On Mon, Aug 05 2019, Jinpu Wang wrote: > Hi Neil, > > For the md higher write IO latency problem, I bisected it to these commits: > > 4ad23a97 MD: use per-cpu counter for writes_pending > 210f7cd percpu-refcount: support synchronous switch to atomic mode. > > Do you maybe have an idea? How can we fix it? Hmmm.... not sure. My guess is that the set_in_sync() call from md_check_recovery() is taking a long time, and is being called too often. Could you try two experiments please. 1/ set /sys/block/md0/md/safe_mode_delay to 20 or more. It defaults to about 0.2. 2/ comment out the call the set_in_sync() in md_check_recovery(). Then run the least separately after each of these changes. I the second one makes a difference, I'd like to know how often it gets called - and why. The test if ( ! ( (mddev->sb_flags & ~ (1<<MD_SB_CHANGE_PENDING)) || test_bit(MD_RECOVERY_NEEDED, &mddev->recovery) || test_bit(MD_RECOVERY_DONE, &mddev->recovery) || (mddev->external == 0 && mddev->safemode == 1) || (mddev->safemode == 2 && !mddev->in_sync && mddev->recovery_cp == MaxSector) )) return; should normally return when doing lots of IO - I'd like to know which condition causes it to not return. Thanks, NeilBrown
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