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

[Stefan Hajnoczi]

On Tue, Sep 13, 2011 at 10:52:44AM +0800, Zhi Yong Wu wrote:
> This is real log when fio issued with bs=128K and bps=1000000(block
> I/O throttling):

I would use 1024 * 1024 instead of 1000000 as the throughput limit.
10^5 is not a multiple of 512 bytes and is not a nice value in KB/s
(976.5625).

> 
>   8,2    0        1     0.000000000 24332  A  WS 79958528 + 256 <-
> (253,2) 71830016

256 blocks = 256 * 512 bytes = 128 KB per request.  We know the maximum
request size from Linux is 128 KB so this makes sense.

> Throughput (R/W): 0KiB/s / 482KiB/s

What throughput do you get without I/O throttling?  Either I/O
throttling is limiting too aggressively here or the physical disk is the
bottleneck (I double that since the write throughput value is very low).
We need to compare against the throughput when throttling is not
enabled.

Stefan