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Re: [Qemu-devel] [PATCH] vmdk: Fix converting to streamOptimized

From: Kevin Wolf
Subject: Re: [Qemu-devel] [PATCH] vmdk: Fix converting to streamOptimized
Date: Tue, 26 Jan 2016 10:22:28 +0100
User-agent: Mutt/1.5.21 (2010-09-15)

Am 26.01.2016 um 04:16 hat Fam Zheng geschrieben:
> On Mon, 01/25 12:16, Kevin Wolf wrote:
> > Am 25.01.2016 um 03:26 hat Fam Zheng geschrieben:
> > > Commit d62d9dc4b8 lifted streamOptimized images's version to 3, but we
> > > now refuse to open version 3 images read-write.  We need to make
> > > streamOptimized an exception to allow converting to it. This fixes the
> > > accidentally broken iotests case 059 for the same reason.
> > > 
> > > Signed-off-by: Fam Zheng <address@hidden>
> > 
> > How different are version 3 images for other subformats? Are we
> > arbitrarily restrictring their use or is it really that they don't work
> > with our driver? And if they don't work with our driver, are we sure
> > that streamOptimized images can't use the features we don't support?
> > 
> > Or is the version defined per subformat and doesn't necessarily exist
> > for other types?
> Version 3 images are undocumented except in the VMware KB article mentioned in
> the comment around this line (http://kb.vmware.com/kb/2064959). A few years
> ago, when users complained that QEMU doesn't support version 3 images, we
> presumed from the article that reading is okay, as the new feature is
> "persistent changed block tracking" (although it didn't say it is the only
> feature enabled by version 3), and went ahead enabling it.
> This time, it seems newer VMware products only accept version 3 if the
> subformat is streamOptimized.  Again, without any documentation/specification
> update. Then our users complains again, so we add another exception to
> mitigate. As this subformat doesn't allow overwrite, the only use case is
> qemu-img converting to it.  So this is pretty safe - it's always operating a
> new image - and the approach is tested by multiple users (both upstream and
> downstream).

I see. Then I guess we can't do much else.

Thanks, applied to the block branch.


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