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########################
Tuning Backup Parameters
########################

Restic offers a few parameters that allow tuning the backup. The default values should
work well in general although specific use cases can benefit from different non-default
values. As the restic commands evolve over time, the optimal value for each parameter
can also change across restic versions.


Disabling Backup Progress Estimation
====================================

When you start a backup, restic will concurrently count the number of files and
their total size, which is used to estimate how long it will take. This will
cause some extra I/O, which can slow down backups of network file systems or
FUSE mounts. To avoid this overhead at the cost of not seeing a progress
estimate, use the ``--no-scan`` option of the ``backup`` command  which disables
this file scanning.

Backend Connections
===================

Restic uses a global limit for the number of concurrent connections to a backend.
This limit can be configured using ``-o <backend-name>.connections=5``, for example for
the REST backend the parameter would be ``-o rest.connections=5`` or for the local backend 
``-o local.connections=2``. By default restic uses ``5`` connections for each backend, 
except for the local backend which uses a limit of ``2``. The defaults should work well in
most cases. For high-latency backends it can be beneficial to increase the number of 
connections. Please be aware that this increases the resource consumption of restic and 
that a too high connection count *will degrade performance*.


CPU Usage
=========

By default, restic uses all available CPU cores. You can set the environment variable
`GOMAXPROCS` to limit the number of used CPU cores. For example to use a single CPU core,
use `GOMAXPROCS=1`. Limiting the number of usable CPU cores, can slightly reduce the memory
usage of restic.


Compression
===========

For a repository using at least repository format version 2, you can configure how data
is compressed with the option ``--compression``. It can be set to ``auto`` (the default,
which will compress very fast), ``max`` (which will trade backup speed and CPU usage for
slightly better compression), or ``off`` (which disables compression). Each setting is
only applied for the single run of restic. The option can also be set via the environment
variable ``RESTIC_COMPRESSION``.


Data Verification
=================

To prevent the upload of corrupted data to the repository, which can happen due
to hardware issues or software bugs, restic verifies that generated files can
be decoded and contain the correct data beforehand. This increases the CPU usage
during backups. If necessary, you can disable this verification using the
``--no-extra-verify`` option of the ``backup`` command. However, in this case
you should verify the repository integrity more actively using
``restic check --read-data`` (or the similar ``--read-data-subset`` option).
Otherwise, data corruption due to hardware issues or software bugs might go
unnoticed.


File Read Concurrency
=====================

When backing up files from fast storage like NVMe disks, it can be beneficial to increase
the read concurrency. This can increase the overall performance of the backup operation
by reading more files in parallel. You can specify the concurrency of file reads with the
``RESTIC_READ_CONCURRENCY`` environment variable or the ``--read-concurrency`` option of
the ``backup`` command.


Pack Size
=========

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.  Larger pack sizes can also improve the backup speed for a repository
stored on a local HDD.  This can be achieved by either using the ``--pack-size`` option
or defining the ``$RESTIC_PACK_SIZE`` environment variable.  Restic currently defaults
to a 16 MiB pack size.

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 ``$TMPDIR`` (except Windows) environment
variable (on Windows use ``$TMP`` or ``$TEMP``).  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 64 MiB, you'll need a *minimum* of 384 MiB
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.

Note that larger pack files increase the chance that the temporary pack files are written
to disk. An operating system usually caches file write operations in memory and writes
them to disk after a short delay. As larger pack files take longer to upload, this
increases the chance of these files being written to disk. This can increase disk wear
for SSDs.


Feature Flags
=============

Feature flags allow disabling or enabling certain experimental restic features. The flags
can be specified via the ``RESTIC_FEATURES`` environment variable. The variable expects a
comma-separated list of ``key[=value],key2[=value2]`` pairs. The key is the name of a feature
flag. The value is optional and can contain either the value ``true`` (default if omitted)
or ``false``. The list of currently available feature flags is shown by the ``features``
command.

Restic will return an error if an invalid feature flag is specified. No longer relevant
feature flags may be removed in a future restic release. Thus, make sure to no longer
specify these flags.

A feature can either be in alpha, beta, stable or deprecated state.

- An _alpha_ feature is disabled by default and may change in arbitrary ways between restic
  versions or be removed.
- A _beta_ feature is enabled by default, but still can change in minor ways or be removed.
- A _stable_ feature is always enabled and cannot be disabled. This allows for a transition
  period after which the flag will be removed in a future restic version.
- A _deprecated_ feature is always disabled and cannot be enabled. The flag will be removed
  in a future restic version.
