A set of patches has been accepted into the repository that develops changes in Btrfs for future Linux kernel branches, implementing a round-robin read operation balancing policy in RAID1. To enable distributed reading operations in a circular cycle, set the value to "round-robin:[min_contiguous_read]" in "\/sys\/fs\/btrfs\/\/read_policy", where the optional parameter "min_contiguous_read" defines the minimum size of requests to be balanced among mirrors (default value is 256 KiB).
Initially, load balancing in RAID1 Btrfs was based on the PID of the process that requested I/O. This scheme worked well in systems where multiple different processes created load; however, it did not allow for maximum performance under single-task loads. For example, tests showed that enabling the "round-robin" policy reduced the defragmentation process time from 18 to 8.8 seconds.
An additional change has been added to implement the "devid:" parameter in "\/sys\/fs\/btrfs\/read_policy". This parameter disables balancing and mandates that reading operations be performed only from the specified disk.
Source: opennet.ru
