread stripes to wait until all full-width stripe- |
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writes are complete. Valid values are 0 to stripe_cache_size. |
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journal_mode (currently raid5 only) |
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The cache mode for raid5. raid5 could include an extra disk for |
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caching. The mode can be "write-throuth" and "write-back". The |
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default is "write-through". |
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ppl_write_hint |
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NVMe stream ID to be set for each PPL write request.
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