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]