Re: [Qemu-devel] Why qemu write/rw speed is so low?

[Stefan Hajnoczi]

On Tue, Sep 13, 2011 at 10:25 AM, Zhi Yong Wu <address@hidden> wrote:
> On Tue, Sep 13, 2011 at 3:14 PM, Stefan Hajnoczi
> <address@hidden> wrote:
>> On Tue, Sep 13, 2011 at 10:52:44AM +0800, Zhi Yong Wu wrote:
>>> This is real log when fio issued with bs=128K and bps=1000000(block
>>> I/O throttling):
>>
>> I would use 1024 * 1024 instead of 1000000 as the throughput limit.
>> 10^5 is not a multiple of 512 bytes and is not a nice value in KB/s
>> (976.5625).
> OK. next time, i will adopt this.
>>
>>>
>>>   8,2    0        1     0.000000000 24332  A  WS 79958528 + 256 <-
>>> (253,2) 71830016
>>
>> 256 blocks = 256 * 512 bytes = 128 KB per request.  We know the maximum
>> request size from Linux is 128 KB so this makes sense.
>>
>>> Throughput (R/W): 0KiB/s / 482KiB/s
>>
>> What throughput do you get without I/O throttling?  Either I/O
>> throttling is limiting too aggressively here or the physical disk is the
>> bottleneck (I double that since the write throughput value is very low).
>> We need to compare against the throughput when throttling is not
>> enabled.
> Without block I/O throttling.
[...]
> test: (g=0): rw=write, bs=128K-128K/128K-128K, ioengine=libaio, iodepth=1
> Starting 1 process
> Jobs: 1 (f=1): [W] [100.0% done] [0K/13M /s] [0/103 iops] [eta 00m:00s]
> test: (groupid=0, jobs=1): err= 0: pid=2734
>  write: io=51,200KB, bw=12,933KB/s, iops=101, runt=  3959msec

This shows that the physical disk is capable of far exceeding 1 MB/s
when I/O is not limited.  So the earlier result where the guest only
gets 482 KiB/s under 1000000 bps limit shows that I/O limits are being
too aggressive.  For some reason the algorithm is causing the guest to
get lower throughput than expected.

It would be interesting to try with bps=$((10 * 1024 * 1024)).  I
wonder if the algorithm has a constant overhead of a couple hundred
KB/s or if it changes with the much larger bps value.

Stefan