Re: Read I/O starvation with writeback RAID controller

From: Martin Svec
Date: Fri Feb 22 2013 - 14:28:21 EST


Dne 21.2.2013 23:01, Nicholas A. Bellinger napsal(a):
Hi Martin,

On Thu, 2013-02-21 at 12:43 +0100, Martin Svec wrote:
> I'm sorry, I forgot to mention hardware details. It isn't aacraid, it
> is megaraid-based Dell PERC H700 w/ 1GB NVRAM and 12x 450GB 15k SAS
> drives in RAID-10. All in Dell R510 server.

Jan Engelhardt (CC'ed) mentioned the currently out-of-tree ROW scheduler
worked for him:

Perhaps this would be worth a shot..?

Yes, I've already tried the ROW scheduler. It helped for some low iodepths depending on quantum settings but generally didn't solve the problem. I think the key issue is that none of the schedulers can throttle I/O according to e.g. average request roundtrip time. Shaohua Li is right here: -- as long as there's free room in device's queue they blindly dispatch requests to it.

Which is exactly what I see in deadline scheduler fifo queues: There're no read requests to be scheduled between writes because all readers are starving. So the scheduler keeps dispatching writes using all the remaining capacity of device queue. Which in turn worses the read starvation. Bigger queue depth and bigger writeback cache means higher chance for read starvation even from a single writer.



> Thanks,
> Martin
> Dne 20.2.2013 21:48, Nicholas A. Bellinger napsal(a):
> > Hi Martin,
> >
> > CC'ing linux-scsi here, as aacraid doesn't have an official maintainer
> > atm.
> >
> > --nab
> >
> > On Wed, 2013-02-20 at 16:38 +0100, Martin Svec wrote:
> >> Hello,
> >>
> >> I've noticed read I/O starvation problems of LIO iSCSI target when
> >> used on top of writeback-enabled HW RAID controller (PERC H700 with
> >> 1GB cache). For intensive mixed read-write workload in virtualized
> >> environments, writes are able to consume over 95% of the IOPS
> >> throughput and cause starvation of reads.
> >>
> >> After a number of tests it seems to me it's a general issue of block
> >> layer I/O scheduling when running on top of a writeback device. If
> >> there is a write-intensive task, all writes go to the writeback cache
> >> with near-zero latency. This allows writer to quickly saturate the
> >> device with thousands of writes while using only a minimal fraction of
> >> queue depth. However, non-cached reads depend on spinning drive
> >> latencies which are orders of magnitude higher than writeback cache
> >> latencies, and so readers cannot submit so many requests per second as
> >> writers. Consequently, I guess the controller has totally wrong view
> >> of the incoming workload pattern, tries to satisfy the write flood
> >> first and the net result is inacceptable starvation of reads, with
> >> latencies up to hundreds of milliseconds.
> >>
> >> A simple fio test with 1TiB block device where one thread does 4k
> >> random sync writes with iodepth=32 and one thread does 4k random reads
> >> with iodepth=32 shows that instead of the theoretical 50:50 IOPS
> >> ratio, the block device runs with 95:5 ratio in favor of writes. In
> >> fact, the imbalance is so high that even write iodepth=2 is enaugh to
> >> achieve the same numbers.
> >>
> >> Real workloads that tend to exhibit this problem are: initial zeroing
> >> of a virtual machine disk, virtual machine migration, virtual machine
> >> cloning, intensive swapping of one virtual machine etc.
> >>
> >> I tried to set WCE=1 on target iblock device, played with queue
> >> depths, tested all three I/O schedulers and their parameters,
> >> controller's parameters, but with no luck. To achieve reasonably good
> >> fairness, the only solution is to set nr_requests to 1 or disable
> >> controller's writeback cache at all -- at the expense of degraded
> >> overall performance :-(
> >>
> >> Regarding nr_requests, there's obvious relation between iodepths and
> >> read starvation: if (nr_requests>= workload iodepth) then starvation
> >> surely occurs. Lowering nr_requests below this threshold slowly starts
> >> improving fairness and for every rd+wr iodepths pair, there exists
> >> sufficiently low nr_requests value at which IOPS ratio is finally
> >> balanced according to rd:wr iodepth ratio. Unfortunately it means
> >> there is no minimal nr_requests value suitable for all workloads. For
> >> iodepths around 2 to 8, only nr_requests=1 provides fair load balancing.
> >>
> >> Is this a known problem? Does anybody find block layer parameters that
> >> elliminate this problem for iscsi-target storage in mixed random
> >> read-write environments like virtualization? Or should I start writing
> >> my own I/O scheduler? ;-)
> >>
> >> Update: I've just found (Read
> >> starvation by sync writes), where Jan Kara describes identical
> >> symptoms. But setting nr_requests=10000 doesn't help in my case.
> >> CC'ing LKML too (I'm not LKML subscriber).
> >>
> >> Thanks,
> >>
> >> Martin
> >>
> >> --
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