Interaction with btrfs

Posted Apr 25, 2017 9:41 UTC (Tue) by zdzichu (subscriber, #17118)
Parent article: Two new block I/O schedulers for 4.12

I wonder how those schedulers are going to interact with btrfs? Those schedulers try to ensure fairness between *processes*. But in my observations, when using btrfs most work seem to be dispatched from kworker, btrfs-worker, btrfs-cleaner, btrfs-transaction and other btrfs-* kernel threads, not from original processes.
This btrfs behaviour makes observing I/O traffic troublesome already. I fear it will wreak havoc with fairness.

Interaction with btrfs

Posted Apr 25, 2017 13:15 UTC (Tue) by DG (subscriber, #16978) [Link]

Unscientific observation : It works well.

I used to see spikes in latency from BTRFS on an internal backup server as it performed rsync --inplace based backups and deleted old snapshots ... which were bad enough to make me try BFQ.

Random details: vanilla 4.9.x kernel + BFQ. Two WD 3TiB Red disks in a BTRFS raid 1 filesystem. 4 core i5 processor with 32gb of RAM. FS contains 150 readonly snapshots. Currently using about 1.79TiB.

Interaction with btrfs

Posted Apr 26, 2017 13:00 UTC (Wed) by masoncl (subscriber, #47138) [Link]

It should work pretty well, but we'll definitely look into any latency problems. For the most part we're able to tag latency sensitive IOs and get good results from the schedulers. On the btrfs side, we try to make sure the btrfs IO threads aren't creating priority inversions of their own.