Read I/O starvation with writeback RAID controller

[Martin Svec]

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

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majo...@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/

[Nicholas A. Bellinger]

Hi Martin,

CC'ing linux-scsi here, as aacraid doesn't have an official maintainer
atm.

--nab

> To unsubscribe from this list: send the line "unsubscribe target-devel" in

[Martin Svec]

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.

Thanks,

Martin

Dne 20.2.2013 21:48, Nicholas A. Bellinger napsal(a):

[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

[Martin Svec]

Hi,

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:
>
> https://lkml.org/lkml/2012/12/11/534
>
> 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: https://lkml.org/lkml/2012/12/11/598 -- 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

[Jan Engelhardt]

On Friday 2013-02-22 20:28, Martin Svec wrote:
>
> 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:
> https://lkml.org/lkml/2012/12/11/598 -- 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.

Sounds just like the bufferbloat problem in networking.
Waiting for codel for the block layer :)

[Chris Friesen]

On 02/22/2013 02:35 PM, Jan Engelhardt wrote:
>
> On Friday 2013-02-22 20:28, Martin Svec wrote:
>>
>> 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:
>> https://lkml.org/lkml/2012/12/11/598 -- 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.
>
> Sounds just like the bufferbloat problem in networking.
> Waiting for codel for the block layer :)

Is there any way to somehow have the reads jump to the head of the queue
in the disk controller?

Otherwise it seems like we might need to minimize the disk cache usage
and do the scheduling in software.

This effectively mirrors what the codel people are doing with using tiny
tx ring buffers to fight bufferbloat. The difference is that with a NIC
all you have to do is make sure the buffer doesn't empty and you get
full speed whereas with a disk the more you stuff in the cache the
better it can schedule things.

Chris