Re: Read I/O starvation with writeback RAID controller

[Nicholas A. Bellinger]

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:

https://lkml.org/lkml/2012/12/11/534

Perhaps this would be worth a shot..?

--nab

> 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 https://lkml.org/lkml/2012/12/10/550 (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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> 
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