From: Nir Soffer <nsoffer@redhat.com>
To: Kevin Wolf <kwolf@redhat.com>
Cc: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>,
"open list:Block layer core" <qemu-block@nongnu.org>,
Markus Armbruster <armbru@redhat.com>,
QEMU Developers <qemu-devel@nongnu.org>,
Max Reitz <mreitz@redhat.com>, Eric Blake <eblake@redhat.com>
Subject: Re: [PATCH v2 2/1] qemu-img: Add "backing":true to unallocated map segments
Date: Tue, 22 Jun 2021 19:56:32 +0300 [thread overview]
Message-ID: <CAMRbyyt5qsVan8dOF=HHvqqo92zwTBRucnfA_UEOOmRMqiyaAQ@mail.gmail.com> (raw)
In-Reply-To: <YNID9rbo+RdwklCf@redhat.com>
On Tue, Jun 22, 2021 at 6:38 PM Kevin Wolf <kwolf@redhat.com> wrote:
>
> Am 11.06.2021 um 21:03 hat Eric Blake geschrieben:
> > To save the user from having to check 'qemu-img info --backing-chain'
> > or other followup command to determine which "depth":n goes beyond the
> > chain, add a boolean field "backing" that is set only for unallocated
> > portions of the disk.
> >
> > Signed-off-by: Eric Blake <eblake@redhat.com>
> > ---
> >
> > Touches the same iotest output as 1/1. If we decide that switching to
> > "depth":n+1 is too risky, and that the mere addition of "backing":true
> > while keeping "depth":n is good enough, then we'd have just one patch,
> > instead of this double churn. Preferences?
>
> I think the additional flag is better because it's guaranteed to be
> backwards compatible, and because you don't need to know the number of
> layers to infer whether a cluster was allocated in the whole backing
> chain. And by exposing ALLOCATED we definitely give access to the whole
> information that exists in QEMU.
>
> However, to continue with the bike shedding: I won't insist on
> "allocated" even if that is what the flag is called internally and
> consistency is usually helpful, but "backing" is misleading, too,
> because intuitively it doesn't cover the top layer or standalone images
> without a backing file. How about something like "present"?
Looks hard to document:
# @present: if present and false, the range is not allocated within the
# backing chain (since 6.1)
And is not consistent with "offset". It would work better as:
# @present: if present, the range is allocated within the backing
# chain (since 6.1)
Or:
# @absent: if present, the range is not allocated within the backing
# chain (since 6.1)
This is used by libnbd now:
https://github.com/libguestfs/libnbd/commit/1d01d2ac4f6443b160b7d81119d555e1aaedb56d
But I'm fine with "backing", It is consistent with BLK_BACKING_FILE,
meaning this area exposes data from a backing file (if one exists).
We use "backing" internally to be consistent with future qemu-img.
next prev parent reply other threads:[~2021-06-22 16:58 UTC|newest]
Thread overview: 19+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-06-11 14:01 [PATCH v2] qemu-img: Make unallocated part of backing chain obvious in map Eric Blake
2021-06-11 14:35 ` Vladimir Sementsov-Ogievskiy
2021-06-11 14:59 ` Eric Blake
2021-06-11 18:13 ` Nir Soffer
2021-06-11 19:03 ` [PATCH v2 2/1] qemu-img: Add "backing":true to unallocated map segments Eric Blake
2021-06-15 8:54 ` Vladimir Sementsov-Ogievskiy
2021-06-15 13:09 ` Nir Soffer
2021-06-22 15:38 ` Kevin Wolf
2021-06-22 16:56 ` Nir Soffer [this message]
2021-06-23 8:57 ` Kevin Wolf
2021-06-23 13:58 ` Nir Soffer
2021-06-23 16:04 ` Kevin Wolf
2021-06-23 16:35 ` Nir Soffer
2021-06-28 17:42 ` Eric Blake
2021-06-29 7:23 ` Vladimir Sementsov-Ogievskiy
2021-06-29 14:40 ` Kevin Wolf
2021-06-29 15:53 ` Nir Soffer
2021-06-22 17:51 ` Vladimir Sementsov-Ogievskiy
2021-06-22 17:04 ` Nir Soffer
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