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rework pack size parameter documentation

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Michael Eischer 2022-06-12 13:03:59 +02:00
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doc/047_tuning_backup_parameters.rst
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@ -54,17 +54,21 @@ variable ``RESTIC_COMPRESSION``.
Pack Size
=========
In certain instances, such as very large repositories, it is desired to have larger pack
sizes to reduce the number of files in the repository. Notable examples are OpenStack
In certain instances, such as very large repositories (in the TiB range) or very fast
upload connections, it is desirable to use larger pack sizes to reduce the number of
files in the repository and improve upload performance. Notable examples are OpenStack
Swift and some Google Drive Team accounts, where there are hard limits on the total
number of files. This can be achieved by either using the ``--min-packsize`` flag
number of files. Larger pack size can also improve the backup speed for a repository
stored on a local HDD. This can be achieved by either using the ``--min-packsize`` flag
or defining the ``$RESTIC_MIN_PACKSIZE`` environment variable. Restic currently defaults
to a 16MB minimum pack size.
to a 16MiB pack size.
The side effect of increasing the pack size is increased client memory usage. A bit of
tuning may be required to strike a balance between memory usage and number of pack files.
Restic uses the majority of it's memory according to the pack size, multiplied by the number
of parallel writers. For example, if you have 4 parallel writers (restic creates one per
available CPU), With a minimum pack size of 64 (Megabytes), you'll get a *minimum* of 256MB
of memory usage.
The side effect of increasing the pack size is requiring more disk space for temporary pack
files created before uploading. The space must be available in the system default temp
directory, unless overwritten by setting the ``$TMPDIR`` environment variable. In addition,
depending on the backend the memory usage can also increase by a similar amount. Restic
requires temporary space according to the pack size, multiplied by the number
of backend connections plus one. For example, if the backend uses 5 connections (the default
for most backends), with a target pack size of 64MiB, you'll need a *minimum* of 384MiB
of space in the temp directory. A bit of tuning may be required to strike a balance between
resource usage at the backup client and the number of pack files in the repository.