Re: [Qemu-devel] [PATCH v13 5/6] raw-posix: Add full preallocation option

[Kevin Wolf]

Am 04.09.2014 um 15:07 hat Richard W.M. Jones geschrieben:
> On Thu, Sep 04, 2014 at 02:52:57PM +0200, Kevin Wolf wrote:
> > Am 04.09.2014 um 14:45 hat Richard W.M. Jones geschrieben:
> > > On Thu, Sep 04, 2014 at 02:35:22PM +0200, Kevin Wolf wrote:
> > > > Please change the code to always write zeros for FULL,
> > > 
> > > How is this useful for anyone?  You don't know if the underlying SAN
> > > is going to detect these zeroes or combine these blocks together.
> > > It's just slow for no reason.
> > 
> > It's slow for the reason that the user has requested it. Do you doubt
> > that users can know what their backend is doing? Why are you insisting
> > on providing only the functionality that you personally need?
> 
> I'm not!  I'm trying to make sure we don't end up with a qemu
> interface which is useless for higher layers.  You're proposing
> preallocation=full which will be slow but not actually give any
> guarantees, or preallocation=meta which is going to be fast but may
> not work, and I'm saying that's a dumb interface that's not useful.

So what you propose is an interface that combines both and may
unpredictably be slow or fast, and doesn't give any guarantees either.
Why would this be any better?

What is your specific use case of full preallocation that wants zero
writing, but only implicitly as a fallback? My expectation is that most
users want cheap preallocation if it's available, but don't bother to
write many gigabytes if it isn't.

Kevin