Re: [Qemu-devel] [PATCH v12 0/6] qcow2, raw: add preallocation=full and preallocation=falloc

[Daniel P. Berrange]

On Fri, Aug 22, 2014 at 01:25:56PM +0100, Richard W.M. Jones wrote:
> 
> On Mon, Jul 28, 2014 at 04:48:46PM +0800, Hu Tao wrote:
> > ping...
> > 
> > All the 6 patches have reviewed-by now.
> > 
> > On Fri, Jul 11, 2014 at 02:09:57PM +0800, Hu Tao wrote:
> > > This series adds two preallocation mode to qcow2 and raw:
> > > 
> > > Option preallocation=full preallocates disk space for image by writing
> > > zeros to disk, this ensures disk space in any cases.
> > > 
> > > Option preallocation=falloc preallocates disk space by calling
> > > posix_fallocate(). This is faster than preallocation=full.
> 
> Sorry if this was discussed before, but why would anyone use
> preallocation=full if preallocation=falloc was possible?
> 
> Shouldn't preallocation=full simply use posix_fallocate if it's
> available, and fall back to writing zeroes if not?

posix_fallocate() will itself fallback to writing zeros to
disk if the filesystem can't do the fast allocation method.
So I can't see any reason to have separate options for this.

Just have a preallocation=full option which - at compile
time - chooses between posix_fallocate and write(). This
is what we've done in libvirt for many years now and no
one has ever asked for more

http://libvirt.org/git/?p=libvirt.git;a=blob;f=src/util/virfile.c;h=b6f5e3f2cbbcc6768d764009b238b2349bee3fee;hb=HEAD#l1037

We actually try to use mmap() before going to write() since
that is slightly faster.

http://log.amitshah.net/2009/03/comparison-of-file-systems-and-speeding-up-applications/

Regards,
Daniel
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