From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([140.186.70.92]:47525) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1R21dw-0007G0-CO for qemu-devel@nongnu.org; Fri, 09 Sep 2011 10:01:21 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1R21dq-0005DI-Jr for qemu-devel@nongnu.org; Fri, 09 Sep 2011 10:01:20 -0400 Received: from mx1.redhat.com ([209.132.183.28]:63853) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1R21dq-0005DE-7Y for qemu-devel@nongnu.org; Fri, 09 Sep 2011 10:01:14 -0400 Message-ID: <4E6A1CD7.70300@redhat.com> Date: Fri, 09 Sep 2011 16:04:07 +0200 From: Kevin Wolf MIME-Version: 1.0 References: <20110909094436.GB23929@f15.cn.ibm.com> <20110909103801.GA26148@stefanha-thinkpad.localdomain> In-Reply-To: Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Subject: Re: [Qemu-devel] Why qemu write/rw speed is so low? List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Stefan Hajnoczi Cc: aliguro@us.ibm.com, Stefan Hajnoczi , Zhi Yong Wu , qemu-devel@nongnu.org, Zhi Yong Wu , ryanh@us.ibm.com Am 09.09.2011 15:54, schrieb Stefan Hajnoczi: > On Fri, Sep 9, 2011 at 2:48 PM, Zhi Yong Wu wrote: >> On Fri, Sep 9, 2011 at 6:38 PM, Stefan Hajnoczi >> 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? > > If the host file system manages space in 4 KB blocks, then a 512 byte > to an unallocated part of the file causes the file system to find 4 KB > of free space for this data. Since the write is only 512 bytes and > does not cover the entire 4 KB region, the file system initializes the > remaining 3.5 KB with zeros and writes out the full 4 KB block. > > Now if a 512 byte write comes in for an allocated 4 KB block, then we > need to read in the existing 4 KB, modify the 512 bytes in place, and > write out the 4 KB block again. This is read-modify-write. In this > worst-case scenario a 512 byte write turns into a 4 KB read followed > by a 4 KB write. But that should only happen with a 4k sector size, otherwise there's no reason for RMW. Kevin