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

[Stefan Hajnoczi]

On Tue, Sep 13, 2011 at 9:31 AM, Zhi Yong Wu <address@hidden> wrote:
> 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?

The blkparse format specifier for blocks is 'n'.  Here is the code to
print it from blkparse_fmt.c:

case 'n':
    fprintf(ofp, strcat(format, "u"), t_sec(t));

And t_sec() is:

#define t_sec(t)        ((t)->bytes >> 9)

So it divides the byte count by 512.  Block size == sector size == 512 bytes.

You can get the blktrace source code here:

http://brick.kernel.dk/snaps/

Stefan