[ext4]08 磁盘布局 - CheckSums

从2012年开始,Ext4和jbd2的元数据中都开始加入checksums。特性标识是metadata_csum。Checksum算法是在super_block中指定:

struct ext4_super_block {

    __u8    s_log_groups_per_flex;  /* FLEX_BG group size */

    __u8   s_checksum_type;  /* metadata checksum algorithm used */

}

但是在当前版本(3.13)中,依旧仅支持一种校验算法CRC32c。

在Ext4系统中,并不是所有的元数据校验值都是保存全部的32校验和,某些元数据因为考虑到数据结构的兼容性问题仅保存校验和的低16位数值。到那时如果开启64bit特性,所有的元数据校验值将保存全部的32位校验和。

Ext4系统中使用CRC32校验的元数据:

Metadata

Length

Ingredients

Superblock

__le32

The entire superblock up to the checksum field. The UUID lives inside the superblock.

MMP

__le32

UUID + the entire MMP block up to the checksum field.

Extended Attributes

__le32

UUID + the entire extended attribute block. The checksum field is set to zero.

Directory Entries

__le32

UUID + inode number + inode generation + the directory block up to the fake entry enclosing the checksum field.

HTREE Nodes

__le32

UUID + inode number + inode generation + all valid extents + HTREE tail. The checksum field is set to zero.

Extents

__le32

UUID + inode number + inode generation + the entire extent block up to the checksum field.

Bitmaps

__le32 or __le16

UUID + the entire bitmap. Checksums are stored in the group descriptor, and truncated if the group descriptor size is 32 bytes (i.e. ^64bit)

Inodes

__le32

UUID + inode number + inode generation + the entire inode. The checksum field is set to zero. Each inode has its own checksum.

Group Descriptors

__le16

If metadata_csum, then UUID + group number + the entire descriptor; else if uninit_bg, then crc16(UUID + group number + the entire descriptor). In all cases, only the lower 16 bits are stored.

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