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

[Kevin Wolf]

Am 22.08.2014 um 14:25 hat Richard W.M. Jones geschrieben:
> 
> 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() is basically metadata preallocation on the file
system level. If any lower levels involve allocations as well, does
posix_fallocate() allocate them there?

Kevin