Hi Jan,
On 01/25/2018 04:48 AM, Jan Kara wrote:
Hi Chris,
On Wed 24-01-18 17:02:47, Chris Mason wrote:
I'm really looking forward to LSF/MM this year. I can bring along a fair
amount of data from production about benchmarking and stability.
We've been expanding our btrfs rollout, and we're also fixing up priority
inversions when cgroup IO controllers are put in place. I think we have
btrfs fixed up, but ext4 seems to be incompatible with IO controllers due to
data=ordered IO.
Yeah, I suspect I know what you hit but still I'd be interested in hearing
more details about your usecase and the problems you see. Maybe it could be
helped.
Both btrfs and ext4 are root drive filesystems for us. The IO
controller is basically making sure the root drive isn't saturated by
lower priority tasks, which might be anything from system updates to log
files to actually part of the workload.
With ext4, the data=ordered IO done during transaction commits makes
priority inversions that I don't see a way around. It's dramatically
better than ext3, but still happens enough that we can't enforce IO
limits at all. It really only takes one low prio IO to sneak into
kjournald's list to wreck everything.
We haven't tried XFS with the controllers yet but I don't think there
will be any major blockers there.
I'm also hoping filesystem slab shrinking gets into the agenda, since we
have a few ugly hacks there to keep production happy.
I'm interested in this as well but I suspect we need someone willing to
spend a long time with slab reclaim guts to improve this. And so far nobody
was bothered enough...
Yeah, we'll keep chipping away at this one, but it's going to take a while.
-chris
