Marc MERLIN
2014-Feb-07 22:20 UTC
btrfs snapshot is killing IO and hanging my device with delayed writes for 10mn+
On my workstation, which unfortunately I can't easily upgrade the kernel on,
so it's running 3.8.0 for now, I've had pretty perpelexing hangs from
btrfs snapshot
when I make a new snapshot every hour.
I see the btrfs snapshot command in iotop for over a minute, and it completes
eventually.
Other times, it just runs with virtually no IO.
When it takes a while, I see this next in iotop for several minutes:
Total DISK READ: 0.00 B/s | Total DISK WRITE: 2.12 M/s
TID PRIO USER DISK READ DISK WRITE SWAPIN IO> COMMAND
1620 be/4 root 0.00 B/s 0.00 B/s 0.00 % 99.99 % [btrfs-transacti]
658 be/3 root 0.00 B/s 28.46 K/s 0.00 % 62.35 % [jbd2/dm-0-8]
The device is indeed on top of an LVM, which likely isn't helping
performance
(I see only 2.12 M/s of writes in the iotop above):
sda 8:0 0 931.5G 0 disk
├─sda1 8:1 0 243M 0 part /boot
├─sda2 8:2 0 1K 0 part
└─sda5 8:5 0 931.3G 0 part
├─moremagic-root (dm-0) 252:0 0 37.3G 0 lvm /
├─moremagic-usr+local (dm-1) 252:1 0 878G 0 lvm
Today, it took about 10mn for IO to go back to normal (stop blocking) to that
partition during my noon snapshot. The later ones didn't hang.
Why would a snapshot cause hangs and so much IO?
hdparm shows that basic IO seems ok:
root@polgara:/tmp# hdparm -t /dev/mapper/moremagic-usr+local
/dev/mapper/moremagic-usr+local:
Timing buffered disk reads: 566 MB in 3.00 seconds = 188.49 MB/sec
root@polgara:/tmp# btrfs fi show
Label: 'btrfs_pool1' uuid: 64eafeba-603e-483f-bff9-395b9907b415
Total devices 1 FS bytes used 443.76GB
devid 1 size 877.98GB used 541.54GB path /dev/dm-1
root@polgara:/tmp# btrfs device stats /mnt/btrfs_pool1
[/dev/mapper/moremagic-usr+local].write_io_errs 0
[/dev/mapper/moremagic-usr+local].read_io_errs 0
[/dev/mapper/moremagic-usr+local].flush_io_errs 0
[/dev/mapper/moremagic-usr+local].corruption_errs 0
[/dev/mapper/moremagic-usr+local].generation_errs 0
Is there other data I can provide?
Thanks,
Marc
--
"A mouse is a device used to point at the xterm you want to type in" -
A.S.R.
Microsoft is to operating systems ....
.... what McDonalds is to gourmet cooking
Home page: http://marc.merlins.org/
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