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. |