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

[Zhi Yong Wu]

On Tue, Sep 13, 2011 at 3:15 PM, Stefan Hajnoczi
<address@hidden> wrote:
> On Tue, Sep 13, 2011 at 10:38:28AM +0800, Zhi Yong Wu wrote:
>> On Fri, Sep 9, 2011 at 6:38 PM, Stefan Hajnoczi
>> <address@hidden> wrote:
>> > On Fri, Sep 09, 2011 at 05:44:36PM +0800, Zhi Yong Wu wrote:
>> >> Today, i did some basical I/O testing, and suddenly found that qemu write 
>> >> and rw speed is so low now, my qemu binary is built on commit 
>> >> 344eecf6995f4a0ad1d887cec922f6806f91a3f8.
>> >>
>> >> Do qemu have regression?
>> >>
>> >> The testing data is shown as below:
>> >>
>> >> 1.) write
>> >>
>> >> test: (g=0): rw=write, bs=512-512/512-512, ioengine=libaio, iodepth=1
>> >
>> > Please post your QEMU command-line.  If your -drive is using
>> > cache=writethrough then small writes are slow because they require the
>> > physical disk to write and then synchronize its write cache.  Typically
>> > cache=none is a good setting to use for local disks.
>> >
>> > The block size of 512 bytes is too small.  Ext4 uses a 4 KB block size,
>> > so I think a 512 byte write from the guest could cause a 4 KB
>> > read-modify-write operation on the host filesystem.
>> >
>> > You can check this by running btrace(8) on the host during the
>> > benchmark.  The blktrace output and the summary statistics will show
>> > what I/O pattern the host is issuing.
>>   8,2    0        1     0.000000000   337  A  WS 425081504 + 8 <-
>> (253,1) 42611360
>
> 8 blocks = 8 * 512 bytes = 4 KB
How do you know each block size is 512 bytes?

>
> So we are not performing 512 byte writes.  Some layer is changing the
> I/O pattern.
>
> Stefan
>



-- 
Regards,

Zhi Yong Wu