[PATCH 19/22] docs: add XFS directory structure to the DS&A book

Darrick J. Wong
2018-10-04T04:20:37Z

.../xfs-data-structures/directories.rst | 1688 ++++++++++++++++++++
.../filesystems/xfs-data-structures/dynamic.rst | 1
2 files changed, 1689 insertions(+)
create mode 100644 Documentation/filesystems/xfs-data-structures/directories.rst

diff --git a/Documentation/filesystems/xfs-data-structures/directories.rst b/Documentation/filesystems/xfs-data-structures/directories.rst
new file mode 100644
index 000000000000..34f34c7aedea
--- /dev/null
+++ b/Documentation/filesystems/xfs-data-structures/directories.rst
@@ -0,0 +1,1688 @@
+.. SPDX-License-Identifier: CC-BY-SA-4.0
+
+Directories
+-----------
+
+ **Note**
+
+ Only v2 directories covered here. v1 directories are obsolete.
+
+ **Note**
+
+ The term "block" in this section will refer to directory blocks, not
+ filesystem blocks unless otherwise specified.
+
+The size of a "directory block" is defined by the
+`superblock’s <#superblocks>`__ sb\_dirblklog value. The size in bytes =
+sb\_blocksize × 2\ :sup:`sb\_dirblklog`. For example, if sb\_blocksize = 4096
+and sb\_dirblklog = 2, the directory block size is 16384 bytes. Directory
+blocks are always allocated in multiples based on sb\_dirblklog. Directory
+blocks cannot be more that 65536 bytes in size.
+
+All directory entries contain the following "data":
+
+- The entry’s name (counted string consisting of a single byte namelen
+ followed by name consisting of an array of 8-bit chars without a NULL
+ terminator).
+
+- The entry’s absolute `inode number <#inode-numbers>`__, which are always 64
+ bits (8 bytes) in size except a special case for shortform directories.
+
+- An offset or tag used for iterative readdir calls.
+
+- If the XFS\_SB\_FEAT\_INCOMPAT\_FTYPE feature flag is set, each directory
+ entry contains an ftype field that caches the inode’s type to avoid having
+ to perform an inode lookup.
+
+.. list-table::
+ :widths: 28 52
+ :header-rows: 1
+
+ * - Flag
+ - Description
+
+ * - XFS_DIR3_FT_UNKNOWN
+ - Entry points to an unknown inode type. This should never appear on
+ disk.
+
+ * - XFS_DIR3_FT_REG_FILE
+ - Entry points to a file.
+
+ * - XFS_DIR3_FT_DIR
+ - Entry points to another directory.
+
+ * - XFS_DIR3_FT_CHRDEV
+ - Entry points to a character device.
+
+ * - XFS_DIR3_FT_BLKDEV
+ - Entry points to a block device.
+
+ * - XFS_DIR3_FT_FIFO
+ - Entry points to a FIFO.
+
+ * - XFS_DIR3_FT_SOCK
+ - Entry points to a socket.
+
+ * - XFS_DIR3_FT_SYMLINK
+ - Entry points to a symbolic link.
+
+ * - XFS_DIR3_FT_WHT
+ - Entry points to an overlayfs whiteout file. This (as far as the author
+ knows) has never appeared on disk.
+
+Table: ftype values
+
+All non-shortform directories also contain two additional structures:
+"leaves"
+and "freespace indexes".
+
+- Leaves contain the sorted hashed name value (xfs\_da\_hashname() in
+ xfs\_da\_btree.c) and associated "address" which points to the
+ effective offset into the directory’s data structures. Leaves are used to
+ optimise lookup operations.
+
+- Freespace indexes contain free space/empty entry tracking for quickly
+ finding an appropriately sized location for new entries. They maintain the
+ largest free space for each "data" block.
+
+A few common types are used for the directory structures:
+
+.. code:: c
+
+ typedef __uint16_t xfs_dir2_data_off_t;
+ typedef __uint32_t xfs_dir2_dataptr_t;
+
+Short Form Directories
+~~~~~~~~~~~~~~~~~~~~~~
+
+- Directory entries are stored within the inode.
+
+- The only data stored is the name, inode number, 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 <#on-disk-inode>`__ size, the number of entries, the length of the
+ entry names, and extended attribute data.
+
+- Once the number of entries exceeds the space available in the inode, the
+ format is converted to a `block directory <#block-directories>`__.
+
+- Shortform directory data is packed as tightly as possible on the disk with
+ the remaining space zeroed:
+
+.. code:: c
+
+ typedef struct xfs_dir2_sf {
+ xfs_dir2_sf_hdr_t hdr;
+ xfs_dir2_sf_entry_t list[1];
+ } xfs_dir2_sf_t;
+
+**hdr**
+ Short form directory header.
+
+**list**
+ An array of variable-length directory entry records.
+
+.. code:: c
+
+ typedef struct xfs_dir2_sf_hdr {
+ __uint8_t count;
+ __uint8_t i8count;
+ xfs_dir2_inou_t parent;
+ } xfs_dir2_sf_hdr_t;
+
+**count**
+ Number of directory entries.
+
+**i8count**
+ Number of directory entries requiring 64-bit entries, if any inode numbers
+ require 64-bits. Zero otherwise.
+
+**parent**
+ The absolute inode number of this directory’s parent.
+
+.. code:: c
+
+ typedef struct xfs_dir2_sf_entry {
+ __uint8_t namelen;
+ xfs_dir2_sf_off_t offset;
+ __uint8_t name[1];
+ __uint8_t ftype;
+ xfs_dir2_inou_t inumber;
+ } xfs_dir2_sf_entry_t;
+
+**namelen**
+ Length of the name, in bytes.
+
+**offset**
+ Offset tag used to assist with directory iteration.
+
+**name**
+ The name of the directory entry. The entry is not NULL-terminated.
+
+**ftype**
+ The type of the inode. This is used to avoid reading the inode while
+ iterating a directory. The XFS\_SB\_VERSION2\_FTYPE feature must be set,
+ or this field will not be present.
+
+**inumber**
+ The inode number that this entry points to. The length is either 32 or 64
+ bits, depending on whether icount or i8count, respectively, are set in the
+ header.
+
+.. figure:: images/39.png
+ :alt: Short form directory layout
+
+ Short form directory layout
+
+- 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 requiring larger inodes.
+ i4count is still the number of entries. The following union covers the
+ shortform inode number structure:
+
+.. code:: c
+
+ 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 Short Form Directory 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:
+
+::
+
+ 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 02 ................
+ 20: 44 ad 3a 83 1d a9 4a d0 44 ad 3a ab 0b c7 a7 d0 D.....J.D.......
+ 30: 44 ad 3a ab 0b c7 a7 d0 00 00 00 00 00 00 00 5e D...............
+ 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 04 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 50 66 72 61 6d 65 30 30 30 30 30 31 2e 74 ..Pframe000001.t
+ 90: 73 74 01 80 00 82 0f 00 70 66 72 61 6d 65 30 30 st......pframe00
+ a0: 30 30 30 32 2e 74 73 74 01 80 00 83 0f 00 90 66 0002.tst........
+ b0: 72 61 6d 65 30 30 30 30 30 33 2e 74 73 74 01 80 rame000003.tst..
+ cO: 00 84 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
+
+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 ................
+
+This is an example of mixed 4-byte and 8-byte inodes in a directory:
+
+::
+
+ xfs_db> inode 1024
+ xfs_db> p
+ core.magic = 0x494e
+ core.mode = 040755
+ core.version = 3
+ core.format = 1 (local)
+ core.nlinkv2 = 9
+ ...
+ core.size = 125
+ core.nblocks = 0
+ core.extsize = 0
+ core.nextents = 0
+ ...
+ u3.sfdir3.hdr.count = 7
+ u3.sfdir3.hdr.i8count = 4
+ u3.sfdir3.hdr.parent.i8 = 1024
+ u3.sfdir3.list[0].namelen = 3
+ u3.sfdir3.list[0].offset = 0x60
+ u3.sfdir3.list[0].name = "git"
+ u3.sfdir3.list[0].inumber.i8 = 1027
+ u3.sfdir3.list[0].filetype = 2
+ u3.sfdir3.list[1].namelen = 4
+ u3.sfdir3.list[1].offset = 0x70
+ u3.sfdir3.list[1].name = "home"
+ u3.sfdir3.list[1].inumber.i8 = 13422826546
+ u3.sfdir3.list[1].filetype = 2
+ u3.sfdir3.list[2].namelen = 10
+ u3.sfdir3.list[2].offset = 0x80
+ u3.sfdir3.list[2].name = "mike"
+ u3.sfdir3.list[2].inumber.i8 = 4299308032
+ u3.sfdir3.list[2].filetype = 2
+ u3.sfdir3.list[3].namelen = 3
+ u3.sfdir3.list[3].offset = 0x98
+ u3.sfdir3.list[3].name = "mtr"
+ u3.sfdir3.list[3].inumber.i8 = 13433252916
+ u3.sfdir3.list[3].filetype = 2
+ u3.sfdir3.list[4].namelen = 3
+ u3.sfdir3.list[4].offset = 0xa8
+ u3.sfdir3.list[4].name = "vms"
+ u3.sfdir3.list[4].inumber.i8 = 16647516355
+ u3.sfdir3.list[4].filetype = 2
+ u3.sfdir3.list[5].namelen = 5
+ u3.sfdir3.list[5].offset = 0xb8
+ u3.sfdir3.list[5].name = "rsync"
+ u3.sfdir3.list[5].inumber.i8 = 3494912
+ u3.sfdir3.list[5].filetype = 2
+ u3.sfdir3.list[6].namelen = 3
+ u3.sfdir3.list[6].offset = 0xd0
+ u3.sfdir3.list[6].name = "tmp"
+ u3.sfdir3.list[6].inumber.i8 = 1593379
+ u3.sfdir3.list[6].filetype = 2
+
+Block Directories
+~~~~~~~~~~~~~~~~~
+
+When the shortform directory space exceeds the space in an inode, the
+directory data is moved into a new single directory block outside the inode.
+The inode’s format is changed from "local" to "extent" Following is a
+list of points about block directories.
+
+- All directory data is stored within the one directory block, including
+ "." and
+ ".." entries which are mandatory.
+
+- The block also contains "leaf" and "freespace index" information.
+
+- The location of the block is defined by the inode’s in-core `extent
+ list <#extent-list>`__: the di\_u.u\_bmx[0] value. The file offset in the
+ extent must always be zero and the length = (directory block size /
+ filesystem block size). The block number points to the filesystem block
+ containing the directory data.
+
+- Block directory data is stored in the following structures:
+
+.. code:: c
+
+ #define XFS_DIR2_DATA_FD_COUNT 3
+ typedef struct xfs_dir2_block {
+ xfs_dir2_data_hdr_t hdr;
+ xfs_dir2_data_union_t u[1];
+ xfs_dir2_leaf_entry_t leaf[1];
+ xfs_dir2_block_tail_t tail;
+ } xfs_dir2_block_t;
+
+**hdr**
+ Directory block header. On a v5 filesystem this is
+ xfs\_dir3\_data\_hdr\_t.
+
+**u**
+ Union of directory and unused entries.
+
+**leaf**
+ Hash values of the entries in this block.
+
+**tail**
+ Bookkeeping for the leaf entries.
+
+.. code:: c
+
+ typedef struct xfs_dir2_data_hdr {
+ __uint32_t magic;
+ xfs_dir2_data_free_t bestfree[XFS_DIR2_DATA_FD_COUNT];
+ } xfs_dir2_data_hdr_t;
+
+**magic**
+ Magic number for this directory block.
+
+**bestfree**
+ An array pointing to free regions in the directory block.
+
+On a v5 filesystem, directory and attribute blocks are formatted with v3
+headers, which contain extra data:
+
+.. code:: c
+
+ struct xfs_dir3_blk_hdr {
+ __be32 magic;
+ __be32 crc;
+ __be64 blkno;
+ __be64 lsn;
+ uuid_t uuid;
+ __be64 owner;
+ };
+
+**magic**
+ Magic number for this directory block.
+
+**crc**
+ Checksum of the directory block.
+
+**blkno**
+ Block number of this directory block.
+
+**lsn**
+ Log sequence number of the last write to this block.
+
+**uuid**
+ The UUID of this block, which must match either sb\_uuid or sb\_meta\_uuid
+ depending on which features are set.
+
+**owner**
+ The inode number that this directory block belongs to.
+
+.. code:: c
+
+ struct xfs_dir3_data_hdr {
+ struct xfs_dir3_blk_hdr hdr;
+ xfs_dir2_data_free_t best_free[XFS_DIR2_DATA_FD_COUNT];
+ __be32 pad;
+ };
+
+**hdr**
+ The v5 directory/attribute block header.
+
+**best\_free**
+ An array pointing to free regions in the directory block.
+
+**pad**
+ Padding to maintain a 64-bit alignment.
+
+Within the block, data structures are as follows:
+
+.. code:: c
+
+ typedef struct xfs_dir2_data_free {
+ xfs_dir2_data_off_t offset;
+ xfs_dir2_data_off_t length;
+ } xfs_dir2_data_free_t;
+
+**offset**
+ Block offset of a free block, in bytes.
+
+**length**
+ Length of the free block, in bytes.
+
+Space inside the directory block can be used for directory entries or unused
+entries. This is signified via a union of the two types:
+
+.. code:: c
+
+ typedef union {
+ xfs_dir2_data_entry_t entry;
+ xfs_dir2_data_unused_t unused;
+ } xfs_dir2_data_union_t;
+
+**entry**
+ A directory entry.
+
+**unused**
+ An unused entry.
+
+.. code:: c
+
+ typedef struct xfs_dir2_data_entry {
+ xfs_ino_t inumber;
+ __uint8_t namelen;
+ __uint8_t name[1];
+ __uint8_t ftype;
+ xfs_dir2_data_off_t tag;
+ } xfs_dir2_data_entry_t;
+
+**inumber**
+ The inode number that this entry points to.
+
+**namelen**
+ Length of the name, in bytes.
+
+**name**
+ The name associated with this entry.
+
+**ftype**
+ The type of the inode. This is used to avoid reading the inode while
+ iterating a directory. The XFS\_SB\_VERSION2\_FTYPE feature must be set,
+ or this field will not be present.
+
+**tag**
+ Starting offset of the entry, in bytes. This is used for directory
+ iteration.
+
+.. code:: c
+
+ typedef struct xfs_dir2_data_unused {
+ __uint16_t freetag; /* 0xffff */
+ xfs_dir2_data_off_t length;
+ xfs_dir2_data_off_t tag;
+ } xfs_dir2_data_unused_t;
+
+**freetag**
+ Magic number signifying that this is an unused entry. Must be 0xFFFF.
+
+**length**
+ Length of this unused entry, in bytes.
+
+**tag**
+ Starting offset of the entry, in bytes.
+
+.. code:: c
+
+ typedef struct xfs_dir2_leaf_entry {
+ xfs_dahash_t hashval;
+ xfs_dir2_dataptr_t address;
+ } xfs_dir2_leaf_entry_t;
+
+**hashval**
+ Hash value of the name of the directory entry. This is used to speed up
+ entry lookups.
+
+**address**
+ Block offset of the entry, in eight byte units.
+
+.. code:: c
+
+ typedef struct xfs_dir2_block_tail {
+ __uint32_t count;
+ __uint32_t stale;
+ } xfs_dir2_block_tail_t;
+
+**count**
+ Number of leaf entries.
+
+**stale**
+ Number of free leaf entries.
+
+Following is a diagram of how these pieces fit together for a block directory.
+
+.. ifconfig:: builder != 'latex'
+
+ .. figure:: images/43.png
+ :alt: Block directory layout
+
+ Block directory layout
+
+.. ifconfig:: builder == 'latex'
+
+ .. figure:: images/43.png
+ :scale: 45%
+ :alt: Block directory layout
+
+ Block directory layout
+
+- The magic number in the header is "XD2B" (0x58443242), or "XDB3"
+ (0x58444233) on a v5 filesystem.
+
+- The tag in the xfs\_dir2\_data\_entry\_t structure stores its offset from
+ the start of the block.
+
+- The start of a free space region is marked with the
+ xfs\_dir2\_data\_unused\_t structure where the freetag is 0xffff. The
+ freetag and length overwrites the inumber for an entry. The tag is located
+ at length - sizeof(tag) from the start of the unused entry on-disk.
+
+- The bestfree array in the header points to as many as three of the largest
+ spaces of free space within the block for storing new entries sorted by
+ largest to third largest. If there are less than 3 empty regions, the
+ remaining bestfree elements are zeroed. The offset specifies the offset
+ from the start of the block in bytes, and the length specifies the size of
+ the free space in bytes. The location each points to must contain the above
+ xfs\_dir2\_data\_unused\_t structure. As a block cannot exceed 64KB in
+ size, each is a 16-bit value. bestfree is used to optimise the time
+ required to locate space to create an entry. It saves scanning through the
+ block to find a location suitable for every entry created.
+
+- The tail structure specifies the number of elements in the leaf array and
+ the number of stale entries in the array. The tail is always located at the
+ end of the block. The leaf data immediately precedes the tail structure.
+
+- The leaf array, which grows from the end of the block just before the tail
+ structure, contains an array of hash/address pairs for quickly looking up a
+ name by a hash value. Hash values are covered by the introduction to
+ directories. The address on-disk is the offset into the block divided by 8
+ (XFS\_DIR2\_DATA\_ALIGN). Hash/address pairs are stored on disk to optimise
+ lookup speed for large directories. If they were not stored, the hashes
+ would have to be calculated for all entries each time a lookup occurs in a
+ directory.
+
+xfs\_db Block Directory Example
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+A directory is created with 8 entries, directory block size = filesystem block
+size:
+
+::
+
+ xfs_db> sb 0
+ xfs_db> p
+ magicnum = 0x58465342
+ blocksize = 4096
+ ...
+ dirblklog = 0
+ ...
+ xfs_db> inode <inode#>
+ xfs_db> p
+ core.magic = 0x494e
+ core.mode = 040755
+ core.version = 1
+ core.format = 2 (extents)
+ core.nlinkv1 = 2
+ ...
+ core.size = 4096
+ core.nblocks = 1
+ core.extsize = 0
+ core.nextents = 1
+ ...
+ u.bmx[0] = [startoff,startblock,blockcount,extentflag] 0:[0,2097164,1,0]
+
+Go to the "startblock" and show the raw disk data:
+
+::
+
+ xfs_db> dblock 0
+ xfs_db> type text
+ xfs_db> p
+ 000: 58 44 32 42 01 30 0e 78 00 00 00 00 00 00 00 00 XD2B.0.x........
+ 010: 00 00 00 00 02 00 00 80 01 2e 00 00 00 00 00 10 ................
+ 020: 00 00 00 00 00 00 00 80 02 2e 2e 00 00 00 00 20 ................
+ 030: 00 00 00 00 02 00 00 81 0f 66 72 61 6d 65 30 30 .........frame00
+ 040: 30 30 30 30 2e 74 73 74 80 8e 59 00 00 00 00 30 0000.tst..Y....0
+ 050: 00 00 00 00 02 00 00 82 0f 66 72 61 6d 65 30 30 .........frame00
+ 060: 30 30 30 31 2e 74 73 74 d0 ca 5c 00 00 00 00 50 0001.tst.......P
+ 070: 00 00 00 00 02 00 00 83 0f 66 72 61 6d 65 30 30 .........frame00
+ 080: 30 30 30 32 2e 74 73 74 00 00 00 00 00 00 00 70 0002.tst.......p
+ 090: 00 00 00 00 02 00 00 84 0f 66 72 61 6d 65 30 30 .........frame00
+ 0a0: 30 30 30 33 2e 74 73 74 00 00 00 00 00 00 00 90 0003.tst........
+ 0b0: 00 00 00 00 02 00 00 85 0f 66 72 61 6d 65 30 30 .........frame00
+ 0c0: 30 30 30 34 2e 74 73 74 00 00 00 00 00 00 00 b0 0004.tst........
+ 0d0: 00 00 00 00 02 00 00 86 0f 66 72 61 6d 65 30 30 .........frame00
+ 0e0: 30 30 30 35 2e 74 73 74 00 00 00 00 00 00 00 d0 0005.tst........
+ 0f0: 00 00 00 00 02 00 00 87 0f 66 72 61 6d 65 30 30 .........frame00
+ 100: 30 30 30 36 2e 74 73 74 00 00 00 00 00 00 00 f0 0006.tst........
+ 110: 00 00 00 00 02 00 00 88 0f 66 72 61 6d 65 30 30 .........frame00
+ 120: 30 30 30 37 2e 74 73 74 00 00 00 00 00 00 01 10 0007.tst........
+ 130: ff ff 0e 78 00 00 00 00 00 00 00 00 00 00 00 00 ...x............
+
+The "leaf" and "tail" structures are stored at the end of the block, so
+as the directory grows, the middle is filled in:
+
+::
+
+ fa0: 00 00 00 00 00 00 01 30 00 00 00 2e 00 00 00 02 .......0........
+ fb0: 00 00 17 2e 00 00 00 04 83 a0 40 b4 00 00 00 0e ................
+ fc0: 93 a0 40 b4 00 00 00 12 a3 a0 40 b4 00 00 00 06 ................
+ fd0: b3 a0 40 b4 00 00 00 0a c3 a0 40 b4 00 00 00 1e ................
+ fe0: d3 a0 40 b4 00 00 00 22 e3 a0 40 b4 00 00 00 16 ................
+ ff0: f3 a0 40 b4 00 00 00 1a 00 00 00 0a 00 00 00 00 ................
+
+In a readable format:
+
+::
+
+ xfs_db> type dir2
+ xfs_db> p
+ bhdr.magic = 0x58443242
+ bhdr.bestfree[0].offset = 0x130
+ bhdr.bestfree[0].length = 0xe78
+ bhdr.bestfree[1].offset = 0
+ bhdr.bestfree[1].length = 0
+ bhdr.bestfree[2].offset = 0
+ bhdr.bestfree[2].length = 0
+ bu[0].inumber = 33554560
+ bu[0].namelen = 1
+ bu[0].name = "."
+ bu[0].tag = 0x10
+ bu[1].inumber = 128
+ bu[1].namelen = 2
+ bu[1].name = ".."
+ bu[1].tag = 0x20
+ bu[2].inumber = 33554561
+ bu[2].namelen = 15
+ bu[2].name = "frame000000.tst"
+ bu[2].tag = 0x30
+ bu[3].inumber = 33554562
+ bu[3].namelen = 15
+ bu[3].name = "frame000001.tst"
+ bu[3].tag = 0x50
+ ...
+ bu[8].inumber = 33554567
+ bu[8].namelen = 15
+ bu[8].name = "frame000006.tst"
+ bu[8].tag = 0xf0
+ bu[9].inumber = 33554568
+ bu[9].namelen = 15
+ bu[9].name = "frame000007.tst"
+ bu[9].tag = 0x110
+ bu[10].freetag = 0xffff
+ bu[10].length = 0xe78
+ bu[10].tag = 0x130
+ bleaf[0].hashval = 0x2e
+ bleaf[0].address = 0x2
+ bleaf[1].hashval = 0x172e
+ bleaf[1].address = 0x4
+ bleaf[2].hashval = 0x83a040b4
+ bleaf[2].address = 0xe
+ ...
+ bleaf[8].hashval = 0xe3a040b4
+ bleaf[8].address = 0x16
+ bleaf[9].hashval = 0xf3a040b4
+ bleaf[9].address = 0x1a
+ btail.count = 10
+ btail.stale = 0
+
+ **Note**
+
+ For block directories, all xfs\_db fields are preceded with "b".
+
+For a simple lookup example, the hash of frame000000.tst is 0xb3a040b4.
+Looking up that value, we get an address of 0x6. Multiply that by 8, it
+becomes offset 0x30 and the inode at that point is 33554561.
+
+When we remove an entry from the middle (frame000004.tst), we can see how the
+freespace details are adjusted:
+
+::
+
+ bhdr.magic = 0x58443242
+ bhdr.bestfree[0].offset = 0x130
+ bhdr.bestfree[0].length = 0xe78
+ bhdr.bestfree[1].offset = 0xb0
+ bhdr.bestfree[1].length = 0x20
+ bhdr.bestfree[2].offset = 0
+ bhdr.bestfree[2].length = 0
+ ...
+ bu[5].inumber = 33554564
+ bu[5].namelen = 15
+ bu[5].name = "frame000003.tst"
+ bu[5].tag = 0x90
+ bu[6].freetag = 0xffff
+ bu[6].length = 0x20
+ bu[6].tag = 0xb0
+ bu[7].inumber = 33554566
+ bu[7].namelen = 15
+ bu[7].name = "frame000005.tst"
+ bu[7].tag = 0xd0
+ ...
+ bleaf[7].hashval = 0xd3a040b4
+ bleaf[7].address = 0x22
+ bleaf[8].hashval = 0xe3a040b4
+ bleaf[8].address = 0
+ bleaf[9].hashval = 0xf3a040b4
+ bleaf[9].address = 0x1a
+ btail.count = 10
+ btail.stale = 1
+
+A new "bestfree" value is added for the entry, the start of the entry is
+marked as unused with 0xffff (which overwrites the inode number for an actual
+entry), and the length of the space. The tag remains intact at the
+offset+length - sizeof(tag). The address for the hash is also cleared. The
+affected areas are highlighted below:
+
+::
+
+ 090: 00 00 00 00 02 00 00 84 0f 66 72 61 6d 65 30 30 ..........frame00
+ 0a0: 30 30 30 33 2e 74 73 74 00 00 00 00 00 00 00 90 0003.tst.........
+ 0b0: ff ff 00 20 02 00 00 85 0f 66 72 61 6d 65 30 30 ..........frame00
+ 0c0: 30 30 30 34 2e 74 73 74 00 00 00 00 00 00 00 b0 0004.tst.........
+ 0d0: 00 00 00 00 02 00 00 86 0f 66 72 61 6d 65 30 30 ..........frame00
+ 0e0: 30 30 30 35 2e 74 73 74 00 00 00 00 00 00 00 0d 0005.tst.........
+ ...
+ fb0: 00 00 17 2e 00 00 00 04 83 a0 40 b4 00 00 00 0e .................
+ fc0: 93 a0 40 b4 00 00 00 12 a3 a0 40 b4 00 00 00 06 .................
+ fd0: b3 a0 40 b4 00 00 00 0a c3 a0 40 b4 00 00 00 1e .................
+ fe0: d3 a0 40 b4 00 00 00 22 e3 a0 40 b4 00 00 00 00 .................
+ ff0: f3 a0 40 b4 00 00 00 1a 00 00 00 0a 00 00 00 01 .................
+
+Leaf Directories
+~~~~~~~~~~~~~~~~
+
+Once a Block Directory has filled the block, the directory data is changed
+into a new format. It still uses `extents <#data-extents>`__ and the same
+basic structures, but the "data" and "leaf" are split up into their own
+extents. The "leaf" information only occupies one extent. As "leaf"
+information is more compact than
+"data" information, more than one "data" extent is common.
+
+- Block to Leaf conversions retain the existing block for the data entries
+ and allocate a new block for the leaf and freespace index information.
+
+- As with all directories, data blocks must start at logical offset zero.
+
+- The "leaf" block has a special offset defined by
+ XFS\_DIR2\_LEAF\_OFFSET. Currently, this is 32GB and in the extent view, a
+ block offset of 32GB / sb\_blocksize. On a 4KB block filesystem, this is
+ 0x800000 (8388608 decimal).
+
+- Blocks with directory entries
+ ("data" extents) have the magic number "X2D2" (0x58443244), or
+ "XDD3" (0x58444433) on a v5 filesystem.
+
+- The "data" extents have a new header (no "leaf" data):
+
+.. code:: c
+
+ typedef struct xfs_dir2_data {
+ xfs_dir2_data_hdr_t hdr;
+ xfs_dir2_data_union_t u[1];
+ } xfs_dir2_data_t;
+
+**hdr**
+ Data block header. On a v5 filesystem, this field is struct
+ xfs\_dir3\_data\_hdr.
+
+**u**
+ Union of directory and unused entries, exactly the same as in a block
+ directory.
+
+- The "leaf" extent uses the following structures:
+
+.. code:: c
+
+ typedef struct xfs_dir2_leaf {
+ xfs_dir2_leaf_hdr_t hdr;
+ xfs_dir2_leaf_entry_t ents[1];
+ xfs_dir2_data_off_t bests[1];
+ xfs_dir2_leaf_tail_t tail;
+ } xfs_dir2_leaf_t;
+
+**hdr**
+ Directory leaf header. On a v5 filesystem this is struct
+ xfs\_dir3\_leaf\_hdr\_t.
+
+**ents**
+ Hash values of the entries in this block.
+
+**bests**
+ An array pointing to free regions in the directory block.
+
+**tail**
+ Bookkeeping for the leaf entries.
+
+.. code:: c
+
+ typedef struct xfs_dir2_leaf_hdr {
+ xfs_da_blkinfo_t info;
+ __uint16_t count;
+ __uint16_t stale;
+ } xfs_dir2_leaf_hdr_t;
+
+**info**
+ Leaf btree block header.
+
+**count**
+ Number of leaf entries.
+
+**stale**
+ Number of stale/zeroed leaf entries.
+
+.. code:: c
+
+ struct xfs_dir3_leaf_hdr {
+ struct xfs_da3_blkinfo info;
+ __uint16_t count;
+ __uint16_t stale;
+ __be32 pad;
+ };
+
+**info**
+ Leaf B+tree block header.
+
+**count**
+ Number of leaf entries.
+
+**stale**
+ Number of stale/zeroed leaf entries.
+
+**pad**
+ Padding to maintain alignment rules.
+
+.. code:: c
+
+ typedef struct xfs_dir2_leaf_tail {
+ __uint32_t bestcount;
+ } xfs_dir2_leaf_tail_t;
+
+**bestcount**
+ Number of best free entries.
+
+- The magic number of the leaf block is XFS\_DIR2\_LEAF1\_MAGIC (0xd2f1); on
+ a v5 filesystem it is XFS\_DIR3\_LEAF1\_MAGIC (0x3df1).
+
+- The size of the ents array is specified by hdr.count.
+
+- The size of the bests array is specified by the tail.bestcount, which is
+ also the number of "data" blocks for  the directory. The bests array
+ maintains each data block’s bestfree[0].length value.
+
+.. ifconfig:: builder != 'latex'
+
+ .. figure:: images/48.png
+ :alt: Leaf directory free entry detail
+
+ Leaf directory free entry detail
+
+.. ifconfig:: builder == 'latex'
+
+ .. figure:: images/48.png
+ :scale: 40%
+ :alt: Leaf directory free entry detail
+
+ Leaf directory free entry detail
+
+xfs\_db Leaf Directory Example
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+For this example, a directory was created with 256 entries (frame000000.tst to
+frame000255.tst). Some files were deleted (frame00005\*, frame00018\* and
+frame000240.tst) to show free list characteristics.
+
+::
+
+ xfs_db> inode <inode#>
+ xfs_db> p
+ core.magic = 0x494e
+ core.mode = 040755
+ core.version = 1
+ core.format = 2 (extents)
+ core.nlinkv1 = 2
+ ...
+ core.size = 12288
+ core.nblocks = 4
+ core.extsize = 0
+ core.nextents = 3
+ ...
+ u.bmx[0-2] = [startoff,startblock,blockcount,extentflag]
+ 0:[0,4718604,1,0]
+ 1:[1,4718610,2,0]
+ 2:[8388608,4718605,1,0]
+
+As can be seen in this example, three blocks are used for
+"data" in two extents,
+and the "leaf" extent has a logical offset of 8388608 blocks (32GB).
+
+Examining the first block:
+
+::
+
+ xfs_db> dblock 0
+ xfs_db> type dir2
+ xfs_db> p
+ dhdr.magic = 0x58443244
+ dhdr.bestfree[0].offset = 0x670
+ dhdr.bestfree[0].length = 0x140
+ dhdr.bestfree[1].offset = 0xff0
+ dhdr.bestfree[1].length = 0x10
+ dhdr.bestfree[2].offset = 0
+ dhdr.bestfree[2].length = 0
+ du[0].inumber = 75497600
+ du[0].namelen = 1
+ du[0].name = "."
+ du[0].tag = 0x10
+ du[1].inumber = 128
+ du[1].namelen = 2
+ du[1].name = ".."
+ du[1].tag = 0x20
+ du[2].inumber = 75497601
+ du[2].namelen = 15
+ du[2].name = "frame000000.tst"
+ du[2].tag = 0x30
+ du[3].inumber = 75497602
+ du[3].namelen = 15
+ du[3].name = "frame000001.tst"
+ du[3].tag = 0x50
+ ...
+ du[51].inumber = 75497650
+ du[51].namelen = 15
+ du[51].name = "frame000049.tst"
+ du[51].tag = 0x650
+ du[52].freetag = 0xffff
+ du[52].length = 0x140
+ du[52].tag = 0x670
+ du[53].inumber = 75497661
+ du[53].namelen = 15
+ du[53].name = "frame000060.tst"
+ du[53].tag = 0x7b0
+ ...
+ du[118].inumber = 75497758
+ du[118].namelen = 15
+ du[118].name = "frame000125.tst"
+ du[118].tag = 0xfd0
+ du[119].freetag = 0xffff
+ du[119].length = 0x10
+ du[119].tag = 0xff0
+
+ **Note**
+
+ The xfs\_db field output is preceded by a "d" for "data".
+
+The next "data" block:
+
+::
+
+ xfs_db> dblock 1
+ xfs_db> type dir2
+ xfs_db> p
+ dhdr.magic = 0x58443244
+ dhdr.bestfree[0].offset = 0x6d0
+ dhdr.bestfree[0].length = 0x140
+ dhdr.bestfree[1].offset = 0xe50
+ dhdr.bestfree[1].length = 0x20
+ dhdr.bestfree[2].offset = 0xff0
+ dhdr.bestfree[2].length = 0x10
+ du[0].inumber = 75497759
+ du[0].namelen = 15
+ du[0].name = "frame000126.tst"
+ du[0].tag = 0x10
+ ...
+ du[53].inumber = 75497844
+ du[53].namelen = 15
+ du[53].name = "frame000179.tst"
+ du[53].tag = 0x6b0
+ du[54].freetag = 0xffff
+ du[54].length = 0x140
+ du[54].tag = 0x6d0
+ du[55].inumber = 75497855
+ du[55].namelen = 15
+ du[55].name = "frame000190.tst"
+ du[55].tag = 0x810
+ ...
+ du[104].inumber = 75497904
+ du[104].namelen = 15
+ du[104].name = "frame000239.tst"
+ du[104].tag = 0xe30
+ du[105].freetag = 0xffff
+ du[105].length = 0x20
+ du[105].tag = 0xe50
+ du[106].inumber = 75497906
+ du[106].namelen = 15
+ du[106].name = "frame000241.tst"
+ du[106].tag = 0xe70
+ ...
+ du[117].inumber = 75497917
+ du[117].namelen = 15
+ du[117].name = "frame000252.tst"
+ du[117].tag = 0xfd0
+ du[118].freetag = 0xffff
+ du[118].length = 0x10
+ du[118].tag = 0xff0
+
+And the last data block:
+
+::
+
+ xfs_db> dblock 2
+ xfs_db> type dir2
+ xfs_db> p
+ dhdr.magic = 0x58443244
+ dhdr.bestfree[0].offset = 0x70
+ dhdr.bestfree[0].length = 0xf90
+ dhdr.bestfree[1].offset = 0
+ dhdr.bestfree[1].length = 0
+ dhdr.bestfree[2].offset = 0
+ dhdr.bestfree[2].length = 0
+ du[0].inumber = 75497918
+ du[0].namelen = 15
+ du[0].name = "frame000253.tst"
+ du[0].tag = 0x10
+ du[1].inumber = 75497919
+ du[1].namelen = 15
+ du[1].name = "frame000254.tst"
+ du[1].tag = 0x30
+ du[2].inumber = 75497920
+ du[2].namelen = 15
+ du[2].name = "frame000255.tst"
+ du[2].tag = 0x50
+ du[3].freetag = 0xffff
+ du[3].length = 0xf90
+ du[3].tag = 0x70
+
+Examining the "leaf" block (with the fields preceded by an "l" for
+"leaf"):
+
+::
+
+ xfs_db> dblock 8388608
+ xfs_db> type dir2
+ xfs_db> p
+ lhdr.info.forw = 0
+ lhdr.info.back = 0
+ lhdr.info.magic = 0xd2f1
+ lhdr.count = 258
+ lhdr.stale = 0
+ lbests[0-2] = 0:0x10 1:0x10 2:0xf90
+ lents[0].hashval = 0x2e
+ lents[0].address = 0x2
+ lents[1].hashval = 0x172e
+ lents[1].address = 0x4
+ lents[2].hashval = 0x23a04084
+ lents[2].address = 0x116
+ ...
+ lents[257].hashval = 0xf3a048bc
+ lents[257].address = 0x366
+ ltail.bestcount = 3
+
+Note how the lbests array correspond with the bestfree[0].length values in the
+"data" blocks:
+
+::
+
+ xfs_db> dblock 0
+ xfs_db> type dir2
+ xfs_db> p
+ dhdr.magic = 0x58443244
+ dhdr.bestfree[0].offset = 0xff0
+ dhdr.bestfree[0].length = 0x10
+ ...
+ xfs_db> dblock 1
+ xfs_db> type dir2
+ xfs_db> p
+ dhdr.magic = 0x58443244
+ dhdr.bestfree[0].offset = 0xff0
+ dhdr.bestfree[0].length = 0x10
+ ...
+ xfs_db> dblock 2
+ xfs_db> type dir2
+ xfs_db> p
+ dhdr.magic = 0x58443244
+ dhdr.bestfree[0].offset = 0x70
+ dhdr.bestfree[0].length = 0xf90
+
+Now after the entries have been deleted:
+
+::
+
+ xfs_db> dblock 8388608
+ xfs_db> type dir2
+ xfs_db> p
+ lhdr.info.forw = 0
+ lhdr.info.back = 0
+ lhdr.info.magic = 0xd2f1
+ lhdr.count = 258
+ lhdr.stale = 21
+ lbests[0-2] = 0:0x140 1:0x140 2:0xf90
+ lents[0].hashval = 0x2e
+ lents[0].address = 0x2
+ lents[1].hashval = 0x172e
+ lents[1].address = 0x4
+ lents[2].hashval = 0x23a04084
+ lents[2].address = 0x116
+ ...
+
+As can be seen, the lbests values have been update to contain each
+hdr.bestfree[0].length values. The leaf’s hdr.stale value has also been
+updated to specify the number of stale entries in the array. The stale entries
+have an address of zero.
+
+TODO: Need an example for where new entries get inserted with several large
+free spaces.
+
+Node Directories
+~~~~~~~~~~~~~~~~
+
+When the "leaf" information fills a block, the extents undergo another
+separation. All "freeindex" information moves into its own extent. Like
+Leaf Directories, the
+"leaf" block maintained the best free space information for
+each "data" block. This is not possible with more than one leaf.
+
+- The "data" blocks stay the same as leaf directories.
+
+- After the "freeindex" data moves to its own block, it is possible for
+ the leaf data to fit within a single leaf block. This single leaf block has
+ a magic number of XFS\_DIR2\_LEAFN\_MAGIC (0xd2ff) or on a v5 filesystem,
+ XFS\_DIR3\_LEAFN\_MAGIC (0x3dff).
+
+- The "leaf" blocks eventually change into a B+tree with the generic B+tree
+ header pointing to directory "leaves" as described in `Leaf
+ Directories <#leaf-directories>`__. Blocks with leaf data still have the
+ LEAFN\_MAGIC magic number as outlined above. The top-level tree blocks are
+ called "nodes" and have a magic number of XFS\_DA\_NODE\_MAGIC
+ (0xfebe), or on a v5 filesystem, XFS\_DA3\_NODE\_MAGIC (0x3ebe).
+
+- Distinguishing between a combined leaf/freeindex block (LEAF1\_MAGIC), a
+ leaf-only block (LEAFN\_MAGIC), and a btree node block (NODE\_MAGIC) can
+ only be done by examining the magic number.
+
+- The new "freeindex" block(s) only contains the bests for each data
+ block.
+
+- The freeindex block uses the following structures:
+
+.. code:: c
+
+ typedef struct xfs_dir2_free_hdr {
+ __uint32_t magic;
+ __int32_t firstdb;
+ __int32_t nvalid;
+ __int32_t nused;
+ } xfs_dir2_free_hdr_t;
+
+**magic**
+ The magic number of the free block, "XD2F" (0x0x58443246).
+
+**firstdb**
+ The starting directory block number for the bests array.
+
+**nvalid**
+ Number of valid elements in the bests array. This number must correspond
+ with the number of directory blocks can fit under the inode di\_size.
+
+**nused**
+ Number of used elements in the bests array. This number must correspond
+ with the number of directory blocks actually mapped under the inode
+ di\_size.
+
+.. code:: c
+
+ typedef struct xfs_dir2_free {
+ xfs_dir2_free_hdr_t hdr;
+ xfs_dir2_data_off_t bests[1];
+ } xfs_dir2_free_t;
+
+**hdr**
+ Free block header.
+
+**bests**
+ An array specifying the best free counts in each directory data block.
+
+- On a v5 filesystem, the freeindex block uses the following structures:
+
+.. code:: c
+
+ struct xfs_dir3_free_hdr {
+ struct xfs_dir3_blk_hdr hdr;
+ __int32_t firstdb;
+ __int32_t nvalid;
+ __int32_t nused;
+ __int32_t pad;
+ };
+
+**hdr**
+ v3 directory block header. The magic number is "XDF3" (0x0x58444633).
+
+**firstdb**
+ The starting directory block number for the bests array.
+
+**nvalid**
+ Number of valid elements in the bests array. This number must correspond
+ with the number of directory blocks can fit under the inode di\_size.
+
+**nused**
+ Number of used elements in the bests array. This number must correspond
+ with the number of directory blocks actually mapped under the inode
+ di\_size.
+
+**pad**
+ Padding to maintain alignment.
+
+.. code:: c
+
+ struct xfs_dir3_free {
+ xfs_dir3_free_hdr_t hdr;
+ __be16 bests[1];
+ };
+
+**hdr**
+ Free block header.
+
+**bests**
+ An array specifying the best free counts in each directory data block.
+
+- The location of the leaf blocks can be in any order, the only way to
+ determine the appropriate is by the node block hash/before values. Given a
+ hash to look up, you read the node’s btree array and first hashval in the
+ array that exceeds the given hash and it can then be found in the block
+ pointed to by the before value.
+
+- The freeindex’s bests array starts from the end of the block and grows to
+ the start of the block.
+
+- When an data block becomes unused (ie. all entries in it have been
+ deleted), the block is freed, the data extents contain a hole, and the
+ freeindex’s hdr.nused value is decremented and the associated bests[] entry
+ is set to 0xffff.
+
+- As the first data block always contains "." and "..", it’s invalid for
+ the directory to have a hole at the start.
+
+- The freeindex’s hdr.nused should always be the same as the number of
+ allocated data directory blocks containing name/inode data and will always
+ be less than or equal to hdr.nvalid. The value of hdr.nvalid should be the
+ same as the index of the last data directory block plus one (i.e. when the
+ last data block is freed, nused and nvalid are decremented).
+
+.. ifconfig:: builder != 'latex'
+
+ .. figure:: images/54.png
+ :alt: Node directory layout
+
+ Node directory layout
+
+.. ifconfig:: builder == 'latex'
+
+ .. figure:: images/54.png
+ :scale: 40%
+ :alt: Node directory layout
+
+ Node directory layout
+
+xfs\_db Node Directory Example
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+With the node directory examples, we are using a filesystems with 4KB block
+size, and a 16KB directory size. The directory has over 2000 entries:
+
+::
+
+ xfs_db> sb 0
+ xfs_db> p
+ magicnum = 0x58465342
+ blocksize = 4096
+ ...
+ dirblklog = 2
+ ...
+ xfs_db> inode <inode#>
+ xfs_db> p
+ core.magic = 0x494e
+ core.mode = 040755
+ core.version = 1
+ core.format = 2 (extents)
+ ...
+ core.size = 81920
+ core.nblocks = 36
+ core.extsize = 0
+ core.nextents = 8
+ ...
+ u.bmx[0-7] = [startoff,startblock,blockcount,extentflag] 0:[0,7368,4,0]
+ 1:[4,7408,4,0] 2:[8,7444,4,0] 3:[12,7480,4,0] 4:[16,7520,4,0]
+ 5:[8388608,7396,4,0] 6:[8388612,7524,8,0] 7:[16777216,7516,4,0]
+
+As can already be observed, all extents are allocated is multiples of 4
+blocks.
+
+Blocks 0 to 19 (16+4-1) are used for directory data blocks. Looking at blocks
+16-19, we can seen that it’s the same as the single-leaf format, except the
+length values are a lot larger to accommodate the increased directory block
+size:
+
+::
+
+ xfs_db> dblock 16
+ xfs_db> type dir2
+ xfs_db> p
+ dhdr.magic = 0x58443244
+ dhdr.bestfree[0].offset = 0xb0
+ dhdr.bestfree[0].length = 0x3f50
+ dhdr.bestfree[1].offset = 0
+ dhdr.bestfree[1].length = 0
+ dhdr.bestfree[2].offset = 0
+ dhdr.bestfree[2].length = 0
+ du[0].inumber = 120224
+ du[0].namelen = 15
+ du[0].name = "frame002043.tst"
+ du[0].tag = 0x10
+ du[1].inumber = 120225
+ du[1].namelen = 15
+ du[1].name = "frame002044.tst"
+ du[1].tag = 0x30
+ du[2].inumber = 120226
+ du[2].namelen = 15
+ du[2].name = "frame002045.tst"
+ du[2].tag = 0x50
+ du[3].inumber = 120227
+ du[3].namelen = 15
+ du[3].name = "frame002046.tst"
+ du[3].tag = 0x70
+ du[4].inumber = 120228
+ du[4].namelen = 15
+ du[4].name = "frame002047.tst"
+ du[4].tag = 0x90
+ du[5].freetag = 0xffff
+ du[5].length = 0x3f50
+ du[5].tag = 0
+
+Next, the "node" block, the fields are preceded with 'n' for node blocks:
+
+::
+
+ xfs_db> dblock 8388608
+ xfs_db> type dir2
+ xfs_db> p
+ nhdr.info.forw = 0
+ nhdr.info.back = 0
+ nhdr.info.magic = 0xfebe
+ nhdr.count = 2
+ nhdr.level = 1
+ nbtree[0-1] = [hashval,before] 0:[0xa3a440ac,8388616] 1:[0xf3a440bc,8388612]
+
+The two following leaf blocks were allocated as part of the directory’s
+conversion to node format. All hashes less than 0xa3a440ac are located at
+directory offset 8,388,616, and hashes less than 0xf3a440bc are located at
+directory offset 8,388,612. Hashes greater or equal to 0xf3a440bc don’t exist
+in this directory.
+
+::
+
+ xfs_db> dblock 8388616
+ xfs_db> type dir2
+ xfs_db> p
+ lhdr.info.forw = 8388612
+ lhdr.info.back = 0
+ lhdr.info.magic = 0xd2ff
+ lhdr.count = 1023
+ lhdr.stale = 0
+ lents[0].hashval = 0x2e
+ lents[0].address = 0x2
+ lents[1].hashval = 0x172e
+ lents[1].address = 0x4
+ lents[2].hashval = 0x23a04084
+ lents[2].address = 0x116
+ ...
+ lents[1021].hashval = 0xa3a440a4
+ lents[1021].address = 0x1fa2
+ lents[1022].hashval = 0xa3a440ac
+ lents[1022].address = 0x1fca
+ xfs_db> dblock 8388612
+ xfs_db> type dir2
+ xfs_db> p
+ lhdr.info.forw = 0
+ lhdr.info.back = 8388616
+ lhdr.info.magic = 0xd2ff
+ lhdr.count = 1027
+ lhdr.stale = 0
+ lents[0].hashval = 0xa3a440b4
+ lents[0].address = 0x1f52
+ lents[1].hashval = 0xa3a440bc
+ lents[1].address = 0x1f7a
+ ...
+ lents[1025].hashval = 0xf3a440b4
+ lents[1025].address = 0x1f66
+ lents[1026].hashval = 0xf3a440bc
+ lents[1026].address = 0x1f8e
+
+An example lookup using xfs\_db:
+
+::
+
+ xfs_db> hash frame001845.tst
+ 0xf3a26094
+
+Doing a binary search through the array, we get address 0x1ce6, which is
+offset 0xe730. Each fsblock is 4KB in size (0x1000), so it will be offset
+0x730 into directory offset 14. From the extent map, this will be fsblock
+7482:
+
+::
+
+ xfs_db> fsblock 7482
+ xfs_db> type text
+ xfs_db> p
+ ...
+ 730: 00 00 00 00 00 01 d4 da 0f 66 72 61 6d 65 30 30 .........frame00
+ 740: 31 38 34 35 2e 74 73 74 00 00 00 00 00 00 27 30 1845.tst.......0
+
+Looking at the freeindex information (fields with an 'f' tag):
+
+::
+
+ xfs_db> fsblock 7516
+ xfs_db> type dir2
+ xfs_db> p
+ fhdr.magic = 0x58443246
+ fhdr.firstdb = 0
+ fhdr.nvalid = 5
+ fhdr.nused = 5
+ fbests[0-4] = 0:0x10 1:0x10 2:0x10 3:0x10 4:0x3f50
+
+Like the Leaf Directory, each of the fbests values correspond to each data
+block’s bestfree[0].length value.
+
+The fbests array is highlighted in a raw block dump:
+
+::
+
+ xfs_db> type text
+ xfs_db> p
+ 000: 58 44 32 46 00 00 00 00 00 00 00 05 00 00 00 05 XD2F............
+ 010: 00 10 00 10 00 10 00 10 3f 50 00 00 1f 01 ff ff .........P......
+
+TODO: Example with a hole in the middle
+
+B+tree Directories
+~~~~~~~~~~~~~~~~~~
+
+When the extent map in an inode grows beyond the inode’s space, the inode
+format is changed to a
+"btree". The inode contains a filesystem block point to the
+B+tree extent map for the directory’s blocks. The B+tree extents contain the
+extent map for the "data", "node", "leaf", and "freeindex"
+information as described in Node Directories.
+
+Refer to the previous section on B+tree `Data Extents <#b-tree-extent-list>`__
+for more information on XFS B+tree extents.
+
+The following properties apply to both node and B+tree directories:
+
+- The node/leaf trees can be more than one level deep.
+
+- More than one freeindex block may exist, but this will be quite rare. It
+ would required hundreds of thousand files with quite long file names (or
+ millions with shorter names) to get a second freeindex block.
+
+xfs\_db B+tree Directory Example
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+A directory has been created with 200,000 entries with each entry being 100
+characters long. The filesystem block size and directory block size are 4KB:
+
+::
+
+ xfs_db> inode <inode#>
+ xfs_db> p
+ core.magic = 0x494e
+ core.mode = 040755
+ core.version = 1
+ core.format = 3 (btree)
+ ...
+ core.size = 22757376
+ core.nblocks = 6145
+ core.extsize = 0
+ core.nextents = 234
+ core.naextents = 0
+ core.forkoff = 0
+ ...
+ u.bmbt.level = 1
+ u.bmbt.numrecs = 1
+ u.bmbt.keys[1] = [startoff] 1:[0]
+ u.bmbt.ptrs[1] = 1:89
+ xfs_db> fsblock 89
+ xfs_db> type bmapbtd
+ xfs_db> p
+ magic = 0x424d4150
+ level = 0
+ numrecs = 234
+ leftsib = null
+ rightsib = null
+ recs[1-234] = [startoff,startblock,blockcount,extentflag]
+ 1:[0,53,1,0] 2:[1,55,13,0] 3:[14,69,1,0] 4:[15,72,13,0]
+ 5:[28,86,2,0] 6:[30,90,21,0] 7:[51,112,1,0] 8:[52,114,11,0]
+ ...
+ 125:[5177,902,15,0] 126:[5192,918,6,0] 127:[5198,524786,358,0]
+ 128:[8388608,54,1,0] 129:[8388609,70,2,0] 130:[8388611,85,1,0]
+ ...
+ 229:[8389164,917,1,0] 230:[8389165,924,19,0] 231:[8389184,944,9,0]
+ 232:[16777216,68,1,0] 233:[16777217,7340114,1,0] 234:[16777218,5767362,1,0]
+
+We have 128 extents and a total of 5555 blocks being used to store name/inode
+pairs. With only about 2000 values that can be stored in the freeindex block,
+3 blocks have been allocated for this information. The firstdb field specifies
+the starting directory block number for each array:
+
+::
+
+ xfs_db> dblock 16777216
+ xfs_db> type dir2
+ xfs_db> p
+ fhdr.magic = 0x58443246
+ fhdr.firstdb = 0
+ fhdr.nvalid = 2040
+ fhdr.nused = 2040
+ fbests[0-2039] = ...
+ xfs_db> dblock 16777217
+ xfs_db> type dir2
+ xfs_db> p
+ fhdr.magic = 0x58443246
+ fhdr.firstdb = 2040
+ fhdr.nvalid = 2040
+ fhdr.nused = 2040
+ fbests[0-2039] = ...
+ xfs_db> dblock 16777218
+ xfs_db> type dir2
+ xfs_db> p
+ fhdr.magic = 0x58443246
+ fhdr.firstdb = 4080
+ fhdr.nvalid = 1476
+ fhdr.nused = 1476
+ fbests[0-1475] = ...
+
+Looking at the root node in the node block, it’s a pretty deep tree:
+
+::
+
+ xfs_db> dblock 8388608
+ xfs_db> type dir2
+ xfs_db> p
+ nhdr.info.forw = 0
+ nhdr.info.back = 0
+ nhdr.info.magic = 0xfebe
+ nhdr.count = 2
+ nhdr.level = 2
+ nbtree[0-1] = [hashval,before] 0:[0x6bbf6f39,8389121] 1:[0xfbbf7f79,8389120]
+ xfs_db> dblock 8389121
+ xfs_db> type dir2
+ xfs_db> p
+ nhdr.info.forw = 8389120
+ nhdr.info.back = 0
+ nhdr.info.magic = 0xfebe
+ nhdr.count = 263
+ nhdr.level = 1
+ nbtree[0-262] = ... 262:[0x6bbf6f39,8388928]
+ xfs_db> dblock 8389120
+ xfs_db> type dir2
+ xfs_db> p
+ nhdr.info.forw = 0
+ nhdr.info.back = 8389121
+ nhdr.info.magic = 0xfebe
+ nhdr.count = 319
+ nhdr.level = 1
+ nbtree[0-318] = [hashval,before] 0:[0x70b14711,8388919] ...
+
+The leaves at each the end of a node always point to the end leaves in
+adjacent nodes. Directory block 8388928 has a forward pointer to block 8388919
+and block 8388919 has a previous pointer to block 8388928, as highlighted in
+the following example:
+
+::
+
+ xfs_db> dblock 8388928
+ xfs_db> type dir2
+ xfs_db> p
+ lhdr.info.forw = 8388919
+ lhdr.info.back = 8388937
+ lhdr.info.magic = 0xd2ff
+ ...
+
+ xfs_db> dblock 8388919
+ xfs_db> type dir2
+ xfs_db> p
+ lhdr.info.forw = 8388706
+ lhdr.info.back = 8388928
+ lhdr.info.magic = 0xd2ff
+ ...
diff --git a/Documentation/filesystems/xfs-data-structures/dynamic.rst b/Documentation/filesystems/xfs-data-structures/dynamic.rst
index 5ba6f3940808..2c12fca905fd 100644
--- a/Documentation/filesystems/xfs-data-structures/dynamic.rst
+++ b/Documentation/filesystems/xfs-data-structures/dynamic.rst
@@ -5,3 +5,4 @@ Dynamic Allocated Structures

.. include:: ondisk_inode.rst
.. include:: data_extents.rst
+.. include:: directories.rst