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

[Stefan Hajnoczi]

On Wed, Sep 14, 2011 at 3:42 AM, Zhi Yong Wu <address@hidden> wrote:
> Log for bps=((10 * 1024 * 1024)).

Okay, I think this data shows that I/O limits is too aggressive.
There seems to be some "overhead" amount so the guest is never able to
reach its bps limit:

> test: (g=0): rw=write, bs=64K-64K/64K-64K, ioengine=libaio, iodepth=1
>  WRITE: io=51,200KB, aggrb=7,073KB/s, minb=7,243KB/s, maxb=7,243KB/s,

> test: (g=0): rw=write, bs=128K-128K/128K-128K, ioengine=libaio, iodepth=1
>  WRITE: io=51,200KB, aggrb=10,112KB/s, minb=10,355KB/s,

> test: (g=0): rw=write, bs=256K-256K/256K-256K, ioengine=libaio, iodepth=1
>  WRITE: io=51,200KB, aggrb=8,882KB/s, minb=9,095KB/s, maxb=9,095KB/s,

bs=128 KB worked nicely.  The 64 KB and 256 KB cases don't look so good.

I worry a little that the benchmark duration is quite short so a
fluctuation would affect the results more than if the benchmark
duration was extended to 30 secs or 1 minute.

Zhi Yong: Do you get similar results each time you run this benchmark
or do they vary by more than +/- 512 KB?  If the results are stable
and the benchmark is able to exceed 10 MB/s when running without I/O
throttling, then it's important to figure out why the guest isn't
achieving 10 MB/s.

Stefan