You can not select more than 25 topics
Topics must start with a letter or number, can include dashes ('-') and can be up to 35 characters long.
109 lines
4.9 KiB
109 lines
4.9 KiB
<title>Rob's ext2 documentation</title>
|
|
|
|
<p>This page focuses on the ext2 on-disk format. The Linux kernel's filesystem
|
|
implementation (the code to read and write it) is documented in the kernel
|
|
source, Documentation/filesystems/ext2.txt.</p>
|
|
|
|
<p>Note: for our purposes, ext3 and ext4 are just ext2 with some extra data
|
|
fields.</p>
|
|
|
|
<h2>Overview</h2>
|
|
|
|
<h2>Blocks and Block Groups</h2>
|
|
|
|
<p>Every ext2 filesystem consists of blocks, which are divided into block
|
|
groups. Blocks can be 1k, 2k, or 4k in length.<super><a href="#1">[1]</a></super>
|
|
All ext2 disk layout is done in terms of these logical blocks, never in
|
|
terms of 512-byte logical blocks.</p>
|
|
|
|
<p>Each block group contains as many blocks as one block can hold a
|
|
bitmap for, so at a 1k block size a block group contains 8192 blocks (1024
|
|
bytes * 8 bits), and at 4k block size a block group contains 32768 blocks.
|
|
Groups are numbered starting at 0, and occur one after another on disk,
|
|
in order, with no gaps between them.</p>
|
|
|
|
<p>Block groups contain the following structures, in order:</p>
|
|
|
|
<ul>
|
|
<li>Superblock (sometimes)</li>
|
|
<li>Group table (sometimes)</li>
|
|
<li>Block bitmap</li>
|
|
<li>Inode bitmap</li>
|
|
<li>Inode table</li>
|
|
<li>Data blocks</li>
|
|
</ul>
|
|
|
|
<p>Not all block groups contain all structures. Specifically, the first two
|
|
(superblock and group table) only occur in some groups, and other block
|
|
groups start with the block bitmap and go from there. This frees up more
|
|
data blocks to hold actual file and directory data, see the superblock
|
|
description for details.</p>
|
|
|
|
<p>Each structure in this list is stored in its' own block (or blocks in the
|
|
case of the group and inode tables), and doesn't share blocks with any other
|
|
structure. This can involve padding the end of the block with zeroes, or
|
|
extending tables with extra entries to fill up the rest of the block.</p>
|
|
|
|
<p>The linux/ext2_fs.h #include file defines struct ext2_super_block,
|
|
struct ext2_group_desc, struct ext2_inode, struct ext2_dir_entry_2, and a lot
|
|
of constants. Toybox doesn't use this file directly, instead it has an e2fs.h
|
|
include of its own containting cleaned-up versions of the data it needs.</p>
|
|
|
|
<h2>Superblock</h2>
|
|
|
|
<p>The superblock contains a 1024 byte structure, which toybox calls
|
|
"struct ext2_superblock". Where exactly this structure is to be found is
|
|
a bit complicated for historical reasons.</p>
|
|
|
|
<p>For copies of the superblock stored in block groups after the first,
|
|
the superblock structure starts at the beginning of the first block of the
|
|
group, with zero padding afterwards if necessary (I.E. if the block size is
|
|
larger than 1k). In modern "sparse superblock" filesystems (everything
|
|
anyone still cares about), the superblock occurs in group 0 and in later groups
|
|
that are powers of 3, 5, and 7. (So groups 0, 1, 3, 5, 7, 9, 25, 27, 49, 81,
|
|
125, 243, 343...) Any block group starting with a superblock will also
|
|
have a group descriptor table, and ones that don't won't.</p>
|
|
|
|
<p>The very first superblock is weird. This is because if you format an entire
|
|
block device (rather than a partition), you stomp the very start of the disk
|
|
which contains the boot sector and the partition table. Back when ext2 on
|
|
floppies was common, this was a big deal.</p>
|
|
|
|
<p>So the very first 1024 bytes of the very first block are always left alone.
|
|
When the block size is 1024 bytes, then that block is left alone and the
|
|
superblock is stored in the second block instead<super><a href="#2">[2]</a>.
|
|
When the block size is larger than 1024 bytes, the first superblock starts
|
|
1024 bytes into the block, with the original data preserved by mke2fs and
|
|
appropriate zero padding added to the end of the block (if necessary).</p>
|
|
|
|
<h2>Group descriptor table</h2>
|
|
<h2>Block bitmap</h2>
|
|
<h2>Inode bitmap</h2>
|
|
<h2>Inode table</h2>
|
|
<h2>Data blocks</h2>
|
|
|
|
<h2>Directories</h2>
|
|
|
|
<p>For performance reasons, directory entries are 4-byte aligned (rec_len is
|
|
a multiple of 4), so up to 3 bytes of padding (zeroes) can be added at the end
|
|
of each name. (This affects rec_len but not the name_len.)</p>
|
|
|
|
<p>The last directory entry in each block is padded up to block size. If there
|
|
isn't enough space for another struct ext2_dentry the last </p>
|
|
|
|
<p>Question: is the length stored in the inode also padded up to block size?</p>
|
|
|
|
<hr />
|
|
<p><a name="1" />Footnote 1: On some systems blocks can be larger than 4k, but
|
|
for implementation reasons not larger than PAGE_SIZE. So the Alpha can have
|
|
8k blocks but most other systems couldn't mount them, thus you don't see this
|
|
out in the wild much anymore.</p>
|
|
|
|
<p><a name="2" />Footnote 2: In this case, the first_data_block field in the
|
|
superblock structure will be set to 1. Otherwise it's always 0. How this
|
|
could POSSIBLY be useful information is an open question, since A) you have to
|
|
read the superblock before you can get this information, so you know where
|
|
it came from, B) the first copy of the superblock always starts at offset 1024
|
|
no matter what, and if your block size is 1024 you already know you skipped the
|
|
first block.</p>
|