[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.