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Re: [Qemu-block] [Qemu-devel] Persistent bitmaps for non-qcow2 formats


From: Fam Zheng
Subject: Re: [Qemu-block] [Qemu-devel] Persistent bitmaps for non-qcow2 formats
Date: Tue, 5 Sep 2017 21:18:58 +0800
User-agent: Mutt/1.8.3 (2017-05-23)

On Tue, 09/05 15:01, Kevin Wolf wrote:
> Am 28.08.2017 um 04:57 hat Fam Zheng geschrieben:
> > On Fri, 08/25 15:44, Max Reitz wrote:
> > > Well, OK.  The main argument against supporting anything but qcow2 is
> > > "if you want features, use qcow2; and we are working on making qcow2 as
> > > fast as possible."  I think that's a very good argument still.  At some
> > > point I (and probably others, too) had the idea of making qcow2 files in
> > > raw layout: 
> > 
> > Yes! I think this idea makes a whole lot of sense, too. Metadata tables can 
> > be
> > generated so old implementation can still use it.
> 
> Maybe a nice way to attack this would be "huge pages", i.e. have a L1
> entry flag that tells "this points directly to a huge cluster instead of
> an L2 table". Gives us 512 MB clusters with a 64k cluster size, or a
> maximum of 512 GB clusters with a 2 MB cluster size.
> 
> Huge clusters would only be used by qemu-img create if the respective
> option is given, and only with preallocation.

Then this image is not usable on old QEMUs, right? So this is going to be an
incompatible_feature, whereas with the linear mapping L1/L2 tables approach, it
can be an autoclear_feature.

Fam



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