ext4文件系统特性浅析

+++++++++++++++++++++++++++ +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
| sb | block group table | block bitmap | inode bitmap | inode table | data blocks |
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|<==================== group =====================>|+++++++++++++++++++++++++++ +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
| sb | block group table | block bitmap | inode bitmap | inode table | data blocks |
+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
|<==================== group (sb和块组描述符备份) =====================>|

Group : (Blocks -)
Primary superblock at , Group descriptors at -
Reserved GDT blocks at -
Block bitmap at (+), Inode bitmap at (+)
Inode table at - (+)
free blocks, free inodes, directories
Free blocks: -
Free inodes: -
Group : (Blocks -)
Backup superblock at , Group descriptors at -
Reserved GDT blocks at -
Block bitmap at (+), Inode bitmap at (+)
Inode table at - (+)
free blocks, free inodes, directories
Free blocks: -
Free inodes: -
Group : (Blocks -)
Block bitmap at (+), Inode bitmap at (+)
Inode table at - (+)
free blocks, free inodes, directories
Free blocks: -
Free inodes: -
Group : (Blocks -)
Backup superblock at , Group descriptors at -
Reserved GDT blocks at -
Block bitmap at (+), Inode bitmap at (+)
Inode table at - (+)
free blocks, free inodes, directories
Free blocks: -
Free inodes: -

Group : (Blocks -) [ITABLE_ZEROED]
Checksum 0xa9e7, unused inodes
Primary superblock at , Group descriptors at -
Reserved GDT blocks at -
Block bitmap at (+), Inode bitmap at (+)
Inode table at - (+)
free blocks, free inodes, directories, unused inodes
Free blocks: -
Free inodes: -
Group : (Blocks -) [INODE_UNINIT, ITABLE_ZEROED]
Checksum 0xd93e, unused inodes
Backup superblock at , Group descriptors at -
Reserved GDT blocks at -
Block bitmap at , Inode bitmap at
Inode table at -
free blocks, free inodes, directories, unused inodes
Free blocks: -
Free inodes: -
Group : (Blocks -) [INODE_UNINIT, BLOCK_UNINIT, ITABLE_ZEROED]
Checksum 0xcde2, unused inodes
Block bitmap at , Inode bitmap at
Inode table at -
free blocks, free inodes, directories, unused inodes
Free blocks: -
Free inodes: -
Group : (Blocks -) [INODE_UNINIT, ITABLE_ZEROED]
Checksum 0xafe0, unused inodes
Backup superblock at , Group descriptors at -
Reserved GDT blocks at -
Block bitmap at , Inode bitmap at
Inode table at -
free blocks, free inodes, directories, unused inodes
Free blocks: -
Free inodes: -

/*
* Each block (leaves and indexes), even inode-stored has header.
*/
struct ext4_extent_header {
__le16 eh_magic; /* probably will support different formats */
__le16 eh_entries; /* number of valid entries */
__le16 eh_max; /* capacity of store in entries */
__le16 eh_depth; /* has tree real underlying blocks? */
__le32 eh_generation; /* generation of the tree */
};

/*
* This is index on-disk structure.
* It's used at all the levels except the bottom.
*/
struct ext4_extent_idx {
__le32 ei_block; /* index covers logical blocks from 'block' */
__le32 ei_leaf_lo; /* pointer to the physical block of the next *
* level. leaf or next index could be there */
__le16 ei_leaf_hi; /* high 16 bits of physical block */
__u16 ei_unused;
};

/*
* This is the extent on-disk structure.
* It's used at the bottom of the tree.
*/
struct ext4_extent {
__le32 ee_block; /* first logical block extent covers */
__le16 ee_len; /* number of blocks covered by extent */
__le16 ee_start_hi; /* high 16 bits of physical block */
__le32 ee_start_lo; /* low 32 bits of physical block */
};