Messages in this thread | | | Subject | Re: Very poor latency when using hard drive (raid1) | From | Mike Galbraith <> | Date | Tue, 16 Apr 2013 09:24:01 +0200 |
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On Tue, 2013-04-16 at 08:49 +0200, lkml@tigusoft.pl wrote: > On 15/04/13 11:59, lkml@tigusoft.pl wrote: > > There are 2 hard drives (normal, magnetic) in software raid 1 > > on 3.2.41 kernel. > > > > When I write into them e.g. using dd from /dev/zero to a local file > > (ext4 on default settings), running 2 dd at once (writing two files) it > > starves all other programs that try to use the disk. > > > > Running ls on any directory on same disk (same fs btw), takes over half > > minute to execute, same for any other disk touching action. > > > > Did anyone seen such problem, where too look, what to test? > > > > What could solve it (other then ionice on applications that I expect to > > use hard drive)? > > I got reply (by e-mail) suggesting to use XFS. > Thanks, possible for other/next server. > > But I fell this should work correctly as well on ext4.
It should not starve readers that badly, something is wrong.
You can try setting low_latency for the devices if you're using CFQ ioscheduler. For my box, that would be..
echo 1 > /sys/devices/pci0000:00/0000:00:1f.2/ata2/host0/target0:0:0/0:0:0:0/block/sda/queue/iosched/low_latency
Or, you can try a different scheduler. cat (blabla)/sda/queue/scheduler to see which choices are available, and echo your choice back to the file to select it.
-Mike
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