# Tuning parameters

Restic offers a few parameters that allow tuning the backup and other operations. 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. This can also result in longer upload times for single temporary packs, which can lead to more disk wear on SSDs (see Pack size).

## 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 `off`, `fastest`, `auto` (default), `better`, or `max`. Each setting uses more CPU but less bandwidth and storage space. This setting is only applied for the single run of restic, but 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, using an integer value for the pack size in MiB. Restic currently defaults to a 16 MiB pack size. Note that this setting should be specified for each restic command that modifies the repository.

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`). 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.