Add a chapter to give an overview of the enhanced metadata integrity fields present on a v5 filesystem. Signed-off-by: Darrick J. Wong <darrick.wong@xxxxxxxxxx> --- .../metadata_integrity.asciidoc | 36 ++++++++++++++++++++ .../xfs_filesystem_structure.asciidoc | 2 + 2 files changed, 38 insertions(+) create mode 100644 design/XFS_Filesystem_Structure/metadata_integrity.asciidoc diff --git a/design/XFS_Filesystem_Structure/metadata_integrity.asciidoc b/design/XFS_Filesystem_Structure/metadata_integrity.asciidoc new file mode 100644 index 0000000..f948d5e --- /dev/null +++ b/design/XFS_Filesystem_Structure/metadata_integrity.asciidoc @@ -0,0 +1,36 @@ += Metadata Integrity + +Prior to version 5, most XFS metadata blocks contained a magic number that +could provide a minimal sanity check that a block read off the disk contained +the same type of data that the code thought it was reading off the disk. +However, this was insufficient -- given a correct type code, it was still +impossible to tell if the block was from a previous filesystem, or happened to +be owned by something else, or had been written to the wrong location on disk. +Furthermore, not all metadata blocks had magic numbers -- remote extended +attributes and extent symbolic links had no protection at all. + +Therefore, the version 5 disk format introduced larger headers for all metadata +types, which enable the filesystem to check information being read from the +disk more rigorously. Metadata integrity fields now include: + +* *Magic* numbers, to classify all types of metadata. This is unchanged from v4. +* A copy of the filesystem *UUID*, to confirm that a given disk block is connected to the superblock. +* The *owner*, to avoid accessing a piece of metadata which belongs to some other part of the filesystem. +* The filesystem *block number*, to detect misplaced writes. +* The *log serial number* of the last write to this block, to avoid replaying obsolete log entries. +* A CRC32c *checksum* of the entire block, to detect minor corruption. + +Metadata integrity coverage has been extended to all metadata blocks in the +filesystem, with the following notes: + +* Inodes can have multiple ``owners'' in the directory tree; therefore the record contains the inode number instead of an owner or a block number. +* Superblocks have no owners. +* The disk quota file has no owner or block numbers. +* Metadata owned by files list the inode number as the owner. +* Per-AG data and B+tree blocks list the AG number as the owner. +* Per-AG header sectors don't list owners or block numbers, since they have fixed locations. +* Remote attribute blocks are not logged and therefore the LSN must be -1. + +This functionality enables XFS to decide that a block contents are so +unexpected that it should stop immediately. Unfortunately checksums do not +allow for automatic correction. Please keep regular backups, as always. diff --git a/design/XFS_Filesystem_Structure/xfs_filesystem_structure.asciidoc b/design/XFS_Filesystem_Structure/xfs_filesystem_structure.asciidoc index 9f4c096..8aacbb7 100644 --- a/design/XFS_Filesystem_Structure/xfs_filesystem_structure.asciidoc +++ b/design/XFS_Filesystem_Structure/xfs_filesystem_structure.asciidoc @@ -46,6 +46,8 @@ log items which are formatted in host order. include::overview.asciidoc[] +include::metadata_integrity.asciidoc[] + include::common_types.asciidoc[] // return titles to normal _______________________________________________ xfs mailing list xfs@xxxxxxxxxxx http://oss.sgi.com/mailman/listinfo/xfs
