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. 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. OK > > Stefan > > -- Regards, Zhi Yong Wu
