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 So we are not performing 512 byte writes. Some layer is changing the I/O pattern. Stefan
