Notice

This document is for a development version of Ceph.

Advanced: Metadata repair tools

Warning

If you do not have expert knowledge of CephFS internals, you will need to seek assistance before using any of these tools.

The tools mentioned here can easily cause damage as well as fixing it.

It is essential to understand exactly what has gone wrong with your file system before attempting to repair it.

If you do not have access to professional support for your cluster, consult the ceph-users mailing list or the #ceph IRC/Slack channel.

Journal export

Before attempting any dangerous operation, make a copy of the journal by running the following command:

cephfs-journal-tool journal export backup.bin

Dentry recovery from journal

If a journal is damaged or for any reason an MDS is incapable of replaying it, attempt to recover file metadata by running the following command:

cephfs-journal-tool event recover_dentries summary

By default, this command acts on MDS rank 0. Pass the option --rank=<n> to the cephfs-journal-tool command to operate on other ranks.

This command writes all inodes and dentries recoverable from the journal into the backing store, but only if these inodes and dentries are higher-versioned than the existing contents of the backing store. Any regions of the journal that are missing or damaged will be skipped.

In addition to writing out dentries and inodes, this command updates the InoTables of each in MDS rank, to indicate that any written inodes’ numbers are now in use. In simple cases, this will result in an entirely valid backing store state.

Warning

The resulting state of the backing store is not guaranteed to be self-consistent, and an online MDS scrub will be required afterwards. The journal contents will not be modified by this command. Truncate the journal separately after recovering what you can.

Journal truncation

Use a command of the following form to truncate any journal that is corrupt or that an MDS cannot replay:

cephfs-journal-tool [--rank=<fs_name>:{mds-rank|all}] journal reset --yes-i-really-really-mean-it

Specify the filesystem and the MDS rank using the --rank option when the file system has or had multiple active MDS daemons.

Warning

Resetting the journal will cause metadata to be lost unless you have extracted it by other means such as recover_dentries. Resetting the journal is likely to leave orphaned objects in the data pool. Resetting the journal may result in the re-allocation of already-written inodes, which means that permissions rules could be violated.

MDS table wipes

After the journal has been reset, it may no longer be consistent with respect to the contents of the MDS tables (InoTable, SessionMap, SnapServer).

Use the following command to reset the SessionMap (this will erase all sessions):

cephfs-table-tool all reset session

This command acts on the tables of all MDS ranks that are in. To operate only on a specified rank, replace all in the above command with an MDS rank.

Of all tables, the session table is the table most likely to require a reset. If you know that you need also to reset the other tables, then replace session with snap or inode.

MDS map reset

When the in-RADOS state of the file system (that is, the contents of the metadata pool) has been somewhat recovered, it may be necessary to update the MDS map to reflect the new state of the metadata pool. Use the following command to reset the MDS map to a single MDS:

ceph fs reset <fs name> --yes-i-really-mean-it

After this command has been run, any in-RADOS state for MDS ranks other than 0 will be ignored. This means that running this command can result in data loss.

There is a difference between the effects of the fs reset command and the fs remove command. The fs reset command leaves rank 0 in the active state so that the next MDS daemon to claim the rank uses the existing in-RADOS metadata. The fs remove command leaves rank 0 in the creating state, which means that existing root inodes on disk will be overwritten. Running the fs remove command will orphan any existing files.

Recovery from missing metadata objects

Depending on which objects are missing or corrupt, you may need to run additional commands to regenerate default versions of the objects.

# Session table
cephfs-table-tool 0 reset session
# SnapServer
cephfs-table-tool 0 reset snap
# InoTable
cephfs-table-tool 0 reset inode
# Journal
cephfs-journal-tool --rank=<fs_name>:0 journal reset --yes-i-really-really-mean-it
# Root inodes ("/" and MDS directory)
cephfs-data-scan init

Finally, you can regenerate metadata objects for missing files and directories based on the contents of a data pool. This is a three-phase process:

Scanning all objects to calculate size and mtime metadata for inodes.
Scanning the first object from every file to collect this metadata and inject it into the metadata pool.
Checking inode linkages and fixing found errors.

cephfs-data-scan scan_extents [<data pool> [<extra data pool> ...]]
cephfs-data-scan scan_inodes [<data pool>]
cephfs-data-scan scan_links

scan_extents and scan_inodes commands may take a very long time if the data pool contains many files or very large files.

To accelerate the process of running scan_extents or scan_inodes, run multiple instances of the tool:

Decide on a number of workers, and pass each worker a number within the range 0-(worker_m - 1) (that is, ‘zero to “worker_m” minus 1’).

The example below shows how to run four workers simultaneously:

# Worker 0
cephfs-data-scan scan_extents --worker_n 0 --worker_m 4
# Worker 1
cephfs-data-scan scan_extents --worker_n 1 --worker_m 4
# Worker 2
cephfs-data-scan scan_extents --worker_n 2 --worker_m 4
# Worker 3
cephfs-data-scan scan_extents --worker_n 3 --worker_m 4

# Worker 0
cephfs-data-scan scan_inodes --worker_n 0 --worker_m 4
# Worker 1
cephfs-data-scan scan_inodes --worker_n 1 --worker_m 4
# Worker 2
cephfs-data-scan scan_inodes --worker_n 2 --worker_m 4
# Worker 3
cephfs-data-scan scan_inodes --worker_n 3 --worker_m 4

It is important to ensure that all workers have completed the scan_extents phase before any worker enters the scan_inodes phase.

After completing the metadata recovery process, you may want to run a cleanup operation to delete ancillary data generated during recovery. Use a command of the following form to run a cleanup operation:

cephfs-data-scan cleanup [<data pool>]

Note

The data pool parameters for scan_extents, scan_inodes and cleanup commands are optional, and usually the tool will be able to detect the pools automatically. Still, you may override this. The scan_extents command requires that all data pools be specified, but the scan_inodes and cleanup commands require only that you specify the main data pool.

Using an alternate metadata pool for recovery

Warning

This procedure has not been extensively tested. It should be undertaken only with great care.

If an existing CephFS file system is damaged and inoperative, then it is possible to create a fresh metadata pool and to attempt the reconstruction the of the damaged and inoperative file system’s metadata into the new pool, while leaving the old metadata in place. This could be used to make a safer attempt at recovery since the existing metadata pool would not be modified.

Caution

During this process, multiple metadata pools will contain data referring to the same data pool. Extreme caution must be exercised to avoid changing the contents of the data pool while this is the case. After recovery is complete, archive or delete the damaged metadata pool.

Take down the existing file system in order to prevent any further modification of the data pool. Unmount all clients. When all clients have been unmounted, use the following command to mark the file system failed:
```
ceph fs fail <fs_name>
```
Note

<fs_name> here and below refers to the original, damaged file system.
Create a recovery file system. This recovery file system will be used to recover the data in the damaged pool. First, the filesystem will have a data pool deployed for it. Then you will attacha new metadata pool to the new data pool. Then you will set the new metadata pool to be backed by the old data pool.
```
ceph osd pool create cephfs_recovery_meta
ceph fs new cephfs_recovery cephfs_recovery_meta <data_pool> --recover --allow-dangerous-metadata-overlay
```
Note

You may rename the recovery metadata pool and file system at a future time. The --recover flag prevents any MDS daemon from joining the new file system.

Create the intial metadata for the file system:

cephfs-table-tool cephfs_recovery:0 reset session

cephfs-table-tool cephfs_recovery:0 reset snap

cephfs-table-tool cephfs_recovery:0 reset inode

cephfs-journal-tool --rank cephfs_recovery:0 journal reset --force --yes-i-really-really-mean-it

Use the following commands to rebuild the metadata pool from the data pool:

cephfs-data-scan init --force-init --filesystem cephfs_recovery --alternate-pool cephfs_recovery_meta

cephfs-data-scan scan_extents --alternate-pool cephfs_recovery_meta --filesystem <fs_name>

cephfs-data-scan scan_inodes --alternate-pool cephfs_recovery_meta --filesystem <fs_name> --force-corrupt

cephfs-data-scan scan_links --filesystem cephfs_recovery

Note

Each of the scan procedures above scans through the entire data pool. This may take a long time. See the previous section on how to distribute this task among workers.

If the damaged file system contains dirty journal data, it may be recovered next with a command of the following form:

cephfs-journal-tool --rank=<fs_name>:0 event recover_dentries list --alternate-pool cephfs_recovery_meta

After recovery, some recovered directories will have incorrect statistics. Ensure that the parameters mds_verify_scatter and mds_debug_scatterstat are set to false (the default) to prevent the MDS from checking the statistics:
```
ceph config rm mds mds_verify_scatter
```
```
ceph config rm mds mds_debug_scatterstat
```
Note

Verify that the config has not been set globally or with a local ceph.conf file.
Allow an MDS daemon to join the recovery file system:
```
ceph fs set cephfs_recovery joinable true
```
Run a forward scrub to repair recursive statistics. Ensure that you have an MDS daemon running and issue the following command:
```
ceph tell mds.cephfs_recovery:0 scrub start / recursive,repair,force
```
Note

The Symbolic link recovery is supported starting in the Quincy release.

Symbolic links were recovered as empty regular files before.

It is recommended that you migrate any data from the recovery file system as soon as possible. Do not restore the old file system while the recovery file system is operational.

Note

If the data pool is also corrupt, some files may not be restored because the backtrace information associated with them is lost. If any data objects are missing (due to issues like lost placement groups on the data pool), the recovered files will contain holes in place of the missing data.

Brought to you by the Ceph Foundation

The Ceph Documentation is a community resource funded and hosted by the non-profit Ceph Foundation. If you would like to support this and our other efforts, please consider joining now.