On Thu, Jun 16, 2016 at 06:48:39PM -0700, Darrick J. Wong wrote:
> Add chapters on the operation of the reverse mapping btree and future
> things we could do with rmap data.
>
> v2: Add magic number to the table.
>
Hi darrick.
Just FYI, I'm not sure why, but this patch came to the list in base64 format,
although the remaining ones are in text-plain.
> Signed-off-by: Darrick J. Wong <darrick.wong@xxxxxxxxxx>
> ---
> .../allocation_groups.asciidoc | 24 +-
> design/XFS_Filesystem_Structure/docinfo.xml | 17 +
> .../journaling_log.asciidoc | 130 +++++++++
> design/XFS_Filesystem_Structure/magic.asciidoc | 3
> .../reconstruction.asciidoc | 53 +++
> design/XFS_Filesystem_Structure/rmapbt.asciidoc | 305 ++++++++++++++++++++
> .../xfs_filesystem_structure.asciidoc | 4
> 7 files changed, 528 insertions(+), 8 deletions(-)
> create mode 100644 design/XFS_Filesystem_Structure/reconstruction.asciidoc
> create mode 100644 design/XFS_Filesystem_Structure/rmapbt.asciidoc
>
>
> diff --git a/design/XFS_Filesystem_Structure/allocation_groups.asciidoc b/design/XFS_Filesystem_Structure/allocation_groups.asciidoc
> index 55bbc50..8ced83a 100644
> --- a/design/XFS_Filesystem_Structure/allocation_groups.asciidoc
> +++ b/design/XFS_Filesystem_Structure/allocation_groups.asciidoc
> @@ -12,6 +12,7 @@ Each AG has the following characteristics:
> * A super block describing overall filesystem info
> * Free space management
> * Inode allocation and tracking
> + * Reverse block-mapping index (optional)
>
> Having multiple AGs allows XFS to handle most operations in parallel without
> degrading performance as the number of concurrent accesses increases.
> @@ -379,6 +380,12 @@ it doesn't understand the flag.
> Free inode B+tree. Each allocation group contains a B+tree to track inode chunks
> containing free inodes. This is a performance optimization to reduce the time
> required to allocate inodes.
> +
> +| +XFS_SB_FEAT_RO_COMPAT_RMAPBT+ |
> +Reverse mapping B+tree. Each allocation group contains a B+tree containing
> +records mapping AG blocks to their owners. See the section about
> +xref:Reconstruction[reconstruction] for more details.
> +
> |=====
>
> *sb_features_incompat*::
> @@ -529,9 +536,7 @@ struct xfs_agf {
> __be32 agf_seqno;
> __be32 agf_length;
> __be32 agf_roots[XFS_BTNUM_AGF];
> - __be32 agf_spare0;
> __be32 agf_levels[XFS_BTNUM_AGF];
> - __be32 agf_spare1;
> __be32 agf_flfirst;
> __be32 agf_fllast;
> __be32 agf_flcount;
> @@ -550,9 +555,10 @@ struct xfs_agf {
> };
> ----
>
> -The rest of the bytes in the sector are zeroed. +XFS_BTNUM_AGF+ is set to 2:
> -index 0 for the free space B+tree indexed by block number; and index 1 for the
> -free space B+tree indexed by extent size.
> +The rest of the bytes in the sector are zeroed. +XFS_BTNUM_AGF+ is set to 3:
> +index 0 for the free space B+tree indexed by block number; index 1 for the free
> +space B+tree indexed by extent size; and index 2 for the reverse-mapping
> +B+tree.
>
> *agf_magicnum*::
> Specifies the magic number for the AGF sector: ``XAGF'' (0x58414746).
> @@ -570,11 +576,13 @@ this could be less than the +sb_agblocks+ value. It is this value that should
> be used to determine the size of the AG.
>
> *agf_roots*::
> -Specifies the block number for the root of the two free space B+trees.
> +Specifies the block number for the root of the two free space B+trees and the
> +reverse-mapping B+tree, if enabled.
>
> *agf_levels*::
> -Specifies the level or depth of the two free space B+trees. For a fresh AG, this
> -will be one, and the ``roots'' will point to a single leaf of level 0.
> +Specifies the level or depth of the two free space B+trees and the
> +reverse-mapping B+tree, if enabled. For a fresh AG, this value will be one,
> +and the ``roots'' will point to a single leaf of level 0.
>
> *agf_flfirst*::
> Specifies the index of the first ``free list'' block. Free lists are covered in
> diff --git a/design/XFS_Filesystem_Structure/docinfo.xml b/design/XFS_Filesystem_Structure/docinfo.xml
> index cc5596d..44f944a 100644
> --- a/design/XFS_Filesystem_Structure/docinfo.xml
> +++ b/design/XFS_Filesystem_Structure/docinfo.xml
> @@ -122,4 +122,21 @@
> </simplelist>
> </revdescription>
> </revision>
> + <revision>
> + <revnumber>3.141</revnumber>
> + <date>June 2016</date>
> + <author>
> + <firstname>Darrick</firstname>
> + <surname>Wong</surname>
> + <email></email>
> + </author>
> + <revdescription>
> + <simplelist>
> + <member>Document the reverse-mapping btree.</member>
> + <member>Move the b+tree info to a separate chapter.</member>
> + <member>Discuss overlapping interval b+trees.</member>
> + <member>Discuss new log items for atomic updates.</member>
> + </simplelist>
> + </revdescription>
> + </revision>
> </revhistory>
> diff --git a/design/XFS_Filesystem_Structure/journaling_log.asciidoc b/design/XFS_Filesystem_Structure/journaling_log.asciidoc
> index 67d209f..0fbbcee 100644
> --- a/design/XFS_Filesystem_Structure/journaling_log.asciidoc
> +++ b/design/XFS_Filesystem_Structure/journaling_log.asciidoc
> @@ -209,6 +209,8 @@ magic number to distinguish themselves. Buffer data items only appear after
> | +XFS_LI_DQUOT+ | 0x123d | xref:Quota_Update_Log_Item[Update Quota]
> | +XFS_LI_QUOTAOFF+ | 0x123e | xref:Quota_Off_Log_Item[Quota Off]
> | +XFS_LI_ICREATE+ | 0x123f | xref:Inode_Create_Log_Item[Inode Creation]
> +| +XFS_LI_RUI+ | 0x1240 | xref:RUI_Log_Item[Reverse Mapping Update Intent]
> +| +XFS_LI_RUD+ | 0x1241 | xref:RUD_Log_Item[Reverse Mapping Update Done]
> |=====
>
> [[Log_Transaction_Headers]]
> @@ -386,6 +388,134 @@ Variable-length array of extents to be freed. The array length is given by
> +xfs_extent_32_t+; this can be determined from the log item size (+oh_len+) and
> the number of extents (+efd_nextents+).
>
> +[[RUI_Log_Item]]
> +=== Reverse Mapping Updates Intent
> +
> +The next two operation types work together to handle deferred reverse mapping
> +updates. Naturally, the mappings to be updated can be expressed in terms of
> +mapping extents:
> +
> +[source, c]
> +----
> +struct xfs_map_extent {
> + __uint64_t me_owner;
> + __uint64_t me_startblock;
> + __uint64_t me_startoff;
> + __uint32_t me_len;
> + __uint32_t me_flags;
> +};
> +----
> +
> +*me_owner*::
> +Owner of this reverse mapping. See the values in the section about
> +xref:Reverse_Mapping_Btree[reverse mapping] for more information.
> +
> +*me_startblock*::
> +Filesystem block of this mapping.
> +
> +*me_startoff*::
> +Logical block offset of this mapping.
> +
> +*me_len*::
> +The length of this mapping.
> +
> +*me_flags*::
> +The lower byte of this field is a type code indicating what sort of
> +reverse mapping operation we want. The upper three bytes are flag bits.
> +
> +.Reverse mapping update log intent types
> +[options="header"]
> +|=====
> +| Value | Description
> +| +XFS_RMAP_EXTENT_MAP+ | Add a reverse mapping for file data.
> +| +XFS_RMAP_EXTENT_MAP_SHARED+ | Add a reverse mapping for file data for a file with shared blocks.
> +| +XFS_RMAP_EXTENT_UNMAP+ | Remove a reverse mapping for file data.
> +| +XFS_RMAP_EXTENT_UNMAP_SHARED+ | Remove a reverse mapping for file data for a file with shared blocks.
> +| +XFS_RMAP_EXTENT_CONVERT+ | Convert a reverse mapping for file data between unwritten and normal.
> +| +XFS_RMAP_EXTENT_CONVERT_SHARED+ | Convert a reverse mapping for file data between unwritten and normal for a file with shared blocks.
> +| +XFS_RMAP_EXTENT_ALLOC+ | Add a reverse mapping for non-file data.
> +| +XFS_RMAP_EXTENT_FREE+ | Remove a reverse mapping for non-file data.
> +|=====
> +
> +.Reverse mapping update log intent flags
> +[options="header"]
> +|=====
> +| Value | Description
> +| +XFS_RMAP_EXTENT_ATTR_FORK+ | Extent is for the attribute fork.
> +| +XFS_RMAP_EXTENT_BMBT_BLOCK+ | Extent is for a block mapping btree block.
> +| +XFS_RMAP_EXTENT_UNWRITTEN+ | Extent is unwritten.
> +|=====
> +
> +The ``rmap update intent'' operation comes first; it tells the log that XFS
> +wants to update some reverse mappings. This record is crucial for correct log
> +recovery because it enables us to spread a complex metadata update across
> +multiple transactions while ensuring that a crash midway through the complex
> +update will be replayed fully during log recovery.
> +
> +[source, c]
> +----
> +struct xfs_rui_log_format {
> + __uint16_t rui_type;
> + __uint16_t rui_size;
> + __uint32_t rui_nextents;
> + __uint64_t rui_id;
> + struct xfs_map_extent rui_extents[1];
> +};
> +----
> +
> +*rui_type*::
> +The signature of an RUI operation, 0x1240. This value is in host-endian order,
> +not big-endian like the rest of XFS.
> +
> +*rui_size*::
> +Size of this log item. Should be 1.
> +
> +*rui_nextents*::
> +Number of reverse mappings.
> +
> +*rui_id*::
> +A 64-bit number that binds the corresponding RUD log item to this RUI log item.
> +
> +*rui_extents*::
> +Variable-length array of reverse mappings to update.
> +
> +[[RUD_Log_Item]]
> +=== Completion of Reverse Mapping Updates
> +
> +The ``reverse mapping update done'' operation complements the ``reverse mapping
> +update intent'' operation. This second operation indicates that the update
> +actually happened, so that log recovery needn't replay the update. The RUD and
> +the actual updates are typically found in a new transaction following the
> +transaction in which the RUI was logged.
> +
> +[source, c]
> +----
> +struct xfs_rud_log_format {
> + __uint16_t rud_type;
> + __uint16_t rud_size;
> + __uint32_t rud_nextents;
> + __uint64_t rud_rui_id;
> + struct xfs_map_extent rud_extents[1];
> +};
> +----
> +
> +*rud_type*::
> +The signature of an RUD operation, 0x1241. This value is in host-endian order,
> +not big-endian like the rest of XFS.
> +
> +*rud_size*::
> +Size of this log item. Should be 1.
> +
> +*rud_nextents*::
> +Number of reverse mappings.
> +
> +*rud_id*::
> +A 64-bit number that binds the corresponding RUI log item to this RUD log item.
> +
> +*rud_extents*::
> +Variable-length array of reverse mappings. The array length is given by
> ++rud_nextents+.
> +
> [[Inode_Log_Item]]
> === Inode Updates
>
> diff --git a/design/XFS_Filesystem_Structure/magic.asciidoc b/design/XFS_Filesystem_Structure/magic.asciidoc
> index 301cfa0..10fd15f 100644
> --- a/design/XFS_Filesystem_Structure/magic.asciidoc
> +++ b/design/XFS_Filesystem_Structure/magic.asciidoc
> @@ -44,6 +44,7 @@ relevant chapters. Magic numbers tend to have consistent locations:
> | +XFS_ATTR_LEAF_MAGIC+ | 0xfbee | | xref:Leaf_Attributes[Leaf Attribute]
> | +XFS_ATTR3_LEAF_MAGIC+ | 0x3bee | | xref:Leaf_Attributes[Leaf Attribute], v5 only
> | +XFS_ATTR3_RMT_MAGIC+ | 0x5841524d | XARM | xref:Remote_Values[Remote Attribute Value], v5 only
> +| +XFS_RMAP_CRC_MAGIC+ | 0x524d4233 | RMB3 | xref:Reverse_Mapping_Btree[Reverse Mapping B+tree], v5 only
> |=====
>
> The magic numbers for log items are at offset zero in each log item, but items
> @@ -61,6 +62,8 @@ are not aligned to blocks.
> | +XFS_LI_DQUOT+ | 0x123d | | xref:Quota_Update_Log_Item[Update Quota Log Item]
> | +XFS_LI_QUOTAOFF+ | 0x123e | | xref:Quota_Off_Log_Item[Quota Off Log Item]
> | +XFS_LI_ICREATE+ | 0x123f | | xref:Inode_Create_Log_Item[Inode Creation Log Item]
> +| +XFS_LI_RUI+ | 0x1240 | | xref:RUI_Log_Item[Reverse Mapping Update Intent]
> +| +XFS_LI_RUD+ | 0x1241 | | xref:RUD_Log_Item[Reverse Mapping Update Done]
> |=====
>
> = Theoretical Limits
> diff --git a/design/XFS_Filesystem_Structure/reconstruction.asciidoc b/design/XFS_Filesystem_Structure/reconstruction.asciidoc
> new file mode 100644
> index 0000000..f172e0f
> --- /dev/null
> +++ b/design/XFS_Filesystem_Structure/reconstruction.asciidoc
> @@ -0,0 +1,53 @@
> +[[Reconstruction]]
> += Metadata Reconstruction
> +
> +[NOTE]
> +This is a theoretical discussion of how reconstruction could work; none of this
> +is implemented as of 2015.
> +
> +A simple UNIX filesystem can be thought of in terms of a directed acyclic graph.
> +To a first approximation, there exists a root directory node, which points to
> +other nodes. Those other nodes can themselves be directories or they can be
> +files. Each file, in turn, points to data blocks.
> +
> +XFS adds a few more details to this picture:
> +
> +* The real root(s) of an XFS filesystem are the allocation group headers
> +(superblock, AGF, AGI, AGFL).
> +* Each allocation group’s headers point to various per-AG B+trees (free space,
> +inode, free inodes, free list, etc.)
> +* The free space B+trees point to unused extents;
> +* The inode B+trees point to blocks containing inode chunks;
> +* All superblocks point to the root directory and the log;
> +* Hardlinks mean that multiple directories can point to a single file node;
> +* File data block pointers are indexed by file offset;
> +* Files and directories can have a second collection of pointers to data blocks
> +which contain extended attributes;
> +* Large directories require multiple data blocks to store all the subpointers;
> +* Still larger directories use high-offset data blocks to store a B+tree of
> +hashes to directory entries;
> +* Large extended attribute forks similarly use high-offset data blocks to store
> +a B+tree of hashes to attribute keys; and
> +* Symbolic links can point to data blocks.
> +
> +The beauty of this massive graph structure is that under normal circumstances,
> +everything known to the filesystem is discoverable (access controls
> +notwithstanding) from the root. The major weakness of this structure of course
> +is that breaking a edge in the graph can render entire subtrees inaccessible.
> ++xfs_repair+ “recovers” from broken directories by scanning for unlinked inodes
> +and connecting them to +/lost+found+, but this isn’t sufficiently general to
> +recover from breaks in other parts of the graph structure. Wouldn’t it be
> +useful to have back pointers as a secondary data structure? The current repair
> +strategy is to reconstruct whatever can be rebuilt, but to scrap anything that
> +doesn't check out.
> +
> +The xref:Reverse_Mapping_Btree[reverse-mapping B+tree] fills in part of the
> +puzzle. Since it contains copies of every entry in each inode’s data and
> +attribute forks, we can fix a corrupted block map with these records.
> +Furthermore, if the inode B+trees become corrupt, it is possible to visit all
> +inode chunks using the reverse-mapping data. Should XFS ever gain the ability
> +to store parent directory information in each inode, it also becomes possible
> +to resurrect damaged directory trees, which should reduce the complaints about
> +inodes ending up in +/lost+found+. Everything else in the per-AG primary
> +metadata can already be reconstructed via +xfs_repair+. Hopefully,
> +reconstruction will not turn out to be a fool's errand.
> diff --git a/design/XFS_Filesystem_Structure/rmapbt.asciidoc b/design/XFS_Filesystem_Structure/rmapbt.asciidoc
> new file mode 100644
> index 0000000..a8a210b
> --- /dev/null
> +++ b/design/XFS_Filesystem_Structure/rmapbt.asciidoc
> @@ -0,0 +1,305 @@
> +[[Reverse_Mapping_Btree]]
> +== Reverse-Mapping B+tree
> +
> +[NOTE]
> +This data structure is under construction! Details may change.
> +
> +If the feature is enabled, each allocation group has its own reverse
> +block-mapping B+tree, which grows in the free space like the free space
> +B+trees. As mentioned in the chapter about
> +xref:Reconstruction[reconstruction], this data structure is another piece of
> +the puzzle necessary to reconstruct the data or attribute fork of a file from
> +reverse-mapping records; we can also use it to double-check allocations to
> +ensure that we are not accidentally cross-linking blocks, which can cause
> +severe damage to the filesystem.
> +
> +This B+tree is only present if the +XFS_SB_FEAT_RO_COMPAT_RMAPBT+
> +feature is enabled. The feature requires a version 5 filesystem.
> +
> +Each record in the reverse-mapping B+tree has the following structure:
> +
> +[source, c]
> +----
> +struct xfs_rmap_rec {
> + __be32 rm_startblock;
> + __be32 rm_blockcount;
> + __be64 rm_owner;
> + __be64 rm_fork:1;
> + __be64 rm_bmbt:1;
> + __be64 rm_unwritten:1;
> + __be64 rm_unused:7;
> + __be64 rm_offset:54;
> +};
> +----
> +
> +*rm_startblock*::
> +AG block number of this record.
> +
> +*rm_blockcount*::
> +The length of this extent.
> +
> +*rm_owner*::
> +A 64-bit number describing the owner of this extent. This is typically the
> +absolute inode number, but can also correspond to one of the following:
> +
> +.Special owner values
> +[options="header"]
> +|=====
> +| Value | Description
> +| +XFS_RMAP_OWN_NULL+ | No owner. This should never appear on disk.
> +| +XFS_RMAP_OWN_UNKNOWN+ | Unknown owner; for EFI recovery. This should never appear on disk.
> +| +XFS_RMAP_OWN_FS+ | Allocation group headers
> +| +XFS_RMAP_OWN_LOG+ | XFS log blocks
> +| +XFS_RMAP_OWN_AG+ | Per-allocation group B+tree blocks. This means free space B+tree blocks, blocks on the freelist, and reverse-mapping B+tree blocks.
> +| +XFS_RMAP_OWN_INOBT+ | Per-allocation group inode B+tree blocks. This includes free inode B+tree blocks.
> +| +XFS_RMAP_OWN_INODES+ | Inode chunks
> +|=====
> +
> +*rm_fork*::
> +If +rm_owner+ describes an inode, this can be 1 if this record is for an
> +attribute fork.
> +
> +*rm_bmbt*::
> +If +rm_owner+ describes an inode, this can be 1 to signify that this record is
> +for a block map B+tree block. In this case, +rm_offset+ has no meaning.
> +
> +*rm_unwritten*::
> +A flag indicating that the extent is unwritten. This corresponds to the flag in
> +the xref:Data_Extents[extent record] format which means +XFS_EXT_UNWRITTEN+.
> +
> +*rm_offset*::
> +The 54-bit logical file block offset, if +rm_owner+ describes an inode.
> +Meaningless otherwise.
> +
> +[NOTE]
> +The single-bit flag values +rm_unwritten+, +rm_fork+, and +rm_bmbt+ are packed
> +into the larger fields in the C structure definition.
> +
> +The key has the following structure:
> +
> +[source, c]
> +----
> +struct xfs_rmap_key {
> + __be32 rm_startblock;
> + __be64 rm_owner;
> + __be64 rm_fork:1;
> + __be64 rm_bmbt:1;
> + __be64 rm_reserved:1;
> + __be64 rm_unused:7;
> + __be64 rm_offset:54;
> +};
> +----
> +
> +For the reverse-mapping B+tree on a filesystem that supports sharing of file
> +data blocks, the key definition is larger than the usual AG block number. On a
> +classic XFS filesystem, each block has only one owner, which means that
> ++rm_startblock+ is sufficient to uniquely identify each record. However,
> +shared block support (reflink) on XFS breaks that assumption; now filesystem
> +blocks can be linked to any logical block offset of any file inode. Therefore,
> +the key must include the owner and offset information to preserve the 1 to 1
> +relation between key and record.
> +
> +* As the reference counting is AG relative, all the block numbers are only
> +32-bits.
> +* The +bb_magic+ value is "RMB3" (0x524d4233).
> +* The +xfs_btree_sblock_t+ header is used for intermediate B+tree node as well
> +as the leaves.
> +* Each pointer is associated with two keys. The first of these is the "low
> +key", which is the key of the smallest record accessible through the pointer.
> +This low key has the same meaning as the key in all other btrees. The second
> +key is the high key, which is the maximum of the largest key that can be used
> +to access a given record underneath the pointer. Recall that each record
> +in the reverse mapping b+tree describes an interval of physical blocks mapped
> +to an interval of logical file block offsets; therefore, it makes sense that
> +a range of keys can be used to find to a record.
> +
> +=== xfs_db rmapbt Example
> +
> +This example shows a reverse-mapping B+tree from a freshly populated root
> +filesystem:
> +
> +----
> +xfs_db> agf 0
> +xfs_db> addr rmaproot
> +xfs_db> p
> +magic = 0x524d4233
> +level = 1
> +numrecs = 43
> +leftsib = null
> +rightsib = null
> +bno = 56
> +lsn = 0x3000004c8
> +uuid = 1977221d-8345-464e-b1f4-aa2ea36895f4
> +owner = 0
> +crc = 0x7cf8be6f (correct)
> +keys[1-43] = [startblock,owner,offset]
> +keys[1-43] = [startblock,owner,offset,attrfork,bmbtblock,startblock_hi,owner_hi,
> + offset_hi,attrfork_hi,bmbtblock_hi]
> + 1:[0,-3,0,0,0,351,4418,66,0,0]
> + 2:[417,285,0,0,0,827,4419,2,0,0]
> + 3:[829,499,0,0,0,2352,573,55,0,0]
> + 4:[1292,710,0,0,0,32168,262923,47,0,0]
> + 5:[32215,-5,0,0,0,34655,2365,3411,0,0]
> + 6:[34083,1161,0,0,0,34895,265220,1,0,1]
> + 7:[34896,256191,0,0,0,36522,-9,0,0,0]
> + ...
> + 41:[50998,326734,0,0,0,51430,-5,0,0,0]
> + 42:[51431,327010,0,0,0,51600,325722,11,0,0]
> + 43:[51611,327112,0,0,0,94063,23522,28375272,0,0]
> +ptrs[1-43] = 1:5 2:6 3:8 4:9 5:10 6:11 7:418 ... 41:46377 42:48784 43:49522
> +----
> +
> +We arbitrarily pick pointer 17 to traverse downwards:
> +
> +----
> +xfs_db> addr ptrs[17]
> +xfs_db> p
> +magic = 0x524d4233
> +level = 0
> +numrecs = 168
> +leftsib = 36284
> +rightsib = 37617
> +bno = 294760
> +lsn = 0x200002761
> +uuid = 1977221d-8345-464e-b1f4-aa2ea36895f4
> +owner = 0
> +crc = 0x2dad3fbe (correct)
> +recs[1-168] = [startblock,blockcount,owner,offset,extentflag,attrfork,bmbtblock]
> + 1:[40326,1,259615,0,0,0,0] 2:[40327,1,-5,0,0,0,0]
> + 3:[40328,2,259618,0,0,0,0] 4:[40330,1,259619,0,0,0,0]
> + ...
> + 127:[40540,1,324266,0,0,0,0] 128:[40541,1,324266,8388608,0,0,0]
> + 129:[40542,2,324266,1,0,0,0] 130:[40544,32,-7,0,0,0,0]
> +----
> +
> +Several interesting things pop out here. The first record shows that inode
> +259,615 has mapped AG block 40,326 at offset 0. We confirm this by looking at
> +the block map for that inode:
> +
> +----
> +xfs_db> inode 259615
> +xfs_db> bmap
> +data offset 0 startblock 40326 (0/40326) count 1 flag 0
> +----
> +
> +Next, notice records 127 and 128, which describe neighboring AG blocks that are
> +mapped to non-contiguous logical blocks in inode 324,266. Given the logical
> +offset of 8,388,608 we surmise that this is a leaf directory, but let us
> +confirm:
> +
> +----
> +xfs_db> inode 324266
> +xfs_db> p core.mode
> +core.mode = 040755
> +xfs_db> bmap
> +data offset 0 startblock 40540 (0/40540) count 1 flag 0
> +data offset 1 startblock 40542 (0/40542) count 2 flag 0
> +data offset 3 startblock 40576 (0/40576) count 1 flag 0
> +data offset 8388608 startblock 40541 (0/40541) count 1 flag 0
> +xfs_db> p core.mode
> +core.mode = 0100644
> +xfs_db> dblock 0
> +xfs_db> p dhdr.hdr.magic
> +dhdr.hdr.magic = 0x58444433
> +xfs_db> dblock 8388608
> +xfs_db> p lhdr.info.hdr.magic
> +lhdr.info.hdr.magic = 0x3df1
> +----
> +
> +Indeed, this inode 324,266 appears to be a leaf directory, as it has regular
> +directory data blocks at low offsets, and a single leaf block.
> +
> +Notice further the two reverse-mapping records with negative owners. An owner
> +of -7 corresponds to +XFS_RMAP_OWN_INODES+, which is an inode chunk, and an
> +owner code of -5 corresponds to +XFS_RMAP_OWN_AG+, which covers free space
> +B+trees and free space. Let's see if block 40,544 is part of an inode chunk:
> +
> +----
> +xfs_db> blockget
> +xfs_db> fsblock 40544
> +xfs_db> blockuse
> +block 40544 (0/40544) type inode
> +xfs_db> stack
> +1:
> + byte offset 166068224, length 4096
> + buffer block 324352 (fsbno 40544), 8 bbs
> + inode 324266, dir inode 324266, type data
> +xfs_db> type inode
> +xfs_db> p
> +core.magic = 0x494e
> +----
> +
> +Our suspicions are confirmed. Let's also see if 40,327 is part of a free space
> +tree:
> +
> +----
> +xfs_db> fsblock 40327
> +xfs_db> blockuse
> +block 40327 (0/40327) type btrmap
> +xfs_db> type rmapbt
> +xfs_db> p
> +magic = 0x524d4233
> +----
> +
> +As you can see, the reverse block-mapping B+tree is an important secondary
> +metadata structure, which can be used to reconstruct damaged primary metadata.
> +Now let's look at an extend rmap btree:
> +
> +----
> +xfs_db> agf 0
> +xfs_db> addr rmaproot
> +xfs_db> p
> +magic = 0x34524d42
> +level = 1
> +numrecs = 5
> +leftsib = null
> +rightsib = null
> +bno = 6368
> +lsn = 0x100000d1b
> +uuid = 400f0928-6b88-4c37-af1e-cef1f8911f3f
> +owner = 0
> +crc = 0x8d4ace05 (correct)
> +keys[1-5] = [startblock,owner,offset,attrfork,bmbtblock,startblock_hi,owner_hi,offset_hi,attrfork_hi,bmbtblock_hi]
> +1:[0,-3,0,0,0,705,132,681,0,0]
> +2:[24,5761,0,0,0,548,5761,524,0,0]
> +3:[24,5929,0,0,0,380,5929,356,0,0]
> +4:[24,6097,0,0,0,212,6097,188,0,0]
> +5:[24,6277,0,0,0,807,-7,0,0,0]
> +ptrs[1-5] = 1:5 2:771 3:9 4:10 5:11
> +----
> +
> +The second pointer stores both the low key [24,5761,0,0,0] and the high key
> +[548,5761,524,0,0], which means that we can expect block 771 to contain records
> +starting at physical block 24, inode 5761, offset zero; and that one of the
> +records can be used to find a reverse mapping for physical block 548, inode
> +5761, and offset 524:
> +
> +----
> +xfs_db> addr ptrs[2]
> +xfs_db> p
> +magic = 0x34524d42
> +level = 0
> +numrecs = 168
> +leftsib = 5
> +rightsib = 9
> +bno = 6168
> +lsn = 0x100000d1b
> +uuid = 400f0928-6b88-4c37-af1e-cef1f8911f3f
> +owner = 0
> +crc = 0xd58eff0e (correct)
> +recs[1-168] = [startblock,blockcount,owner,offset,extentflag,attrfork,bmbtblock]
> +1:[24,525,5761,0,0,0,0]
> +2:[24,524,5762,0,0,0,0]
> +3:[24,523,5763,0,0,0,0]
> +...
> +166:[24,360,5926,0,0,0,0]
> +167:[24,359,5927,0,0,0,0]
> +168:[24,358,5928,0,0,0,0]
> +----
> +
> +Observe that the first record in the block starts at physical block 24, inode
> +5761, offset zero, just as we expected. Note that this first record is also
> +indexed by the highest key as provided in the node block; physical block 548,
> +inode 5761, offset 524 is the very last block mapped by this record. Furthermore,
> +note that record 168, despite being the last record in this block, has a lower
> +maximum key (physical block 382, inode 5928, offset 23) than the first record.
> diff --git a/design/XFS_Filesystem_Structure/xfs_filesystem_structure.asciidoc b/design/XFS_Filesystem_Structure/xfs_filesystem_structure.asciidoc
> index 62502b3..1b8658d 100644
> --- a/design/XFS_Filesystem_Structure/xfs_filesystem_structure.asciidoc
> +++ b/design/XFS_Filesystem_Structure/xfs_filesystem_structure.asciidoc
> @@ -48,6 +48,8 @@ include::overview.asciidoc[]
>
> include::metadata_integrity.asciidoc[]
>
> +include::reconstruction.asciidoc[]
> +
> include::common_types.asciidoc[]
>
> include::magic.asciidoc[]
> @@ -66,6 +68,8 @@ include::btrees.asciidoc[]
>
> include::allocation_groups.asciidoc[]
>
> +include::rmapbt.asciidoc[]
> +
> include::journaling_log.asciidoc[]
>
> include::internal_inodes.asciidoc[]
>
> _______________________________________________
> xfs mailing list
> xfs@xxxxxxxxxxx
> http://oss.sgi.com/mailman/listinfo/xfs
--
Carlos
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