ext2文件系统解析---透过dumpe2fs看ext2文件系统

个人随笔 (Owed by: 春夜喜雨 http://blog.csdn.net/chunyexiyu)

参考资料

参考:https://www.nongnu.org/ext2-doc/ext2.pdf

参考:https://developer.aliyun.com/article/297657

参考:https://www.bilibili.com/read/cv17345430/

参考:https://github.com/torvalds/linux/blob/master/fs/ext2/ext2.h

参考:https://baike.baidu.com/item/e2fsprogs/6202376?fr=ge_ala

参考:https://www.easeus.com/partition-master/ext2-ext3-ext4-file-system-format-and-difference.html

1. 引言

对于ext2文件系统,我们大多数会有所耳闻的,那我们是否知道,该文件系统的内部存储结构是什么样的?

另外对于一个制定大小的文件系统,它的存储大小和文件数有约束吗?

一个10MB的ext2文件系统中:

能支持建立多少个文件?能建立1000个?10000个?和什么有关系?

能最大存储多大的内容,是满10MB吗?还是9MB?由什么来确定的?

这些问题,我们可以借助dump2fs命令的使用,来做一个初步的探究。

2. dumpe2fs查看信息

创建出loop device之后,使用命令如下,来查看ext2设备的信息。

创建loop device的方法,参考文章下面附录中方法。

shell 复制代码
[root@ls_CxhK1nVN ~]# dumpe2fs /dev/loop0
[root@ls_CxhK1nVN ~]# dumpe2fs /dev/loop1

下面是一个新创建的10MB的loop deivce的信息:

10MB的内容如下:

shell 复制代码
[root@ls_CxhK1nVN ~]# dumpe2fs /dev/loop1
dumpe2fs 1.45.6 (20-Mar-2020)
Filesystem volume name:   <none>
Last mounted on:          /root/mongodir
Filesystem UUID:          10f36aab-aa44-4d96-8733-ee31bd6bed5f
Filesystem magic number:  0xEF53
Filesystem revision #:    1 (dynamic)
Filesystem features:      ext_attr resize_inode dir_index filetype sparse_super large_file
Filesystem flags:         signed_directory_hash 
Default mount options:    user_xattr acl
Filesystem state:         not clean
Errors behavior:          Continue
Filesystem OS type:       Linux
Inode count:              2560
Block count:              10240
Reserved block count:     512
Free blocks:              9819
Free inodes:              2549
First block:              1
Block size:               1024
Fragment size:            1024
Reserved GDT blocks:      39
Blocks per group:         8192
Fragments per group:      8192
Inodes per group:         1280
Inode blocks per group:   160
Filesystem created:       Thu Mar 14 15:11:03 2024
Last mount time:          Thu Mar 14 15:11:48 2024
Last write time:          Thu Mar 14 15:11:48 2024
Mount count:              1
Maximum mount count:      -1
Last checked:             Thu Mar 14 15:11:03 2024
Check interval:           0 (<none>)
Reserved blocks uid:      0 (user root)
Reserved blocks gid:      0 (group root)
First inode:              11
Inode size:               128
Default directory hash:   half_md4
Directory Hash Seed:      3bd0f015-62fe-4eb2-b815-0fed1e1ebf11


Group 0: (Blocks 1-8192)
  Primary superblock at 1, Group descriptors at 2-2
  Reserved GDT blocks at 3-41
  Block bitmap at 42 (+41)
  Inode bitmap at 43 (+42)
  Inode table at 44-203 (+43)
  7975 free blocks, 1269 free inodes, 2 directories
  Free blocks: 218-8192
  Free inodes: 12-1280
Group 1: (Blocks 8193-10239)
  Backup superblock at 8193, Group descriptors at 8194-8194
  Reserved GDT blocks at 8195-8233
  Block bitmap at 8234 (+41)
  Inode bitmap at 8235 (+42)
  Inode table at 8236-8395 (+43)
  1844 free blocks, 1280 free inodes, 0 directories
  Free blocks: 8396-10239
  Free inodes: 1281-2560

3. 信息解析

通过上述信息,我们关注到:

  1. Block总量,Block count * Block size = 10MB,正好是一个block1KB,10240个block。

    Block count: 10240

    Block size: 1024

  2. Inode节点数量,Inode count * Inode size = 320 * 1024;

    将会占用320个block,正好有2个Group,1个Group存储160个,和下面的信息相互印证了。

    Inode count: 2560

    Inode size: 128

    Inode blocks per group: 160

    Group 0: (Blocks 1-8192)

    Inode table at 44-203 (+43)

    Group 1: (Blocks 8193-10239)

    Inode table at 8236-8395 (+43)

  3. 关注初始创建后的空闲block数量,空闲inode数量

    空闲block数量将决定存储容量的门限:9819 * 1024 = 9819KB,比10240少421个Blocks。

    空闲inodes数量将决定可创建文件数量的门限:2549个文件

    Free blocks: 9819

    Free inodes: 2549

  4. 关注Block count / Inode count比值 = 4,不考虑文件系统占用情况下,每4KB会支持建立一个文件;

    推算得出:基于10MB是2560个文件,那么1GB就是256,000, 1TB就是256,000,000个文件,支持的文件数量还是非常多的。

    Block count: 10240

    Block size: 1024

    Inode count: 2560

  5. Group Table的大小与Bitmap关系,Blocks per group * 1024 = 8MB。

    每个Group大小是8MB;当前总大小10MB,拆分处了一个8MB,一个约2MB的group。

    为什么Group大小采用8MB,这个和Bitmap有关系吗?是有关系的,Bitmap占据一个block,也就是1024字节,也是1024*8=8192比特。

    Bitmap最多能指示8192个block的使用情况,而Group Table也正式采用的8192个block组成,刚好Bitmap可以全部指示出来。

    Blocks per group: 8192

    Block size: 1024

    Group 0: (Blocks 1-8192)

    Group 1: (Blocks 8193-10239)

4. 总体结构解析

基于上面的分析,侧面也可以佐证,ext2文件系统的layout结构信息

对于上面10MB的信息结构解析如下:

  1. filesystem boot header
    开头的1024字节,也即block 0,作为boot预留字节,通常是全0值。
Block Offset Length Description
byte 0 byte 512 boot record (if present)
512 bytes byte 512 additional boot record data (if present)
  1. block group 0, blocks 1 to 8192
    第0个table group,空间充足情况下,每个table group都有8192个block组成。
    地0个table group的第一个block放的是superblock,之后是block group desciptor table。
Block Offset Length Description
byte 1024 1024 bytes superblock
block 2 1 block block group descriptor table
block 3 39 block Reserved GDT blocks
block 42 1 block block bitmap
block 43 1 block inode bitmap
block 44 160 blocks inode table
block 204 7989 blocks data blocks
  1. block group 1, blocks 8193 to 10239
    第1个table group,空间不充情况下,剩余的block组成最后一个table group。
    第1, 3, 5, 7,以及3, 5, 7的幂中会放入backup super block以及后续的block group desciptor;
    非这些的剩余group中,直接放的是block bitmap及剩余的信息,这点可以参见30MB时的第2个group的信息。
Block Offset Length Description
block 8193 1024 bytes Backup superblock
block 8194 1 block block group descriptor table
block 8195 39 block Reserved GDT blocks
block 8234 1 block block bitmap
block 8235 1 block inode bitmap
block 8236 160 blocks inode table
block 8396 1844 blocks data blocks

5. 附录:对照一个30MB的文件dumpe2fs信息

下面是使用dump2fs命令,来查看一个30MB的loop device内容,可以作为和上面信息的对比参考。

30MB的内容如下:

shell 复制代码
[root@ls_CxhK1nVN ~]# dumpe2fs /dev/loop2
dumpe2fs 1.45.6 (20-Mar-2020)
Filesystem volume name:   <none>
Last mounted on:          /root/appledir
Filesystem UUID:          66ccabd0-1809-4cae-a1d4-503fd0c5bb42
Filesystem magic number:  0xEF53
Filesystem revision #:    1 (dynamic)
Filesystem features:      ext_attr resize_inode dir_index filetype sparse_super large_file
Filesystem flags:         signed_directory_hash 
Default mount options:    user_xattr acl
Filesystem state:         not clean
Errors behavior:          Continue
Filesystem OS type:       Linux
Inode count:              7680
Block count:              30720
Reserved block count:     1536
Free blocks:              29374
Free inodes:              7669
First block:              1
Block size:               1024
Fragment size:            1024
Reserved GDT blocks:      119
Blocks per group:         8192
Fragments per group:      8192
Inodes per group:         1920
Inode blocks per group:   240
Filesystem created:       Fri Mar 15 11:40:10 2024
Last mount time:          Fri Mar 15 11:40:15 2024
Last write time:          Fri Mar 15 11:40:15 2024
Mount count:              1
Maximum mount count:      -1
Last checked:             Fri Mar 15 11:40:10 2024
Check interval:           0 (<none>)
Reserved blocks uid:      0 (user root)
Reserved blocks gid:      0 (group root)
First inode:              11
Inode size:               128
Default directory hash:   half_md4
Directory Hash Seed:      e5834267-fbd0-4b52-a1ea-a1ae4418bb95


Group 0: (Blocks 1-8192)
  Primary superblock at 1, Group descriptors at 2-2
  Reserved GDT blocks at 3-121
  Block bitmap at 122 (+121)
  Inode bitmap at 123 (+122)
  Inode table at 124-363 (+123)
  7815 free blocks, 1909 free inodes, 2 directories
  Free blocks: 378-8192
  Free inodes: 12-1920
Group 1: (Blocks 8193-16384)
  Backup superblock at 8193, Group descriptors at 8194-8194
  Reserved GDT blocks at 8195-8313
  Block bitmap at 8314 (+121)
  Inode bitmap at 8315 (+122)
  Inode table at 8316-8555 (+123)
  7829 free blocks, 1920 free inodes, 0 directories
  Free blocks: 8556-16384
  Free inodes: 1921-3840
Group 2: (Blocks 16385-24576)
  Block bitmap at 16385 (+0)
  Inode bitmap at 16386 (+1)
  Inode table at 16387-16626 (+2)
  7950 free blocks, 1920 free inodes, 0 directories
  Free blocks: 16627-24576
  Free inodes: 3841-5760
Group 3: (Blocks 24577-30719)
  Backup superblock at 24577, Group descriptors at 24578-24578
  Reserved GDT blocks at 24579-24697
  Block bitmap at 24698 (+121)
  Inode bitmap at 24699 (+122)
  Inode table at 24700-24939 (+123)
  5780 free blocks, 1920 free inodes, 0 directories
  Free blocks: 24940-30719
  Free inodes: 5761-7680

5. 附录:如何创建一个10MB的loop device

如何创建一个10MB的loop device,方法如下:

shell 复制代码
[root@ls_CxhK1nVN ~]# dd if=/dev/zero of=./mongo bs=1K count=10240
10240+0 records in
10240+0 records out
10485760 bytes (10 MB, 10 MiB) copied, 0.0155913 s, 673 MB/s
[root@ls_CxhK1nVN ~]# losetup /dev/loop1 ./mongo
[root@ls_CxhK1nVN ~]# losetup -a
/dev/loop1: [64769]:395659 (/root/mongo)
/dev/loop0: [64769]:395656 (/bean)
[root@ls_CxhK1nVN ~]# mke2fs /dev/loop1
mke2fs 1.45.4 (23-Sep-2019)
Discarding device blocks: done                            
Creating filesystem with 10240 1k blocks and 2560 inodes
Filesystem UUID: 10f36aab-aa44-4d96-8733-ee31bd6bed5f
Superblock backups stored on blocks: 
        8193

Allocating group tables: done                            
Writing inode tables: done                            
Writing superblocks and filesystem accounting information: done

[root@ls_CxhK1nVN ~]# mkdir mongodir 
[root@ls_CxhK1nVN ~]# mount -t ext2 /dev/loop1 mongodir
[root@ls_CxhK1nVN ~]# dumpe2fs /dev/loop1

6. 附录:如何写一个脚本来验证创建文件的个数限制:

创建文件的shell脚本如下,执行该脚本,创建文件测试文件的支持数量;

当创建文件失败时,会报错:

touch: cannot touch 'files/file_2548.txt': No space left on device

使用chatgpt输出的脚本;

shell 复制代码
[root@ls_CxhK1nVN mongodir]# cat createfile.sh 
#!/bin/bash

# 指定文件夹路径
folder="files"

# 创建文件夹
mkdir -p "$folder"

# 循环创建1000个文件
for ((i=1; i<=2549; i++))
do
    # 生成文件名
    filename="$folder/file_$i.txt"
    
    # 创建文件
    touch "$filename"
    
    # 输出创建的文件名
    echo "Created file: $filename"
done

个人随笔 (Owed by: 春夜喜雨 http://blog.csdn.net/chunyexiyu)

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