[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.