To the repository developing changes to Btrfs for future kernel branches LinuxA patch set has been adopted implementing a round-robin read balancing policy for RAID1. To enable round-robin read balancing in "/sys/fs/btrfs/" /read_policy" you need to set the value "round-robin:[min_contiguous_read]", in which the optional parameter "min_contiguous_read" defines the minimum size of requests to be balanced between mirrors (the default value is 256 KiB).
Initially, RAID1 load balancing in Btrfs was performed based on the PID of the process that requested the I/O. This scheme worked well in systems where the load was created by many different processes, but with a single-task load it did not allow achieving maximum performance. For example, in the tests conducted, enabling the "round-robin" policy reduced the execution time of the defragmentation process from 18 to 8.8 seconds.
Additionally, a change has been added that implements the "devid:" parameter. " in "/sys/fs/btrfs//read_policy". This parameter disables balancing and instructs to perform read operations only from the specified disk.
Source: opennet.ru
