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207 lines
6.8 KiB
ReStructuredText
207 lines
6.8 KiB
ReStructuredText
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Disaster recovery
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=================
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.. danger::
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The notes in this section are aimed at experts, making a best effort
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to recovery what they can from damaged filesystems. These steps
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have the potential to make things worse as well as better. If you
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are unsure, do not proceed.
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Journal export
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--------------
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Before attempting dangerous operations, make a copy of the journal like so:
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::
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cephfs-journal-tool journal export backup.bin
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Note that this command may not always work if the journal is badly corrupted,
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in which case a RADOS-level copy should be made (http://tracker.ceph.com/issues/9902).
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Dentry recovery from journal
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----------------------------
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If a journal is damaged or for any reason an MDS is incapable of replaying it,
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attempt to recover what file metadata we can like so:
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::
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cephfs-journal-tool event recover_dentries summary
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This command by default acts on MDS rank 0, pass --rank=<n> to operate on other ranks.
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This command will write any inodes/dentries recoverable from the journal
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into the backing store, if these inodes/dentries are higher-versioned
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than the previous contents of the backing store. If any regions of the journal
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are missing/damaged, they will be skipped.
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Note that in addition to writing out dentries and inodes, this command will update
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the InoTables of each 'in' MDS rank, to indicate that any written inodes' numbers
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are now in use. In simple cases, this will result in an entirely valid backing
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store state.
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.. warning::
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The resulting state of the backing store is not guaranteed to be self-consistent,
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and an online MDS scrub will be required afterwards. The journal contents
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will not be modified by this command, you should truncate the journal
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separately after recovering what you can.
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Journal truncation
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------------------
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If the journal is corrupt or MDSs cannot replay it for any reason, you can
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truncate it like so:
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::
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cephfs-journal-tool journal reset
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.. warning::
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Resetting the journal *will* lose metadata unless you have extracted
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it by other means such as ``recover_dentries``. It is likely to leave
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some orphaned objects in the data pool. It may result in re-allocation
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of already-written inodes, such that permissions rules could be violated.
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MDS table wipes
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---------------
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After the journal has been reset, it may no longer be consistent with respect
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to the contents of the MDS tables (InoTable, SessionMap, SnapServer).
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To reset the SessionMap (erase all sessions), use:
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::
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cephfs-table-tool all reset session
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This command acts on the tables of all 'in' MDS ranks. Replace 'all' with an MDS
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rank to operate on that rank only.
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The session table is the table most likely to need resetting, but if you know you
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also need to reset the other tables then replace 'session' with 'snap' or 'inode'.
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MDS map reset
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-------------
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Once the in-RADOS state of the filesystem (i.e. contents of the metadata pool)
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is somewhat recovered, it may be necessary to update the MDS map to reflect
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the contents of the metadata pool. Use the following command to reset the MDS
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map to a single MDS:
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::
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ceph fs reset <fs name> --yes-i-really-mean-it
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Once this is run, any in-RADOS state for MDS ranks other than 0 will be ignored:
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as a result it is possible for this to result in data loss.
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One might wonder what the difference is between 'fs reset' and 'fs remove; fs new'. The
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key distinction is that doing a remove/new will leave rank 0 in 'creating' state, such
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that it would overwrite any existing root inode on disk and orphan any existing files. In
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contrast, the 'reset' command will leave rank 0 in 'active' state such that the next MDS
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daemon to claim the rank will go ahead and use the existing in-RADOS metadata.
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Recovery from missing metadata objects
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--------------------------------------
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Depending on what objects are missing or corrupt, you may need to
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run various commands to regenerate default versions of the
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objects.
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::
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# Session table
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cephfs-table-tool 0 reset session
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# SnapServer
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cephfs-table-tool 0 reset snap
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# InoTable
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cephfs-table-tool 0 reset inode
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# Journal
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cephfs-journal-tool --rank=0 journal reset
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# Root inodes ("/" and MDS directory)
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cephfs-data-scan init
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Finally, you can regenerate metadata objects for missing files
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and directories based on the contents of a data pool. This is
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a two-phase process. First, scanning *all* objects to calculate
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size and mtime metadata for inodes. Second, scanning the first
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object from every file to collect this metadata and inject
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it into the metadata pool.
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::
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cephfs-data-scan scan_extents <data pool>
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cephfs-data-scan scan_inodes <data pool>
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This command may take a very long time if there are many
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files or very large files in the data pool. To accelerate
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the process, run multiple instances of the tool. Decide on
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a number of workers, and pass each worker a number within
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the range 0-(N_workers - 1), like so:
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::
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# Worker 0
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cephfs-data-scan scan_extents --worker_n 0 --worker_m 1 <data pool>
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# Worker 1
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cephfs-data-scan scan_extents --worker_n 1 --worker_m 1<data pool> 1 1
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# Worker 0
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cephfs-data-scan scan_inodes --worker_n 0 --worker_m 1 <data pool>
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# Worker 1
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cephfs-data-scan scan_inodes --worker_n 1 --worker_m 1 <data pool>
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It is important to ensure that all workers have completed the
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scan_extents phase before any workers enter the scan_inodes phase.
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After completing the metadata recovery, you may want to run cleanup
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operation to delete ancillary data geneated during recovery.
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cephfs-data-scan cleanup <data pool>
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Finding files affected by lost data PGs
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---------------------------------------
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Losing a data PG may affect many files. Files are split into many objects,
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so identifying which files are affected by loss of particular PGs requires
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a full scan over all object IDs that may exist within the size of a file.
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This type of scan may be useful for identifying which files require
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restoring from a backup.
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.. danger::
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This command does not repair any metadata, so when restoring files in
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this case you must *remove* the damaged file, and replace it in order
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to have a fresh inode. Do not overwrite damaged files in place.
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If you know that objects have been lost from PGs, use the ``pg_files``
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subcommand to scan for files that may have been damaged as a result:
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::
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cephfs-data-scan pg_files <path> <pg id> [<pg id>...]
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For example, if you have lost data from PGs 1.4 and 4.5, and you would like
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to know which files under /home/bob might have been damaged:
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::
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cephfs-data-scan pg_files /home/bob 1.4 4.5
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The output will be a list of paths to potentially damaged files, one
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per line.
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Note that this command acts as a normal CephFS client to find all the
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files in the filesystem and read their layouts, so the MDS must be
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up and running.
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