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@@ -91,8 +91,9 @@ Here are the available checks from fastest to slowest:
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* `repository`: Checks the consistency of the repository itself.
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* `archives`: Checks all of the archives in the repository.
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- * `extract`: Performs an extraction dry-run of the most recent archive.
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+ * `extract`: Performs an extraction dry-run of the latest archive.
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* `data`: Verifies the data integrity of all archives contents, decrypting and decompressing all data.
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+ * `spot`: Compares file counts and contents between your source files and the latest archive.
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Note that the `data` check is a more thorough version of the `archives` check,
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so enabling the `data` check implicitly enables the `archives` check as well.
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@@ -102,6 +103,80 @@ documentation](https://borgbackup.readthedocs.io/en/stable/usage/check.html)
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for more information.
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+### Spot check
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+
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+The various consistency checks all have trade-offs around speed and
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+thoroughness, but most of them don't even look at your original source
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+files—arguably one important way to ensure your backups contain the files
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+you'll ultimately want to restore in the case of catastrophe (or just an
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+accidentally deleted file). Because if something goes wrong with your source
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+files, most consistency checks will still pass with flying colors and you
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+won't discover there's a problem until you go to restore.
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+
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+<span class="minilink minilink-addedin">New in version 1.8.10</span> <span
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+class="minilink minilink-addedin">Beta feature</span> That's where the spot
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+check comes in. This check actually compares your source files counts and data
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+against those in the latest archive, potentially catching problems like
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+incorrect excludes, inadvertent deletes, files changed by malware, etc.
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+
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+However, because an exhaustive comparison of all source files against the
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+latest archive might be too slow, the spot check supports sampling a
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+percentage of your source files for the comparison, ensuring it falls within
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+configured tolerances.
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+
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+Here's how to use it. Start by installing the `xxhash` OS package if you don't
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+already have it, so the spot check can run the `xxh64sum` command and
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+efficiently hash files for comparison. Then add something like the following
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+to your borgmatic configuration:
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+
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+```yaml
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+checks:
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+ - name: spot
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+ count_tolerance_percentage: 10
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+ data_sample_percentage: 1
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+ data_tolerance_percentage: 0.5
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+```
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+
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+The `count_tolerance_percentage` is the percentage delta between the source
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+directories file count and the latest backup archive file count that is
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+allowed before the entire consistency check fails. For instance, if the spot
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+check runs and finds 100 source files and 105 files in the latest archive,
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+that would be within a 10% count tolerance and the check would succeed. But if
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+there were 100 source files and 200 archive files, the check would fail. (100
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+source files and only 50 archive files would also fail.)
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+
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+The `data_sample_percentage` is the percentage of total files in the source
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+directories to randomly sample and compare to their corresponding files in the
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+latest backup archive. The comparison is performed by hashing the selected
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+files in each of the source paths and the backup archive and counting hashes
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+that don't match. For instance, if you have 1,000 source files and your sample
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+percentage is 1%, then only 10 source files will be compared against the
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+latest archive. These sampled files are selected randomly each time, so in
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+effect the spot check is a probabilistic check.
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+
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+The `data_tolerance_percentage` is the percentage of total files in the source
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+directories that can fail a spot check data comparison without failing the
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+entire consistency check. The value must be lower than or equal to the
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+`contents_sample_percentage`.
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+
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+All three options are required when using the spot check. And because the spot
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+check relies on these configured tolerances, it may not be a
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+set-it-and-forget-it type of consistency check, at least until you get the
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+tolerances dialed in so there are minimal false positives or negatives. For
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+certain workloads where your source files experience wild swings of changed
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+data or file counts, the spot check may not suitable at all.
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+
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+What if you change or add or delete a bunch of your source files and you don't
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+want the spot check to fail the next time it's run? Run `borgmatic create` to
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+create a new backup, thereby allowing the spot check to run against an archive
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+that contains your source file changes.
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+
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+While the spot check feature is currently in beta, it may be subject to
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+breaking changes. But feel free to use it in production if you're okay with
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+that caveat, and please [provide any
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+feedback](https://torsion.org/borgmatic/#issues) you have on this feature.
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+
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+
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### Check frequency
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<span class="minilink minilink-addedin">New in version 1.6.2</span> You can
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Dan Helfman