borg check

borg [common options] check [options] [REPOSITORY_OR_ARCHIVE]
positional arguments
  REPOSITORY_OR_ARCHIVE repository or archive to check consistency of
optional arguments
  --repository-only only perform repository checks
  --archives-only only perform archives checks
  --verify-data perform cryptographic archive data integrity verification (conflicts with --repository-only)
  --repair attempt to repair any inconsistencies found
  --save-space work slower, but using less space
  --max-duration SECONDS do only a partial repo check for max. SECONDS seconds (Default: unlimited)

Common options

Archive filters — Archive filters can be applied to repository targets.
  -P PREFIX, --prefix PREFIX only consider archive names starting with this prefix.
  -a GLOB, --glob-archives GLOB only consider archive names matching the glob. sh: rules apply, see “borg help patterns”. --prefix and --glob-archives are mutually exclusive.
  --sort-by KEYS Comma-separated list of sorting keys; valid keys are: timestamp, name, id; default is: timestamp
  --first N consider first N archives after other filters were applied
  --last N consider last N archives after other filters were applied

Description

The check command verifies the consistency of a repository and the corresponding archives.

First, the underlying repository data files are checked:

  • For all segments the segment magic (header) is checked
  • For all objects stored in the segments, all metadata (e.g. crc and size) and all data is read. The read data is checked by size and CRC. Bit rot and other types of accidental damage can be detected this way.
  • If we are in repair mode and a integrity error is detected for a segment, we try to recover as many objects from the segment as possible.
  • In repair mode, it makes sure that the index is consistent with the data stored in the segments.
  • If you use a remote repo server via ssh:, the repo check is executed on the repo server without causing significant network traffic.
  • The repository check can be skipped using the --archives-only option.

Second, the consistency and correctness of the archive metadata is verified:

  • Is the repo manifest present? If not, it is rebuilt from archive metadata chunks (this requires reading and decrypting of all metadata and data).
  • Check if archive metadata chunk is present. if not, remove archive from manifest.
  • For all files (items) in the archive, for all chunks referenced by these files, check if chunk is present. If a chunk is not present and we are in repair mode, replace it with a same-size replacement chunk of zeros. If a previously lost chunk reappears (e.g. via a later backup) and we are in repair mode, the all-zero replacement chunk will be replaced by the correct chunk. This requires reading of archive and file metadata, but not data.
  • If we are in repair mode and we checked all the archives: delete orphaned chunks from the repo.
  • if you use a remote repo server via ssh:, the archive check is executed on the client machine (because if encryption is enabled, the checks will require decryption and this is always done client-side, because key access will be required).
  • The archive checks can be time consuming, they can be skipped using the --repository-only option.

The --verify-data option will perform a full integrity verification (as opposed to checking the CRC32 of the segment) of data, which means reading the data from the repository, decrypting and decompressing it. This is a cryptographic verification, which will detect (accidental) corruption. For encrypted repositories it is tamper-resistant as well, unless the attacker has access to the keys.

It is also very slow.