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

From: Zhi Yong Wu
Subject: Re: [Qemu-devel] Why qemu write/rw speed is so low?
Date: Fri, 9 Sep 2011 21:48:59 +0800

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.
Now i can not access my workstation in the office.
-drive if=virtio,cache=none,file=xxxx

> 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 mean RCU? What is its work procedure? Can you explain in more
details if you are available?
> 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.
OK, i will try next Tuesday.
> I suggest changing your fio block size to 8 KB if you want to try a
> small block size.  If you want a large block size, try 64 KB or 128 KB.
> Stefan


Zhi Yong Wu

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