Merge pull request #4792 from kylinstorage/wip-writeback-throttling-for-cache-tiering

Merge pull request #4792 from kylinstorage/wip-writeback-throttling-for-cache-tiering

Wip writeback throttling for cache tiering

This patch is to do write back throttling for cache tiering, which is similar to what the Linux kernel does for  page cache write back.  A paramter 'cache_target_dirty_high_ratio'  (default 0.6) is introduced as the high speed flushing threshold, while leave the 'cache_target_dirty_ratio' (default 0.4) to represent the low speed  threshold. The flush speed is controlled by limiting the parallelism of flushing. The maximum parallelism under low speed is half of the parallelism under high speed. If there is at least one PG such that the dirty ratio beyond the high threshold, full speed mode is entered; If there is no PG such that dirty ratio beyond the low threshold, idle mode is entered; In other cases, slow speed mode is entered.

Signed-off-by: Mingxin Liu <mingxinliu@ubuntukylin.com>
Reviewed-by: Li Wang <liwang@ubuntukylin.com>
Suggested-by: Nick Fisk <nick@fisk.me.uk>
Tested-by: Kefu Chai <kchai@redhat.com>