Re: [Qemu-devel] Why qemu write/rw speed is so low?

[Christoph Hellwig]

On Fri, Sep 09, 2011 at 04:04:07PM +0200, Kevin Wolf wrote:
> > 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.

The might not be a need for RMW, but if you're doing 512 byte writes to a
sparse file on 4k filesystem that filesystem will have to serialize the
I/O to prevent races from happening during block allocation.