# 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. (deprecated)
| `-a GLOB`, `--glob-archives GLOB` | only consider archive names matching the glob. sh: rules apply (without actually using the sh: prefix), see “borg help patterns”.
| `--sort-by KEYS` | Comma-separated list of sorting keys; valid keys are: timestamp, archive, 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 its archives. It consists of two major steps:

1. Checking the consistency of the repository itself. This includes checking the segment magic headers, and both the metadata and data of all objects in the segments. The read data is checked by size and CRC. Bit rot and other types of accidental damage can be detected this way. Running the repository check can be split into multiple partial checks using `--max-duration`. When checking a remote repository, please note that the checks run on the server and do not cause significant network traffic.

2. Checking consistency and correctness of the archive metadata and optionally archive data (requires `--verify-data`). This includes ensuring that the repository manifest exists, the archive metadata chunk is present, and that all chunks referencing files (items) in the archive exist. This requires reading archive and file metadata, but not data. To cryptographically verify the file (content) data integrity pass `--verify-data`, but keep in mind that this requires reading all data and is hence very time consuming. When checking archives of a remote repository, archive checks run on the client machine because they require decrypting data and therefore the encryption key.

Both steps can also be run independently. Pass `--repository-only` to run the repository checks only, or pass `--archives-only` to run the archive checks only.

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 full repository has been checked. Assuming a complete check would take 7 hours, then running a daily check with `--max-duration=3600` (1 hour) would result in one full repository check per week. Doing a full repository check aborts any previous partial check; the next partial check will restart from the beginning. With partial repository checks you can run neither archive checks, nor enable repair mode. Consequently, if you want to use `--max-duration` you must also pass `--repository-only`, and must not pass `--archives-only`, nor `--repair`.

Warning: Please note that partial repository checks (i.e. running it with `--max-duration`) can only perform non-cryptographic checksum checks on the segment files. A full repository check (i.e. without `--max-duration`) can also do a repository index check. Enabling partial repository checks excepts archive checks for the same reason. Therefore partial checks may be useful with very large repositories only where a full check would take too long.

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. It is a complete cryptographic verification and hence very time consuming, but will detect any accidental and malicious corruption. Tamper-resistance is only guaranteed for encrypted repositories against attackers without access to the keys. You can not use `--verify-data` with `--repository-only`.

### About repair mode

The check command is a readonly task by default. If any corruption is found, Borg will report the issue and proceed with checking. To actually repair the issues found, pass `--repair`.

Note

`--repair` is a POTENTIALLY DANGEROUS FEATURE and might lead to data loss! This does not just include data that was previously lost anyway, but might include more data 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 Borg with stress testing software. This especially includes storage and memory testers. Unreliable hardware might lead to additional data loss.

It is highly recommended to create a backup of your repository before running in repair mode (i.e. running it with `--repair`).

Repair mode will attempt to fix any corruptions found. Fixing corruptions does not mean recovering lost data: Borg can not magically restore data lost due to e.g. a hardware failure. Repairing a repository means sacrificing some data for the sake of the repository as a whole and the remaining data. Hence it is, by definition, a potentially lossy task.

In practice, repair mode hooks into both the repository and archive checks:

1. When checking the repository’s consistency, repair mode will try to recover as many objects from segments with integrity errors as possible, and ensure that the index is consistent with the data stored in the segments.

2. When checking the consistency and correctness of archives, repair mode might remove whole archives from the manifest if their archive metadata chunk is corrupt or lost. On a chunk level (i.e. the contents of files), repair mode will replace corrupt or lost chunks with a same-size replacement chunk of zeroes. If a previously zeroed chunk reappears, repair mode will restore this lost chunk using the new chunk. Lastly, repair mode will also delete orphaned chunks (e.g. caused by read errors while creating the archive).

Most steps taken by repair mode have a one-time effect on the repository, like removing a lost archive from the repository. However, replacing a corrupt or lost chunk with an all-zero replacement will have an ongoing effect on the repository: When attempting to extract a file referencing an all-zero chunk, the `extract` command will distinctly warn about it. The FUSE filesystem created by the `mount` command will reject reading such a “zero-patched” file unless a special mount option is given.

As mentioned earlier, Borg might be able to “heal” a “zero-patched” file in repair mode, if all its previously lost chunks reappear (e.g. via a later backup). This is achieved by Borg not only keeping track of the all-zero replacement chunks, but also by keeping metadata about the lost chunks. In repair mode Borg will check whether a previously lost chunk reappeared and will replace the all-zero replacement chunk by the reappeared chunk. If all lost chunks of a “zero-patched” file reappear, this effectively “heals” the file. Consequently, if lost chunks were repaired earlier, it is advised to run `--repair` a second time after creating some new backups.