[Qemu-devel] Write requests, during read operations

[Qemu-devel] Write requests, during read operations

[Gleb Stepanov]

Hello, all.

   We are testing disk I/O perfomance using fio tool. I've measured
performance on qcow2 image mounted to nb0 device. I run tests and
launch iostat -x 1 command to browse I/O operations. Test consists of
sequential read operations with 1 MB size. I've get following output.

Device:         rrqm/s   wrqm/s     r/s     w/s    rkB/s    wkB/s
sda               0.00     5.00    0.00  677.00     0.00 22812.00
sdb               0.00     0.00    0.00  545.00     0.00     0.00
dm-0              0.00     0.00    0.00  674.00     0.00 22784.00
dm-1              0.00     0.00    0.00    7.00     0.00    28.00
dm-2              0.00     0.00    0.00    0.00     0.00     0.00
nb0               0.00     0.00  432.00  182.00 55296.00 23296.00

So, reading produce a lot of write operations. What is the reason why
these write operations occured?
How to determina block size that qemu operates , because if i've
writtien 55 mb with 1 mb blocks
i should get 55 operations instead of 432.

Kind regards, Gleb Stepanov.

Re: [Qemu-devel] Write requests, during read operations

[Stefan Hajnoczi]

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On Tue, Jul 14, 2015 at 05:39:13PM +0300, Gleb Stepanov wrote:
> I've measured performance on qcow2 image mounted to nb0 device.

nb0 device?

> I run tests and
> launch iostat -x 1 command to browse I/O operations. Test consists of
> sequential read operations with 1 MB size. I've get following output.
> 
> Device:         rrqm/s   wrqm/s     r/s     w/s    rkB/s    wkB/s
> sda               0.00     5.00    0.00  677.00     0.00 22812.00
> sdb               0.00     0.00    0.00  545.00     0.00     0.00
> dm-0              0.00     0.00    0.00  674.00     0.00 22784.00
> dm-1              0.00     0.00    0.00    7.00     0.00    28.00
> dm-2              0.00     0.00    0.00    0.00     0.00     0.00
> nb0               0.00     0.00  432.00  182.00 55296.00 23296.00
> 
> So, reading produce a lot of write operations. What is the reason why
> these write operations occured?
> How to determina block size that qemu operates , because if i've
> writtien 55 mb with 1 mb blocks
> i should get 55 operations instead of 432.

Please post details of the benchmark:
 * fio job file and fio command-line
 * commands used to launch QEMU or qemu-nbd

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