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

From: Eric Blake
Subject: Re: [Qemu-devel] [RFC] Proposed qcow2 extension: subcluster allocation
Date: Tue, 11 Apr 2017 10:30:03 -0500
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.8.0

On 04/11/2017 10:18 AM, Max Reitz wrote:

> Hm, yeah, although you have to keep in mind that the padding is almost
> pretty much the same as the the data bits we need, effectively doubling
> the size of the L2 tables:
> padding = 2^{n+2} - 2^{n+1} - 64 (=2^6)
>         = 2^{n+1} - 64
> So that's not so nice, but if it's the only thing we can do...

Or we mix-and-match your ideas: since our subclusters are ternary
encoding, if you want 128 subclusters, instead of asking for 256 bits,
you ask for ld(3^128) = 203 bits, then use 11 padding bits of your
original 64, and you have something that still fits in 256 bits instead
of 512...

Okay, just kidding.  Anything larger than 32 subclusters does get
awkward fast.

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

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