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

[Kevin Wolf]

Am 22.08.2014 um 17:34 hat Richard W.M. Jones geschrieben:
> 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.

Isn't the first one exactly preallocation=falloc and the second is
preallocation=full, except that we're not writing non-zero blocks? (And
probably shouldn't, because that would change the content.)

Kevin