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PERFORMANCE

mergerfs is at its core just a proxy and therefore its theoretical max performance is that of the underlying devices. However, given it is a FUSE filesystem working from userspace there is an increase in overhead relative to kernel based solutions. That said the performance can match the theoretical max but it depends greatly on the system's configuration. Especially when adding network filesystems into the mix there are many variables which can impact performance. Device speeds and latency, network speeds and latency, general concurrency, read/write sizes, etc. Unfortunately, given the number of variables it has been difficult to find a single set of settings which provide optimal performance. If you're having performance issues please look over the suggestions below (including the benchmarking section.)

NOTE: be sure to read about these features before changing them to understand what behaviors it may impact

  • disable security_capability and/or xattr
  • increase cache timeouts cache.attr, cache.entry, cache.negative_entry
  • enable (or disable) page caching (cache.files)
  • enable parallel-direct-writes
  • enable cache.writeback
  • enable cache.statfs
  • enable cache.symlinks
  • enable cache.readdir
  • change the number of worker threads
  • disable posix_acl
  • disable async_read
  • test theoretical performance using nullrw or mounting a ram disk
  • use symlinkify if your data is largely static and read-only
  • use tiered cache devices
  • use LVM and LVM cache to place a SSD in front of your HDDs
  • increase readahead: readahead=1024

If you come across a setting that significantly impacts performance please contact trapexit so he may investigate further. Please test both against your normal setup, a singular branch, and with nullrw=true