For the btrfs file system asynchronous implementation of the DISCARD operation (marking freed blocks that can no longer be physically stored), developed by Facebook engineers.
The essence of the problem: in the original implementation, DISCARD is performed synchronously with other operations, which can lead to performance issues in certain cases, as it has to wait for the completion of the corresponding commands from the storage devices, requiring additional time. This can become a problem if the DISCARD implementation on the storage device operates slowly.
With asynchronous implementation, there is no need to wait for the storage device to complete DISCARD during normal FS operations, thereby eliminating the issue and turning this operation into a background task. The presented implementation also performs some optimizations. For instance, it waits for a certain period, considering that the block may soon be used again, making the execution of the DISCARD procedure unnecessary, and also tries to merge regions before actual DISCARD execution to reduce the overall number of operations.
Kilde: opennet.ru
