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Ext4 Disk Layout

From Ext4

is document aempts to describe the on-disk format for ext4 ﬁlesystems. e same general

ideas should apply to ext2/3 ﬁlesystems as well, though they do not support all the features that

ext4 supports, and the ﬁelds will be shorter.

NOTE: is is a work in progress, based on notes that the author (djwong) made while picking

apart a ﬁlesystem by hand. e data structure deﬁnitions were pulled out of Linux 3.11 and

e2fsprogs-1.42.8. He welcomes all comments and corrections, since there is undoubtedly plenty

of lore that doesn't necessarily show up on freshly created demonstration ﬁlesystems.

Contents

1 Terminology

2 Overview

2.1 Blocks

2.2 Layout

2.3 Flexible Block Groups

2.4 Meta Block Groups

2.5 Lazy Block Group Initialization

2.6 Special inodes

2.7 Block and Inode Allocation Policy

2.8 Checksums

2.9 Bigalloc

2.10 Inline Data

2.10.1 Inline Directories

3 e Super Block

4 Block Group Descriptors

5 Block and inode Bitmaps

6 Inode Table

6.1 Finding an Inode

6.2 Inode Size

6.3 Inode Timestamps

7 e Contents of inode.i_block

7.1 Symbolic Links

Ext4 Disk Layout - Ext4 hps://ext4.wiki.kernel.org/index.php/Ext4_Disk_Layout

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block groups necessarily host a redundant copy (see following paragraph for more details). If

the group does not have a redundant copy, the block group begins with the data block bitmap.

Note also that when the ﬁlesystem is freshly formaed, mkfs will allocate "reserve GDT block"

space aer the block group descriptors and before the start of the block bitmaps to allow for

future expansion of the ﬁlesystem. By default, a ﬁlesystem is allowed to increase in size by a

factor of 1024x over the original ﬁlesystem size.

e location of the inode table is given by grp.bg_inode_table_*. It is continuous range of

blocks large enough to contain sb.s_inodes_per_group * sb.s_inode_size bytes.

As for the ordering of items in a block group, it is generally established that the super block and

the group descriptor table, if present, will be at the beginning of the block group. e bitmaps

and the inode table can be anywhere, and it is quite possible for the bitmaps to come aer the

inode table, or for both to be in diﬀerent groups (ﬂex_bg). Leover space is used for ﬁle data

blocks, indirect block maps, extent tree blocks, and extended aributes.

Flexible Block Groups

Starting in ext4, there is a new feature called ﬂexible block groups (ﬂex_bg). In a ﬂex_bg, several

block groups are tied together as one logical block group; the bitmap spaces and the inode table

space in the ﬁrst block group of the ﬂex_bg are expanded to include the bitmaps and inode

tables of all other block groups in the ﬂex_bg. For example, if the ﬂex_bg size is 4, then group 0

will contain (in order) the superblock, group descriptors, data block bitmaps for groups 0-3,

inode bitmaps for groups 0-3, inode tables for groups 0-3, and the remaining space in group 0 is

for ﬁle data. e eﬀect of this is to group the block metadata close together for faster loading,

and to enable large ﬁles to be continuous on disk. Backup copies of the superblock and group

descriptors are always at the beginning of block groups, even if ﬂex_bg is enabled. e number

of block groups that make up a ﬂex_bg is given by 2 ^ sb.s_log_groups_per_flex.

Meta Block Groups

Without the option META_BG, for safety concerns, all block group descriptors copies are kept

in the ﬁrst block group. Given the default 128MiB(2^27 bytes) block group size and 64-byte

group descriptors, ext4 can have at most 2^27/64 = 2^21 block groups. is limits the entire

ﬁlesystem size to 2^21 ∗ 2^27 = 2^48bytes or 256TiB.

e solution to this problem is to use the metablock group feature (META_BG), which is already

in ext3 for all 2.6 releases. With the META_BG feature, ext4 ﬁlesystems are partitioned into

many metablock groups. Each metablock group is a cluster of block groups whose group

descriptor structures can be stored in a single disk block. For ext4 ﬁlesystems with 4 KB block

size, a single metablock group partition includes 64 block groups, or 8 GiB of disk space. e

metablock group feature moves the location of the group descriptors from the congested ﬁrst

Ext4 Disk Layout - Ext4 hps://ext4.wiki.kernel.org/index.php/Ext4_Disk_Layout

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