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

[Richard W.M. Jones]

On Fri, Aug 22, 2014 at 05:22:33PM +0200, Kevin Wolf wrote:
> It's still useful because it happens to reduce the overhead in most
> implementations and it's a relatively quick operation, but the best way
> I know of to actually _fully_ preallocate is still writing zeros. Which
> of the two the user wants, is a decision that qemu can't make for them.

This is a difficult situation.  Possibly the choice is between

 - efficiently make the file fully allocated, that works in the vast
   majority of cases, but don't go crazy (ie. fallocate)

 - really really try as hard as possible to make sure that future
   allocations will never fail (ie. write random non-zero data to the
   file)

Note that neither of these is the preallocation=... option as
specified in this patch.

Rich.

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