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EXT4 数据结构

🕗 发布于 2024-07-26 13:32 数据结构  

高级数据结构设计

Blocks

EXT4 以块为单位分配存储空间。块是一组扇区介于 1KiB 和 64KiB 之间，扇区数量必须为 2 的整数幂。块又被分组成更大的单元，称为块组。默认情况下，文件系统可以包含 2^32 个块，启用 64 bits feature，则文件系统可以有 2^64 个块。

Layout

标准块组的布局：

Group 0 Padding ext4 Super Block Group Descriptors Reserved GDT Blocks Data Block Bitmap inode Bitmap inode Table Data Blocks
1024 bytes 1 block many blocks many blocks 1 block 1 block many blocks many more blocks

对于块组 0 的特殊情况，前 1024 个字节未使用， 允许安装 x86 引导扇区。 超级块将从偏移量 1024 字节开始。如果块大小 = 1024，则块 0 标记为使用中，超级块就处于块 1。

Flexible Block Groups

ext4 开始，flex_bg 中几个块组被捆绑在一起作为一个 逻辑块组，bitmap 空间和 inode Table 空间 flex_bg 的第一个块组被扩展为包括位图以及 flex_bg 中所有其他块组的 inode 表。

Meta Block Groups

Special inodes

ext4 为特殊功能预留了一些 inode，如下所示：

inode 编号 目的
0 没有 inode 0。
1 有缺陷的块的列表。
2 Root directory
3 User quota
4 Group quota
5 Boot loader
6 Undelete directory
7 Reserved group descriptors inode（调整 Inode）
8 Journal inode
9 exclude inode，用于快照
10 Replica inode
11 lost+found

Checksums

Inline Data

  • 文件小于 60 字节，数据就会内嵌存储在 inode.i_block 中。
  • 文件的其余部分可以容纳在扩展属性空间内，那么就可以在 inode 主体（“ibody EA”）中找到扩展属性 “system.data”。 当然，这也限制了一个 inode 可以附加的扩展属性数量。
  • 数据大小超过 i_block + ibody EA，就会分配一个常规块，并将内容移至该块。
Inline Directories

i_block 的前四个字节是父目录的 inode 编号。 之后是 56 字节的目录条目数组空间；参见 struct ext4_dir_entry。 如果 inode 主体中有 "system.data "属性，则 EA 值也是 struct ext4_dir_entry 数组。 请注意，对于内联目录，i_block 和 EA 空间被视为独立的 dirent 块；目录条目不能跨越这两个块。 内联目录条目不进行校验和，因为 inode 校验和应保护所有内联数据内容。

全局数据结构

Super Block

Offset Size Name Description
0x0 __le32 s_inodes_count 节点总数
0x4 __le32 s_blocks_count_lo 区块总数
0x8 __le32 s_r_blocks_count_lo This number of blocks can only be allocated by the super-user.
0xC __le32 s_free_blocks_count_lo 空闲块计数
0x10 __le32 s_free_inodes_count 空闲 inode 数量
0x14 __le32 s_first_data_block 第一个数据块， 对于 1k 块的文件系统，该值必须至少为 1，对于所有其他块大小，该值通常为 0
0x18 __le32 s_log_block_size 块大小为 2 ^ (10 + s_log_block_size)。
0x1C __le32 s_log_cluster_size Cluster size is 2 ^ (10 + s_log_cluster_size) blocks if bigalloc is enabled. Otherwise s_log_cluster_size must equal s_log_block_size.
0x20 __le32 s_blocks_per_group 每组块数
0x24 __le32 s_clusters_per_group Clusters per group, if bigalloc is enabled. Otherwise s_clusters_per_group must equal s_blocks_per_group.
0x28 __le32 s_inodes_per_group 每个组的 Inodes 数
0x2C __le32 s_mtime Mount time, in seconds since the epoch.
0x30 __le32 s_wtime Write time, in seconds since the epoch.
0x34 __le16 s_mnt_count Number of mounts since the last fsck.
0x36 __le16 s_max_mnt_count Number of mounts beyond which a fsck is needed.
0x38 __le16 s_magic 标识签名, 0xEF53
0x3A __le16 s_state File system state. See super_state for more info.
0x3C __le16 s_errors Behaviour when detecting errors. See super_errors for more info.
0x3E __le16 s_minor_rev_level Minor revision level.
0x40 __le32 s_lastcheck Time of last check, in seconds since the epoch.
0x44 __le32 s_checkinterval Maximum time between checks, in seconds.
0x48 __le32 s_creator_os Creator OS. See the table super_creator for more info.
0x4C __le32 s_rev_level Revision level. See the table super_revision for more info.
0x50 __le16 s_def_resuid Default uid for reserved blocks.
0x52 __le16 s_def_resgid Default gid for reserved blocks.
这些字段仅适用于 EXT4_DYNAMIC_REV 超级块。
0x54 __le32 s_first_ino First non-reserved inode.
0x58 __le16 s_inode_size inode 结构的大小（字节）
0x5A __le16 s_block_group_nr Block group # of this superblock.
0x5C __le32 s_feature_compat Compatible feature set flags. Kernel can still read/write this fs even if it doesn’t understand a flag; fsck should not do that. See the super_compat table for more info.
0x60 __le32 s_feature_incompat Incompatible feature set. If the kernel or fsck doesn’t understand one of these bits, it should stop. See the super_incompat table for more info.
0x64 __le32 s_feature_ro_compat Readonly-compatible feature set. If the kernel doesn’t understand one of these bits, it can still mount read-only. See the super_rocompat table for more info.
0x68 __u8 s_uuid[16] 128-bit UUID for volume.
0x78 char s_volume_name[16] 卷标
0x88 char s_last_mounted[64] Directory where filesystem was last mounted.
0xC8 __le32 s_algorithm_usage_bitmap For compression (Not used in e2fsprogs/Linux)
Performance hints. Directory preallocation should only happen if the EXT4_FEATURE_COMPAT_DIR_PREALLOC flag is on.
0xCC __u8 s_prealloc_blocks #. of blocks to try to preallocate for … files? (Not used in e2fsprogs/Linux)
0xCD __u8 s_prealloc_dir_blocks #. of blocks to preallocate for directories. (Not used in e2fsprogs/Linux)
0xCE __le16 s_reserved_gdt_blocks Number of reserved GDT entries for future filesystem expansion.
Journalling support is valid only if EXT4_FEATURE_COMPAT_HAS_JOURNAL is set.
0xD0 __u8 s_journal_uuid[16] UUID of journal superblock
0xE0 __le32 s_journal_inum inode number of journal file.
0xE4 __le32 s_journal_dev Device number of journal file, if the external journal feature flag is set.
0xE8 __le32 s_last_orphan Start of list of orphaned inodes to delete.
0xEC __le32 s_hash_seed[4] HTREE hash seed.
0xFC __u8 s_def_hash_version Default hash algorithm to use for directory hashes. See super_def_hash for more info.
0xFD __u8 s_jnl_backup_type If this value is 0 or EXT3_JNL_BACKUP_BLOCKS (1), then the s_jnl_blocks field contains a duplicate copy of the inode’s i_block[] array and i_size.
0xFE __le16 s_desc_size Size of group descriptors, in bytes, if the 64bit incompat feature flag is set.
0x100 __le32 s_default_mount_opts Default mount options. See the super_mountopts table for more info.
0x104 __le32 s_first_meta_bg First metablock block group, if the meta_bg feature is enabled.
0x108 __le32 s_mkfs_time When the filesystem was created, in seconds since the epoch.
0x10C __le32 s_jnl_blocks[17]

原文地址：https://blog.csdn.net/qq_41577017/article/details/140694363

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