From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([140.186.70.92]:43696) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1R21XR-0005YU-Ei for qemu-devel@nongnu.org; Fri, 09 Sep 2011 09:54:39 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1R21XQ-0004HG-A3 for qemu-devel@nongnu.org; Fri, 09 Sep 2011 09:54:37 -0400 Received: from mail-gy0-f173.google.com ([209.85.160.173]:59652) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1R21XQ-0004H7-5c for qemu-devel@nongnu.org; Fri, 09 Sep 2011 09:54:36 -0400 Received: by gye5 with SMTP id 5so1899985gye.4 for ; Fri, 09 Sep 2011 06:54:35 -0700 (PDT) MIME-Version: 1.0 In-Reply-To: References: <20110909094436.GB23929@f15.cn.ibm.com> <20110909103801.GA26148@stefanha-thinkpad.localdomain> Date: Fri, 9 Sep 2011 14:54:35 +0100 Message-ID: From: Stefan Hajnoczi Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: quoted-printable 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: Zhi Yong Wu Cc: kwolf@redhat.com, aliguro@us.ibm.com, Stefan Hajnoczi , Zhi Yong Wu , qemu-devel@nongnu.org, ryanh@us.ibm.com 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 wri= te and rw speed is so low now, my qemu binary is built on commit 344eecf699= 5f4a0ad1d887cec922f6806f91a3f8. >>> >>> Do qemu have regression? >>> >>> The testing data is shown as below: >>> >>> 1.) write >>> >>> test: (g=3D0): rw=3Dwrite, bs=3D512-512/512-512, ioengine=3Dlibaio, iod= epth=3D1 >> >> Please post your QEMU command-line. =A0If your -drive is using >> cache=3Dwritethrough then small writes are slow because they require the >> physical disk to write and then synchronize its write cache. =A0Typicall= y >> cache=3Dnone is a good setting to use for local disks. > Now i can not access my workstation in the office. > -drive if=3Dvirtio,cache=3Dnone,file=3Dxxxx > >> >> The block size of 512 bytes is too small. =A0Ext4 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. Stefan