ext4 extent 寻址方案

  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 2^32, and the smallest n
 that satisfies 4*(((blocksize - 12)/12)^n) >= 2^32 is 5.
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 ee_len - 32768. Therefore, the maximum length of a initialized extent is 32768 blocks, and the maximum length of an uninitialized extent is 32767.
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)