When the attribute's extent map in an inode grows beyond the available space, the inode's attribute format is changed to a "btree". The inode contains root node of the extent B+tree which then address the leaves that contains the extent arrays for the attribute data. The attribute data itself in the allocated filesystem blocks use the same layout and structures as described in Node Attributes.
Refer to the previous section on B+tree Data Extents for more information on XFS B+tree extents.
xfs_db Example:
Added 2000 attributes with 729 byte values to a file:
xfs_db> inode <inode#>
xfs_db> p
...
core.nblocks = 640
core.extsize = 0
core.nextents = 1
core.naextents = 274
core.forkoff = 15
core.aformat = 3 (btree)
...
a.bmbt.level = 1
a.bmbt.numrecs = 2
a.bmbt.keys[1-2] = [startoff] 1:[0] 2:[219]
a.bmbt.ptrs[1-2] = 1:83162 2:109968
xfs_db> fsblock 83162
xfs_db> type bmapbtd
xfs_db> p
magic = 0x424d4150
level = 0
numrecs = 127
leftsib = null
rightsib = 109968
recs[1-127] = [startoff,startblock,blockcount,extentflag]
1:[0,81870,1,0]
...
xfs_db> fsblock 109968
xfs_db> type bmapbtd
xfs_db> p
magic = 0x424d4150
level = 0
numrecs = 147
leftsib = 83162
rightsib = null
recs[1-147] = [startoff,startblock,blockcount,extentflag]
...
(which is fsblock 81870)
xfs_db> ablock 0
xfs_db> p
hdr.info.forw = 0
hdr.info.back = 0
hdr.info.magic = 0xfebe
hdr.count = 2
hdr.level = 2
btree[0-1] = [hashval,before] 0:[0x343612a6,513] 1:[0x3e686fad,512]
The extent B+tree has two leaves that specify the 274 extents used for the attributes. Looking at the first block, it can be seen that the attribute B+tree is two levels deep. The two blocks at offset 513 and 512 (ie. access using the ablock command) are intermediate xfs_da_intnode_t nodes that index all the attribute leaves.
