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.

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. 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.

File Read Concurrency

When backing up 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 flag for 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 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 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.