On Tue, Sep 13, 2011 at 3:15 PM, Stefan Hajnoczi <stefa...@linux.vnet.ibm.com> wrote: > On Tue, Sep 13, 2011 at 10:38:28AM +0800, Zhi Yong Wu wrote: >> On Fri, Sep 9, 2011 at 6:38 PM, Stefan Hajnoczi >> <stefa...@linux.vnet.ibm.com> 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. >> > >> > 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 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. >> 8,2 0 1 0.000000000 337 A WS 425081504 + 8 <- >> (253,1) 42611360 > > 8 blocks = 8 * 512 bytes = 4 KB How do you know each block size is 512 bytes?
> > So we are not performing 512 byte writes. Some layer is changing the > I/O pattern. > > Stefan > -- Regards, Zhi Yong Wu
