# borg check¶

borg [common options] check [options]

 options --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 --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. -a PATTERN, --match-archives PATTERN only consider archive names matching the pattern. see “borg help match-archives”. --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 --oldest TIMESPAN consider archives between the oldest archive’s timestamp and (oldest + TIMESPAN), e.g. 7d or 12m. --newest TIMESPAN consider archives between the newest archive’s timestamp and (newest - TIMESPAN), e.g. 7d or 12m. --older TIMESPAN consider archives older than (now - TIMESPAN), e.g. 7d oder 12m. --newer TIMESPAN consider archives newer than (now - TIMESPAN), e.g. 7d or 12m.

## Description¶

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

check --repair is a potentially dangerous function and might lead to data loss (for kinds of corruption it is not capable of dealing with). BE VERY CAREFUL!

Pursuant to the previous warning it is also highly recommended to test the reliability of the hardware running this software with stress testing software such as memory testers. Unreliable hardware can also lead to data loss especially when this command is run in repair mode.

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.
• In repair mode, if an integrity error is detected in a segment, try to recover as many objects from the segment as possible.
• In repair mode, make sure that the index is consistent with the data stored in the segments.
• If checking a remote repo via ssh:, the repo check is executed on the server without causing significant network traffic.
• The repository check can be skipped using the --archives-only option.
• A repository check can be time consuming. Partial checks are possible with the --max-duration 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. In repair mode, if a chunk is not present, replace it with a same-size replacement chunk of zeroes. If a previously lost chunk reappears (e.g. via a later backup), 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.
• In repair mode, when all the archives were checked, orphaned chunks are deleted from the repo. One cause of orphaned chunks are input file related errors (like read errors) in the archive creation process.
• In verify-data mode, a complete cryptographic verification of the archive data integrity is performed. This conflicts with --repository-only as this mode only makes sense if the archive checks are enabled. The full details of this mode are documented below.
• If checking a remote repo via ssh:, the archive check is executed on the client machine because it requires decryption, and this is always done client-side as key access is needed.
• The archive checks can be time consuming; they can be skipped using the --repository-only option.

The --max-duration option can be used to split a long-running repository check into multiple partial checks. After the given number of seconds the check is interrupted. The next partial check will continue where the previous one stopped, until the complete repository has been checked. Example: Assuming a complete check took 7 hours, then running a daily check with --max-duration=3600 (1 hour) resulted in one completed check per week.

Attention: A partial --repository-only check can only do way less checking than a full --repository-only check: only the non-cryptographic checksum checks on segment file entries are done, while a full --repository-only check would also do a repo index check. A partial check cannot be combined with the --repair option. Partial checks may therefore be useful only with very large repositories where a full check would take too long. Doing a full repository check aborts a partial check; the next partial check will restart from the beginning.

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.