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

From: Christoph Hellwig
Subject: Re: [Qemu-devel] Why qemu write/rw speed is so low?
Date: Sun, 11 Sep 2011 15:32:54 +0200
User-agent: Mutt/1.5.17 (2007-11-01)

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.

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