Extent Tree
In ext4, the file to logical block map has been replaced with an extent tree. Under the old scheme, allocating a contiguous run of 1,000 blocks requires an indirect block to map all 1,000 entries; with extents, the mapping is reduced to a single struct ext4_extent with ee_len = 1000. If flex_bg is enabled, it is possible to allocate very large files with a single extent, at a considerable reduction in metadata block use, and some improvement in disk efficiency. The inode must have the extents flag (0x80000) flag set for this feature to be in use.
Extents are arranged as a tree. Each node of the tree begins with a struct ext4_extent_header. If the node is an interior node (eh.eh_depth > 0), the header is followed by eh.eh_entries instances of struct ext4_extent_idx; each of these index entries points to a block containing more nodes in the extent tree. If the node is a leaf node (eh.eh_depth == 0), then the header is followed by eh.eh_entries instances of struct ext4_extent; these instances point to the file's data blocks. The root node of the extent tree is stored in inode.i_block, which allows for the first four extents to be recorded without the use of extra metadata blocks.
The extent tree header is recorded in struct ext4_extent_header, which is 12 bytes long:
Offset | Size | Name | Description |
0x0 | __le16 | eh_magic | Magic number, 0xF30A. |
0x2 | __le16 | eh_entries | Number of valid entries following the header. |
0x4 | __le16 | eh_max | Maximum number of entries that could follow the header. |
0x6 | __le16 | eh_depth | Depth of this extent node in the extent tree. 0 = this extent node points to data blocks; otherwise, this extent node points to other extent nodes. The extent tree can be at most 5 levels deep: a logical block number can be at most that satisfies |
0x8 | __le32 | eh_generation | Generation of the tree. (Used by Lustre, but not standard ext4). |
Internal nodes of the extent tree, also known as index nodes, are recorded as struct ext4_extent_idx, and are 12 bytes long:
Offset | Size | Name | Description |
0x0 | __le32 | ei_block | This index node covers file blocks from 'block' onward. |
0x4 | __le32 | ei_leaf_lo | Lower 32-bits of the block number of the extent node that is the next level lower in the tree. The tree node pointed to can be either another internal node or a leaf node, described below. |
0x8 | __le16 | ei_leaf_hi | Upper 16-bits of the previous field. |
0xA | __u16 | ei_unused | |
Leaf nodes of the extent tree are recorded as struct ext4_extent, and are also 12 bytes long:
Offset | Size | Name | Description |
0x0 | __le32 | ee_block | First file block number that this extent covers. |
0x4 | __le16 | ee_len | Number of blocks covered by extent. If the value of this field is <= 32768, the extent is initialized. If the value of the field is > 32768, the extent is uninitialized and the actual extent length is |
0x6 | __le16 | ee_start_hi | Upper 16-bits of the block number to which this extent points. |
0x8 | __le32 | ee_start_lo | Lower 32-bits of the block number to which this extent points. |
Prior to the introduction of metadata checksums, the extent header + extent entries always left at least 4 bytes of unallocated space at the end of each extent tree data block (because (2^x % 12) >= 4). Therefore, the 32-bit checksum is inserted into this space. The 4 extents in the inode do not need checksumming, since the inode is already checksummed. The checksum is calculated against the FS UUID, the inode number, the inode generation, and the entire extent block leading up to (but not including) the checksum itself.
struct ext4_extent_tail is 4 bytes long:
Offset | Size | Name | Description |
0x0 | __le32 | eb_checksum | Checksum of the extent block, crc32c(uuid+inum+igeneration+extentblock) |