Re: [Qemu-devel] [RFC] Proposed qcow2 extension: subcluster allocation

[Eric Blake <eblake@redhat.com>]

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On 04/11/2017 09:59 AM, Max Reitz wrote:

> 
> Good point, but that also means that (with (2)) you can only use
> subcluster configurations where the L2 entry size increases by a power
> of two. Unfortunately, only one of those configurations itself is a
> power of two, and that is 32.
> 
> (With 32 subclusters, you take up 64 bits, which means an L2 entry will
> take 128 bits; with any higher 2^n, you'd take up 2^{n+1} bits and the
> L2 entry would take 2^{n+1} + 64 which is impossible to be a power of two.)

Or we add padding. If you want 64 subclusters, you burn 256 bits per
entry, even though only 192 of those bits are used.

> 
> I don't know how useful non-power-of-two subcluster configurations are.
> Probably not at all.
> 
> Since using subcluster would always result in the L2 table taking more
> than 512 bytes, you could therefore never guarantee that there is no
> entry overlapping a sector border (except with 32 subclusters).

Yes, there's definite benefits to keeping whatever structure we end up
with aligned so that it naturally falls into sector boundaries, even if
it means more padding bits.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org


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