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Re: [Qemu-devel] [PATCH v7 3/6] qcow2: Reduce REFT_OFFSET_MASK

From: Kevin Wolf
Subject: Re: [Qemu-devel] [PATCH v7 3/6] qcow2: Reduce REFT_OFFSET_MASK
Date: Fri, 29 Jun 2018 17:43:08 +0200
User-agent: Mutt/1.9.1 (2017-09-22)

Am 29.06.2018 um 17:22 hat Eric Blake geschrieben:
> On 06/29/2018 03:44 AM, Kevin Wolf wrote:
> > Am 28.06.2018 um 21:07 hat Eric Blake geschrieben:
> > > Match our code to the spec change in the previous patch - there's
> > > no reason for the refcount table to allow larger offsets than the
> > > L1/L2 tables.
> > 
> > What about internal snapshots? And anyway, because of the metadata
> > overhead, the physical image size of a fully allocated image is always
> > going to be at least minimally larger than the virtual disk size.
> > 
> > I'm not necessarily opposed to making the change if there is a good
> > reason to make it, but I don't see a real need for it and the
> > justification used here and also in the previous patch is incorrect.
> The fact that ext4 cannot hold an image this large is already an indication
> that setting this limit on the refcount table is NOT going to bite real
> users.
> Yes, you can argue that with lots of metadata, including internal snapshots,
> and on a capable file system (such as tmpfs) that can even hold files this
> large to begin with, then yes, allowing the refcount to exceed this limit
> will allow slightly more metadata to be crammed into a single image.  But
> will it actually help anyone?
> Do I need to respin the series to split patch 2 into the obvious changes
> (stuff unrelated to capping refcount size) vs. the controversial stuff
> (refcount cap and this code change)?
> > 
> > Kevin
> > 
> > > In practice, no image has more than 64PB of
> > > allocated clusters anyways, as anything beyond that can't be
> > > expressed via L2 mappings to host offsets.
> If you're opposed to an exact 56-bit limit on the grounds that 56-bit guest
> data plus minimal metadata should still be expressable, would it be better
> to cap refcount at 57-bits?

You're arguing that the patch doesn't hurt in practice, but what I'm
really looking for is what do we gain from it?

We generally don't apply patches just because they don't hurt, but
because they are helpful for something. In the simple, fully compatible
cases "helpful" could just mean "we like the code better".

This is a slightly different category: "it's technically incompatible,
but we don't think anyone uses this". We have done such changes before
when they allowed us to improve something substantially, but I think the
requirements for this are at least a little higher than just "because we
like it better". And I'm not even sure yet if I like it better because
it seems like an arbitrary restriction.

So apart from not hurting in practice, what does the restriction buy us?


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