45 lines
1.7 KiB
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45 lines
1.7 KiB
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.. 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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