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44 lines
1.7 KiB
44 lines
1.7 KiB
.. SPDX-License-Identifier: GPL-2.0 |
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About this Book |
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=============== |
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This document attempts to describe the on-disk format for ext4 |
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filesystems. The same general ideas should apply to ext2/3 filesystems |
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as well, though they do not support all the features that ext4 supports, |
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and the fields will be shorter. |
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**NOTE**: This is a work in progress, based on notes that the author |
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(djwong) made while picking apart a filesystem by hand. The data |
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structure definitions should be current as of Linux 4.18 and |
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e2fsprogs-1.44. All comments and corrections are welcome, since there is |
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undoubtedly plenty of lore that might not be reflected in freshly |
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created demonstration filesystems. |
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License |
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------- |
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This book is licensed under the terms of the GNU Public License, v2. |
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Terminology |
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----------- |
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ext4 divides a storage device into an array of logical blocks both to |
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reduce bookkeeping overhead and to increase throughput by forcing larger |
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transfer sizes. Generally, the block size will be 4KiB (the same size as |
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pages on x86 and the block layer's default block size), though the |
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actual size is calculated as 2 ^ (10 + ``sb.s_log_block_size``) bytes. |
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Throughout this document, disk locations are given in terms of these |
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logical blocks, not raw LBAs, and not 1024-byte blocks. For the sake of |
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convenience, the logical block size will be referred to as |
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``$block_size`` throughout the rest of the document. |
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When referenced in ``preformatted text`` blocks, ``sb`` refers to fields |
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in the super block, and ``inode`` refers to fields in an inode table |
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entry. |
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Other References |
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---------------- |
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Also see https://www.nongnu.org/ext2-doc/ for quite a collection of |
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information about ext2/3. Here's another old reference: |
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http://wiki.osdev.org/Ext2
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