Linux filesystem(ext4), inode and blocks
2025-02-22
2025-04-20
data structure
block group: partition consists of multiple block groups
block group 0: a little bit special. first 1024 bytes are reserved for boot sector
super block (sb): is a record of the characteristics of a filesystem. primary superblock is in block group 0. Backup superblock is in block groups whose group number is power of 1,3,5,7
group descriptor: records the location of block bitmap and inode bitmap. counter for free blocks and inodes. number of directories. block bitmap: each bit represents block. max size is 8*4096(block size)
inode: index nodes
inode table: array of inodes
inode bitmap: each bit represents a inode in inodetable. max size is 8*4096(block size)
commands
dumpe2fs(8) - retrieve filesystem information
$ sudo dumpe2fs -h /dev/nvme0n1p4
Filesystem OS type: Linux
Inode count: 24453120
Block count: 97792081
Reserved block count: 4889604
Overhead clusters: 1814658
Free blocks: 81687378
Free inodes: 23866982
First block: 0
Block size: 4096
Fragment size: 4096
Group descriptor size: 64
Reserved GDT blocks: 1024
Blocks per group: 32768
Fragments per group: 32768
Inodes per group: 8192
Inode blocks per group: 512
dumpe2fs(8) - where is primary and backup superblocks (1,3,5,7,...)
$ sudo dumpe2fs /dev/nvme0n1p4 | grep -B 1 -i superblock
dumpe2fs 1.47.2 (1-Jan-2025)
Group 0: (Blocks 0-32767) csum 0x6bd4 [ITABLE_ZEROED]
Primary superblock at 0, Group descriptors at 1-47
--
Group 1: (Blocks 32768-65535) csum 0xab1c [ITABLE_ZEROED]
Backup superblock at 32768, Group descriptors at 32769-32815
--
Group 3: (Blocks 98304-131071) csum 0x992a [ITABLE_ZEROED]
Backup superblock at 98304, Group descriptors at 98305-98351
--
Group 5: (Blocks 163840-196607) csum 0x8083 [ITABLE_ZEROED]
Backup superblock at 163840, Group descriptors at 163841-163887
--
Group 7: (Blocks 229376-262143) csum 0x84c7 [ITABLE_ZEROED]
Backup superblock at 229376, Group descriptors at 229377-229423
--
tune2fs(8) - modify filesystem parameters
$ sudo tune2fs -l /dev/nvmen01p4
tune2fs 1.47.2 (1-Jan-2025)
Filesystem volume name: endeavouros
Last mounted on: /sysroot
Filesystem UUID: 9b6cd942-2a06-48b1-b9fb-cf00862cb5d0
Filesystem magic number: 0xEF53
Filesystem revision #: 1 (dynamic)
Filesystem features: has_journal ext_attr resize_inode dir_index orphan_file filetype needs_recovery extent 64bit flex_bg metadata_csum_seed sparse_super large_file huge_file dir_nlink extra_isize metadata_csum orphan_present
Filesystem flags: signed_directory_hash
Default mount options: user_xattr acl
Filesystem state: clean
Errors behavior: Continue
Filesystem OS type: Linux
Inode count: 24453120
Block count: 97792081
Reserved block count: 4889604
Overhead clusters: 1814658
Free blocks: 81687378
Free inodes: 23866982
First block: 0
Block size: 4096
Fragment size: 4096
Group descriptor size: 64
Reserved GDT blocks: 1024
Blocks per group: 32768
Fragments per group: 32768
Inodes per group: 8192
Inode blocks per group: 512
Flex block group size: 16
Filesystem created: Mon Nov 27 02:07:25 2023
Last mount time: Sun Feb 16 04:23:30 2025
Last write time: Sun Feb 23 23:39:58 2025
Mount count: 32
Maximum mount count: -1
Last checked: Tue Dec 31 14:40:33 2024
Check interval: 0 (<none>)
Lifetime writes: 1030 GB
Reserved blocks uid: 0 (user root)
Reserved blocks gid: 0 (group root)
First inode: 11
Inode size: 256
Required extra isize: 32
Desired extra isize: 32
Journal inode: 8
Default directory hash: half_md4
Directory Hash Seed: de0781a2-b353-41ab-9a47-2cfdaefb0ec8
Journal backup: inode blocks
Checksum type: crc32c
Checksum: 0xb125b965
Checksum seed: 0x0bc9aa60
Orphan file inode: 12
stat(1) - print file or file system status
stat calculates "Blocks" based on virtual block(512 bytes), that’s why you see 24 blocks below.
$ stat -fc "%n type=%T freeblk=%f totalblk=%b blksz=%S" /root
/root type=ext2/ext3 freeblk=81921201 totalblk=95977423 blksz=4096
$ dd if=/dev/urandom of=a count=3 bs=4096
$ stat a
File: a
Size: 12288 Blocks: 24 IO Block: 4096 regular file
Device: 259,4 Inode: 21644642 Links: 1
Access: (0644/-rw-r--r--) Uid: ( 1000/ hjl) Gid: ( 1000/ hjl)
Access: 2025-04-19 23:19:07.283020996 +0800
Modify: 2025-04-19 23:20:00.730320206 +0800
Change: 2025-04-19 23:20:00.730320206 +0800
Birth: 2025-04-19 23:19:07.283020996 +0800
ls(1) - list file and directory inode
$ dd if=/dev/urandom of=a count=3 bs=4096
$ ls -i a
21644642 a
$ mkdir b
$ ls -id .
21657519 .
$ ls -id b
21774445 b
$ ls -lia b
total 8
21774445 drwxr-xr-x 2 hjl hjl 4096 Apr 20 03:18 .
21657519 drwxr-xr-x 3 hjl hjl 4096 Apr 20 03:18 ..
df(1) - filesystem space usage
$ df /dev/nvme0n1p4
Filesystem 1K-blocks Used Available Use% Mounted on
/dev/nvme0n1p4 383909692 59225448 305109444 17% /
$ df -B 4096 /dev/nvme0n1p4
Filesystem 4K-blocks Used Available Use% Mounted on
/dev/nvme0n1p4 95977423 14806327 76277396 17% /
blockdev(8) - block device ioctls
from "EXTENTS" we can see file a occupies 3 blocks
$ sudo blockdev --getbsz /dev/nvme0n1p4
4096
$ dd if=/dev/urandom of=a count=3 bs=4096
$ ls -i a
21644642 a
$ sudo debugfs -R "stat <21644642>" /dev/nvme0n1p4
debugfs 1.47.2 (1-Jan-2025)
Inode: 21644642 Type: regular Mode: 0644 Flags: 0x80000
Generation: 1360945917 Version: 0x00000000:00000009
User: 1000 Group: 1000 Project: 0 Size: 12288
File ACL: 0
Links: 1 Blockcount: 24
Fragment: Address: 0 Number: 0 Size: 0
ctime: 0x6803bf20:ae1f3538 -- Sat Apr 19 23:20:00 2025
atime: 0x6803beeb:437a3b10 -- Sat Apr 19 23:19:07 2025
mtime: 0x6803bf20:ae1f3538 -- Sat Apr 19 23:20:00 2025
crtime: 0x6803beeb:437a3b10 -- Sat Apr 19 23:19:07 2025
Size of extra inode fields: 32
Inode checksum: 0x44c771d9
EXTENTS:
(0-2):5850527-5850529
Reference
https://www.kernel.org/doc/html/v6.13/filesystems/ext4/overview.html#layout
https://www.kernel.org/doc/html/v6.13/filesystems/ext4/dynamic.html#dynamic-structures