Directory entries are stored within the inode.
Only data stored is the name, inode # and offset, no "leaf" or "freespace index" information is required as an inode can only store a few entries.
"." is not stored (as it's in the inode itself), and ".." is a dedicated parent field in the header.
The number of directories that can be stored in an inode depends on the inode size (
Chapter 4, On-disk Inode), the number of entries, the length of the entry names and extended attribute data.
Once the number of entries exceed the space available in the inode, the format is converted to a "Block Directory".
Shortform directory data is packed as tightly as possible on the disk with the remaining space zeroed:
typedef struct xfs_dir2_sf {
xfs_dir2_sf_hdr_t hdr;
xfs_dir2_sf_entry_t list[1];
} xfs_dir2_sf_t;
typedef struct xfs_dir2_sf_hdr {
__uint8_t count;
__uint8_t i8count;
xfs_dir2_inou_t parent;
} xfs_dir2_sf_hdr_t;
typedef struct xfs_dir2_sf_entry {
__uint8_t namelen;
xfs_dir2_sf_off_t offset;
__uint8_t name[1];
xfs_dir2_inou_t inumber;
} xfs_dir2_sf_entry_t;
Inode numbers are stored using 4 or 8 bytes depending on whether all the inode numbers for the directory fit in 4 bytes (32 bits) or not. If all inode numbers fit in 4 bytes, the header's count value specifies the number of entries in the directory and i8count will be zero. If any inode number exceeds 4 bytes, all inode numbers will be 8 bytes in size and the header's i8count value specifies the number of entries and count will be zero. The following union covers the shortform inode number structure:
typedef struct { __uint8_t i[8]; } xfs_dir2_ino8_t;
typedef struct { __uint8_t i[4]; } xfs_dir2_ino4_t;
typedef union {
xfs_dir2_ino8_t i8;
xfs_dir2_ino4_t i4;
} xfs_dir2_inou_t;
xfs_db Example:
A directory is created with 4 files, all inode numbers fitting within 4 bytes:
xfs_db> inode <inode#>
xfs_db> p
core.magic = 0x494e
core.mode = 040755
core.version = 1
core.format = 1 (local)
core.nlinkv1 = 2
...
core.size = 94
core.nblocks = 0
core.extsize = 0
core.nextents = 0
...
u.sfdir2.hdr.count = 4
u.sfdir2.hdr.i8count = 0
u.sfdir2.hdr.parent.i4 = 128 /* parent = root inode */
u.sfdir2.list[0].namelen = 15
u.sfdir2.list[0].offset = 0x30
u.sfdir2.list[0].name = "frame000000.tst"
u.sfdir2.list[0].inumber.i4 = 25165953
u.sfdir2.list[1].namelen = 15
u.sfdir2.list[1].offset = 0x50
u.sfdir2.list[1].name = "frame000001.tst"
u.sfdir2.list[1].inumber.i4 = 25165954
u.sfdir2.list[2].namelen = 15
u.sfdir2.list[2].offset = 0x70
u.sfdir2.list[2].name = "frame000002.tst"
u.sfdir2.list[2].inumber.i4 = 25165955
u.sfdir2.list[3].namelen = 15
u.sfdir2.list[3].offset = 0x90
u.sfdir2.list[3].name = "frame000003.tst"
u.sfdir2.list[3].inumber.i4 = 25165956
The raw data on disk with the first entry highlighted. The six byte header precedes the first entry:
Next, an entry is deleted (frame000001.tst), and any entries after the deleted entry are moved or compacted to "cover" the hole:
xfs_db> inode <inode#>
xfs_db> p
core.magic = 0x494e
core.mode = 040755
core.version = 1
core.format = 1 (local)
core.nlinkv1 = 2
...
core.size = 72
core.nblocks = 0
core.extsize = 0
core.nextents = 0
...
u.sfdir2.hdr.count = 3
u.sfdir2.hdr.i8count = 0
u.sfdir2.hdr.parent.i4 = 128
u.sfdir2.list[0].namelen = 15
u.sfdir2.list[0].offset = 0x30
u.sfdir2.list[0].name = "frame000000.tst"
u.sfdir2.list[0].inumber.i4 = 25165953
u.sfdir2.list[1].namelen = 15
u.sfdir2.list[1].offset = 0x70
u.sfdir2.list[1].name = "frame000002.tst"
u.sfdir2.list[1].inumber.i4 = 25165955
u.sfdir2.list[2].namelen = 15
u.sfdir2.list[2].offset = 0x90
u.sfdir2.list[2].name = "frame000003.tst"
u.sfdir2.list[2].inumber.i4 = 25165956
Raw disk data, the space beyond the shortform entries is invalid and could be non-zero:
xfs_db> type text
xfs_db> p
00: 49 4e 41 ed 01 01 00 02 00 00 00 00 00 00 00 00 INA.............
10: 00 00 00 02 00 00 00 00 00 00 00 00 00 00 00 03 ................
20: 44 b2 45 a2 09 fd e4 50 44 b2 45 a3 12 ee b5 d0 D.E....PD.E.....
30: 44 b2 45 a3 12 ee b5 d0 00 00 00 00 00 00 00 48 D.E............H
40: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
50: 00 00 00 02 00 00 00 00 00 00 00 00 00 00 00 00 ................
60: ff ff ff ff 03 00 00 00 00 80 0f 00 30 66 72 61 ............0fra
70: 6d 65 30 30 30 30 30 30 2e 74 73 74 01 80 00 81 me000000.tst....
80: 0f 00 70 66 72 61 6d 65 30 30 30 30 30 32 2e 74 ..pframe000002.t
90: 73 74 01 80 00 83 0f 00 90 66 72 61 6d 65 30 30 st.......frame00
a0: 30 30 30 33 2e 74 73 74 01 80 00 84 0f 00 90 66 0003.tst.......f
b0: 72 61 6d 65 30 30 30 30 30 33 2e 74 73 74 01 80 rame000003.tst..
c0: 00 84 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
TODO: 8-byte inode number example
