xfs_repair is a xfs tool that repairs corrupt or damaged XFS  filesystems. To be noted is that the  filesystem  to be repaired must be unmounted, otherwise, the resulting filesystem may be inconsistent or corrupt. Thus, the device name should be name of the disk partition, volume, or a block device that containing the file system. Here is the syntax:

xfs_repair  [  -dfLnPv  ]  [ -m maxmem ] [ -c subopt=value ] [ -o sub-
       opt[=value] ] [ -t interval ] [ -l logdev ] [ -r rtdev ] device
       xfs_repair -V

xfs_repair read only mode with '-n' option

-n     No modify mode. Specifies that xfs_repair should not modify the
        filesystem  but  should  only  scan the filesystem and indicate
        what repairs would have been made

# xfs_repair -n /dev/disk/by-vdev/c5t7
Phase 1 - find and verify superblock...
Phase 2 - using internal log
        - scan filesystem freespace and inode maps...
        - found root inode chunk
Phase 3 - for each AG...
        - scan (but don't clear) agi unlinked lists...
        - process known inodes and perform inode discovery...
        - agno = 0
        - agno = 1
        - agno = 2
        - agno = 3
        - process newly discovered inodes...
Phase 4 - check for duplicate blocks...
        - setting up duplicate extent list...
        - check for inodes claiming duplicate blocks...
        - agno = 0
        - agno = 1
        - agno = 2
        - agno = 3
No modify flag set, skipping phase 5
Phase 6 - check inode connectivity...
        - traversing filesystem ...
        - traversal finished ...
        - moving disconnected inodes to lost+found ...
Phase 7 - verify link counts...
No modify flag set, skipping filesystem flush and exiting.

Note:

The no-modify mode (-n option) is not completely  accurate.  It  does  not  catch  inconsistencies  in the freespace and inode maps, particularly lost blocks or subtly corrupted maps (trees).

The no-modify mode can generate repeated warnings about the same problems because it cannot fix the problems as they are encountered.

xfs_repair verbose output, '-v' option

-v option gives you xfs_repair verbose outpt, inaddition to the regular repair message +extra verbosity, it has a summary for each repair phase.
 XFS_REPAIR Summary    Wed Dec 21 22:49:43 2016

Phase        Start        End        Duration
Phase 1:    12/21 22:49:39    12/21 22:49:39    
Phase 2:    12/21 22:49:39    12/21 22:49:43    4 seconds
Phase 3:    12/21 22:49:43    12/21 22:49:43    
Phase 4:    12/21 22:49:43    12/21 22:49:43    
Phase 5:    12/21 22:49:43    12/21 22:49:43    
Phase 6:    12/21 22:49:43    12/21 22:49:43    
Phase 7:    12/21 22:49:43    12/21 22:49:43    

xfs_repair default repair

Without any option, xfs_repair will check and repair errors that it encounters. Here is the list of Inconsistency Checks Performed by xfs_repair:

1. Inode  and inode blockmap (addressing) checks:

bad magic number  in inode, bad magic numbers in inode blockmap  blocks,  extents out  of  order,  incorrect  number of records in inode blockmap blocks, blocks claimed that are not in a legal data area of the filesystem, blocks that are claimed by more than one inode.


2. Inode  allocation  map  checks:

bad  magic number in inode map blocks, inode state as indicated by map (free or in-use) inconsistent with state indicated by the inode, inodes referenced by the filesystem that do not appear in the inode allocation  map, inode  allocation  map referencing blocks that do not appear to contain inodes.

3. Size checks:

number of blocks  claimed  by  inode  inconsistent with  inode  size, directory size not block aligned, inode size not consistent with inode format.

4. Directory checks:

bad magic numbers in directory blocks, incorrect  number  of  entries  in  a directory block, bad freespace information in a directory leaf block,  entry  pointing  to  an unallocated  (free) or out of range inode, overlapping entries, missing or incorrect dot and dotdot  entries,  entries  out  of hashvalue  order, incorrect internal directory pointers, directory type not consistent with inode format and size.

5. Pathname checks:

files or directories not referenced by a pathname starting from the filesystem root, illegal pathname components.


6. Link count checks:

link counts that do not agree with the  number of directory references to the inode.

7. Freemap  checks: 

blocks  claimed  free by the freemap but also claimed by an inode, blocks unclaimed  by  any  inode  but  not appearing in the freemap.


8. Super  Block  checks:

total free block and/or free i-node count incorrect, filesystem geometry inconsistent, secondary and primary superblocks contradictory.

Orphaned  files  and  directories (allocated, in-use but unreferenced) are reconnected by placing them in the lost+found directory.  The name  assigned is the inode number.

 

xfs_repair dangeously with '-d' option

Repair dangerously. Allow xfs_repair to repair an XFS  filesystem  mounted  read  only.  This  is  typically  done  on a root filesystem from single user mode,  immediately followed by a reboot.

# xfs_repair -d /

xfs_repair '-L' option, force zero log

Force  Log Zeroing.  Forces xfs_repair to zero the log even if it is dirty (contains metadata changes). When using this option the filesystem will likely appear  to  be  corrupt, and can cause the loss of user files and/or data.

# xfs_repair -L  /dev/disk/by-vdev/c5t7

xfs_pair '-l' option, external log

<p">Specifies  the  device  special file where the filesystem's external log resides. Only for those filesystems which use an external log.

xfs_repair a filesystem image with '-f' option

<p">Specifies that the filesystem image to be processed is stored in a regular file at  device . This might happen if an image copy of a filesystem has been copied or written into an ordinary file.  This option implies that any  external  log or realtime section is also in an ordinary file.

A filesystem image could be created by

# mkfs.xfs -d file=<file image>

or xfs_copy, both source and target can be device name or file system image

# xfs_copy <source> <target>

xfs_repair '-P' option, disable prefetching

Disable prefetching of inode and directory blocks. Use this option if you find  xfs_repair gets stuck and stops proceeding. Note: Interrupting a stuck xfs_repair is safe.

xfs_repair reporting interval with '-t' option

Modify reporting interval, specified in seconds. During long runs xfs_repair  outputs  its progress every 15 minutes. Reporting is only activated when ag_stride is enabled.

xfs_repair max memory usage with '-m' option

Specifies  the  approximate maximum amount of memory, in megabytes, to use for xfs_repair. xfs_repair has its own internal block cache which will scale out up to the lesser  of  the process's  virtual  address  limit or about 75% of the system's physical RAM. This option overrides these limits.

xfs_repair '-o', override the

-o subopt[=value]
              Override what the program might conclude about the filesystem if left to its own devices.

              The suboptions supported are:

                 bhash=bhashsize
                        overrides  the  default buffer cache hash size. The total number of buffer cache
                        entries are limited to 8 times this amount. The default size is set  to  use  up
                        the remainder of 75% of the system's physical RAM size.

                 ag_stride=ags_per_concat_unit
                        This  creates  additional  processing  threads to parallel process AGs that span
                        multiple concat units. This can significantly  reduce  repair  times  on  concat
                        based filesystems.

                 force_geometry
                        Check the filesystem even if geometry information could not be validated.  Geom-
                        etry information can not be validated if only a single allocation  group  exists
                        and thus we do not have a backup superblock available, or if there are two allo-
                        cation groups and the two superblocks do not agree on the  filesystem  geometry.
                        Only  use  this  option if you validated the geometry yourself and know what you
                        are doing.  If In doubt run in no modify mode first.

xfs_repair change filesystem parameters with '-c'

  -c subopt=value change filesystem parameters. Refer to xfs_admin(8) for information on changing filesystem parameters.

xfs_repair EXIT STATUS

xfs_repair -n (no modify node) will return a status of 1 if filesystem corruption  was  detected  and  0  if  no  filesystem  corruption  was detected. 

xfs_repair run without the -n option will always  return  a status code of 0.

xfs_repair on Disk Errors

xfs_repair  aborts  on  most  disk I/O errors. Therefore, if you are trying to repair a filesystem that was damaged due to a disk drive failure, steps should be taken to ensure that all blocks in the filesystem are readable and writable before attempting to use xfs_repair to repair the filesystem. A possible method is using dd(8) to copy the data onto a good disk.

xfs_repair lost+foud

The directory lost+found does not have to already exist in the filesystem being repaired.  If the directory does not exist, it is automatically  created  if  required.  

If  it  already exists,  it  will be checked for consistency and if valid will be used for additional orphaned files. Invalid lost+found directories are removed and recreated. Existing files in a valid lost+found are not removed or renamed.

xfs_repair Corrupted Superblocks

XFS has both primary and secondary superblocks.  xfs_repair uses information in the primary superblock to automatically find and validate the primary superblock  against  the  secondary superblocks  before  proceeding.   Should  the primary be too corrupted to be useful in locating the secondary superblocks, the program scans the filesystem until it finds and validates some secondary superblocks.  At that point, it generates a primary superblock.

xfs_repair Quotas

If quotas are in use, it is possible that xfs_repair will clear some or all of the filesystem quota information.  If so, the program issues a warning just before it terminates.  If  all quota information is lost, quotas are disabled and the program issues a warning to that effect.

More info about xfs_repair diagnosis

xfs_repair issues informative messages as it proceeds indicating what it has found that is abnormal or any corrective action that it has taken.  Most  of  the  messages  are  completely understandable  only  to those who are knowledgeable about the structure of the filesystem.  Some of the more common messages are explained here.  Note that the language of the messages is slightly different if xfs_repair is run in no-modify mode because the program is not changing anything on disk.  No-modify mode indicates what it would do to repair the filesystem if  run without the no-modify flag.

disconnected inode ino, moving to lost+found

An  inode  numbered  ino  was  not  connected to the filesystem directory tree and was reconnected to the lost+found directory. The inode is assigned the name of its inode number(ino).  If a lost+found directory does not exist, it is automatically created.

disconnected dir inode ino, moving to lost+found
As above only the inode is a directory inode.  If a directory inode is attached to lost+found, all of its children (if any) stay attached to the directory and therefore get automatically reconnected when the directory is reconnected.

imap claims in-use inode ino is free, correcting imap

The  inode  allocation  map  thinks that inode ino is free whereas examination of the inode indicates that the inode may be in use (although it may be disconnected).  The program updates the inode allocation map.

imap claims free inode ino is in use, correcting imap

The inode allocation map thinks that inode ino is in use whereas examination of the inode indicates that the inode is not in use and therefore is free.  The program  updates  the inode allocation map.

resetting inode ino nlinks from x to y

The  program detected a mismatch between the number of valid directory entries referencing inode ino and the number of references recorded in the inode and corrected the the number in the inode.

fork-type fork in ino ino claims used block bno

Inode ino claims a block bno that is used (claimed) by either another inode or the filesystem itself for metadata storage. The fork-type is either data or attr indicating whether the problem lies in the portion of the inode that tracks regular data or the portion of the inode that stores XFS attributes.  If the inode is a real-time (rt) inode, the message says so.  Any inode that claims blocks used by the filesystem is deleted.  If two or more inodes claim the same block, they are both deleted.

fork-type fork in ino ino claims dup extent ...

Inode ino claims a block in an extent known to be claimed more than once.  The offset in the inode, start and length of the extent is given.  The message is slightly different if the inode is a real-time (rt) inode and the extent is therefore a real-time (rt) extent.

inode ino - bad extent ...

An  extent  record  in  the  blockmap  of  inode ino claims blocks that are out of the legal range of the filesystem.  The message supplies the start, end, and file offset of the extent.  The message is slightly different if the extent is a real-time (rt) extent.

bad fork-type fork in inode ino

There was something structurally wrong or inconsistent with the data structures that map offsets to filesystem blocks.

cleared inode ino

There was something wrong with the inode that was uncorrectable so the program freed the inode.  This usually happens because the inode claims blocks that are used  by  something else or the inode itself is badly corrupted. Typically, this message is preceded by one or more messages indicating why the inode needed to be cleared.

bad attribute fork in inode ino, clearing attr fork

There  was  something  wrong  with  the  portion  of  the inode that stores XFS attributes (the attribute fork) so the program reset the attribute fork.  As a result of this, all attributes on that inode are lost.

correcting nextents for inode ino, was x - counted y

The program found that the number of extents used to store the data in the inode is wrong and corrected the number.  The message refers to nextents if the count is wrong  on  the number of extents used to store attribute information.

entry name in dir dir_ino not consistent with .. value (xxxx) in dir ino ino, junking entry name in directory inode dir_ino

The  entry  name  in  directory  inode  dir_ino references a directory inode ino.  However, the .. entry in directory ino does not point back to directory dir_ino, so the program deletes the entry name in directory inode dir_ino.  If the directory inode ino winds up becoming a disconnected inode as a result of this, it is moved to lost+found later.

entry name in dir dir_ino references already connected dir ino ino, junking entry name in directory inode dir_ino
The entry name in directory inode dir_ino points to a directory inode ino that is known to be a child of another directory.  Therefore, the entry is invalid and is deleted.  This message refers to an entry in a small directory.  If this were a large directory, the last phrase would read "will clear entry".

entry references free inode ino in directory dir_ino, will clear entry

An  entry  in  directory inode dir_ino references an inode ino that is known to be free. The entry is therefore invalid and is deleted.  This message refers to a large directory. If the directory were small, the message would read "junking entry ...".

xfs_repair bugs and concerns:

The filesystem to be checked and repaired must have been unmounted cleanly using normal system administration procedures (the umount(8) command or system shutdown), not as a result of a crash or system reset.  If the filesystem has not been unmounted cleanly, mount it and unmount it cleanly before running xfs_repair.

xfs_repair does not do a thorough job on XFS extended attributes.  The structure of the attribute fork will be consistent, but only the contents of attribute forks that will fit into an inode are checked.  This limitation will be fixed in the future.