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44 lines
1.9 KiB
44 lines
1.9 KiB
.. SPDX-License-Identifier: GPL-2.0 |
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============== |
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Fuse I/O Modes |
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============== |
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Fuse supports the following I/O modes: |
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- direct-io |
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- cached |
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+ write-through |
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+ writeback-cache |
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The direct-io mode can be selected with the FOPEN_DIRECT_IO flag in the |
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FUSE_OPEN reply. |
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In direct-io mode the page cache is completely bypassed for reads and writes. |
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No read-ahead takes place. Shared mmap is disabled. |
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In cached mode reads may be satisfied from the page cache, and data may be |
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read-ahead by the kernel to fill the cache. The cache is always kept consistent |
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after any writes to the file. All mmap modes are supported. |
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The cached mode has two sub modes controlling how writes are handled. The |
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write-through mode is the default and is supported on all kernels. The |
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writeback-cache mode may be selected by the FUSE_WRITEBACK_CACHE flag in the |
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FUSE_INIT reply. |
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In write-through mode each write is immediately sent to userspace as one or more |
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WRITE requests, as well as updating any cached pages (and caching previously |
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uncached, but fully written pages). No READ requests are ever sent for writes, |
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so when an uncached page is partially written, the page is discarded. |
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In writeback-cache mode (enabled by the FUSE_WRITEBACK_CACHE flag) writes go to |
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the cache only, which means that the write(2) syscall can often complete very |
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fast. Dirty pages are written back implicitly (background writeback or page |
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reclaim on memory pressure) or explicitly (invoked by close(2), fsync(2) and |
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when the last ref to the file is being released on munmap(2)). This mode |
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assumes that all changes to the filesystem go through the FUSE kernel module |
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(size and atime/ctime/mtime attributes are kept up-to-date by the kernel), so |
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it's generally not suitable for network filesystems. If a partial page is |
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written, then the page needs to be first read from userspace. This means, that |
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even for files opened for O_WRONLY it is possible that READ requests will be |
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generated by the kernel.
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