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Re: [Qemu-devel] Very poor IO performance which looks like some design p

From: ein
Subject: Re: [Qemu-devel] Very poor IO performance which looks like some design problem.
Date: Sat, 11 Apr 2015 19:10:08 +0200
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On 04/11/2015 03:09 PM, Paolo Bonzini wrote:
> On 10/04/2015 22:38, ein wrote:
>> Qemu creates more than 70 threads and everyone of them tries to write to
>> disk, which results in:
>> 1. High I/O time.
>> 2. Large latency.
>> 2. Poor sequential read/write speeds.
>> When I limited number of cores, I guess I limited number of threads as
>> well. That's why I got better numbers.
>> I've tried to combine AIO native and thread setting with deadline
>> scheduler. Native AIO was much more worse.
>> The final question, is there any way to prevent Qemu for making so large
>> number of processes when VM does only one sequential R/W operation?
> Use "aio=native,cache=none".  If that's not enough, you'll need to use
> XFS or a block device; ext4 suffers from spinlock contention on O_DIRECT
> I/O.
Hello Paolo and thank you for reply.

Firstly, I do use ext2 now, which gave me more MiB/s than XFS in the
past. I've tried combination with XFS and block_device with NTFS (4KB)
on it. I did tests with AIO=native,cache=none. Results in this workload
was significantly worse. I don't have numbers on me right now but if
somebody is interested, I'll redo the tests. From my experience I can
say that disabling every software caches gives significant boost in
sequential RW ops. I mean: Qemu cache, linux kernel dirty pages or even
caching on VM itself. It makes somehow speed of data flow softer and
more stable. Using cache creates hiccups. Firstly there's enormous speed
for couple of seconds, more than hardware is capable of, then flush and
no data flow at all (or very little) in few / over a dozen / seconds.

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