Gareth Pye posted on Tue, 05 Apr 2016 09:36:48 +1000 as excerpted: > I've got a btrfs file system set up on 6 drbd disks running on 2Tb > spinning disks. The server is moderately loaded with various regular > tasks that use a fair bit of disk IO, but I've scheduled my weekly btrfs > scrub for the best quiet time in the week. > > The command that is run is: > /usr/local/bin/btrfs scrub start -Bd -c idle /data > > Which is my best attempt to try and get it to have a low impact on user > operations > > But iotop shows me: > > 1765 be/4 root 14.84 M/s 0.00 B/s 0.00 % 96.65 % btrfs scrub > start -Bd -c idle /data > 1767 be/4 root 14.70 M/s 0.00 B/s 0.00 % 95.35 % btrfs > scrub start -Bd -c idle /data > 1768 be/4 root 13.47 M/s 0.00 B/s 0.00 % 92.59 % btrfs > scrub start -Bd -c idle /data > 1764 be/4 root 12.61 M/s 0.00 B/s 0.00 % 88.77 % btrfs > scrub start -Bd -c idle /data > 1766 be/4 root 11.24 M/s 0.00 B/s 0.00 % 85.18 % btrfs > scrub start -Bd -c idle /data > 1763 be/4 root 7.79 M/s 0.00 B/s 0.00 % 63.30 % btrfs > scrub start -Bd -c idle /data > 28858 be/4 root 0.00 B/s 810.50 B/s 0.00 % 61.32 % [kworker/ u16:25] > > > Which doesn't look like an idle priority to me. And the system sure > feels like a system with a lot of heavy io going on. Is there something > I'm doing wrong?
Two points: 1) It appears btrfs scrub start's -c option only takes numeric class, so try -c3 instead of -c idle. Works for me with the numeric class (same results as you with spelled out class), tho I'm on ssd with multiple independent btrfs on partitions, the biggest of which is 24 GiB, 18.something GiB used, which scrubs in all of 20 seconds, so I don't need and hadn't tried the -c option at all until now. 2) What a difference an ssd makes! $$ sudo btrfs scrub start -c3 /p scrub started on /p, [...] $$ sudo iotop -obn1 Total DISK READ : 626.53 M/s | Total DISK WRITE : 0.00 B/s Actual DISK READ: 596.93 M/s | Actual DISK WRITE: 0.00 B/s TID PRIO USER DISK READ DISK WRITE SWAPIN IO COMMAND 872 idle root 268.40 M/s 0.00 B/s 0.00 % 0.00 % btrfs scrub start -c3 /p 873 idle root 358.13 M/s 0.00 B/s 0.00 % 0.00 % btrfs scrub start -c3 /p CPU bound, 0% IOWait even at idle IO priority, in addition to the hundreds of M/s values per thread/device, here. You OTOH are showing under 20 M/s per thread/device on spinning rust, with an IOWait near 90%, thus making it IO bound. -- Duncan - List replies preferred. No HTML msgs. "Every nonfree program has a lord, a master -- and if you use the program, he is your master." Richard Stallman -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
