Date:	Thu, 2 Dec 2010 11:23:07 +0100 (CET)
From:	Lukas Czerner <lczerner@...hat.com>
To:	Amir Goldstein <amir73il@...il.com>
cc:	Lukas Czerner <lczerner@...hat.com>, tytso@....edu,
	adilger@...ger.ca, sandeen@...hat.com, linux-ext4@...r.kernel.org
Subject: Re: [PARCH 0/1 RFC] e2image: Add support for QCOW2 image format

On Thu, 2 Dec 2010, Amir Goldstein wrote:

> On Thu, Dec 2, 2010 at 10:23 AM, Lukas Czerner <lczerner@...hat.com> wrote:
> > On Wed, 1 Dec 2010, Amir Goldstein wrote:
> >
> >> On Wed, Dec 1, 2010 at 7:41 PM, Lukas Czerner <lczerner@...hat.com> wrote:
> >> >
> >> > QCOW2 format supports other neat features like compression, encryption and
> >> > snapshots. None of those feature are supported in this patch and the question
> >> > is, do we need it ? I can imagine for example snapshots to be useful for
> >> > roll-back changes made by e2fsck, but this is hardly problem of e2image itself.
> >> >
> >> > So, please look at the patch, try it and let me know what else you thing is
> >> > useful to implement, or what would you like to change.
> >> >
> >>
> >> Hi Lukas,
> >>
> >> This is very cool :-)
> >> My wish is to export next3/ext4 snapshots as qcow2 snapshots.
> >> I actually thought of implementing e2image -x <snapid> as a way to
> >> export a snapshot image,
> >> but using qcow2, file system can be exported with all of it's snapshots.
> >> It should be quite trivial since next3 snapshots contain a map of changed blocks
> >> and I suppose qcow2 snapshots should be the same.
> >> Now I only need to find the time to do it...
> >>
> >> Amir.
> >
> > Hi Amir,
> >
> > I seems like a cool feature, so when QCOW2 infrastructure is in place
> > adding snapshots to it should not be a big deal (not that QCOW2
> > infrastructure is :)), however I was wondering are you planning to
> > export only metadata, or even data itself ?
> >
> 
> There are 2 applications I can think of for exporting snapshots.
> One needs only the metadata and the other requires the data as well.
> 
> 1. Rollback to snapshot (metadata only)
> ----------------------------------------------------------
> Unlike ZFS/Btrfs, there is no inherent way to "rollback to snapshot"
> with Ext4 snapshots.
> Instead, all changed metadata needs to be copied over from snapshot.
> The only plausible way to do this is to un-mount the file system (or
> re-mount read-only),

So, you're saying that it would not be possible to implement in-kernel
snapshot roll-back for ext4 with snapshots ? I find this hard to
believe, because we can always freeze the filesystem a do whatever we
want to do, or am I missing something ? If it would be possible this
would be much better solution then exporting and restoring it from
QCOW2 image.

> use e2image to export a metadata image of a snapshot to a different
> location and then
> overwrite the filesystem with snapshot metadata (data blocks are
> already in-place).
> 
> This method can be applied today, even without qcow2 support, but with
> qcow2 snapshots,
> you can generate a single e2image, which can be used to rollback to
> any snapshot and to
> restore the original filesystem (as long as the rolled back version
> wasn't modifed).
> 
> 2. Filesystem replication (data + metadata)
> --------------------------------------------------------------
> This application is inspired by ZFS replication:
> http://wikitech-static.wikimedia.org/articles/z/f/s/Zfs_replication.html
> The idea is to start with a remote replica by transferring a full copy
> of snapshot N1
> and then push incremental differences N1..N2 to roll the replica to snapshot N2.
> So if e2image has the ability to export a full snapshot image (including data)
> and the capability to export incremental qcow2 snapshots, those could
> be transferred
> to the remote location and be used to create and update the replicated
> filesystem.

I am sorry but I do not really get it, can't you just export
filesystem data + metadata into qcow2 image without any qcow2 snapshot
functionality and then recreate the whole filesystem from this image ?
It seems to me that you do not need qcow2 snapshot functionality to do
this, however I might be missing something.

> 
> So to answer your original question ("what else you thing is useful to
> implement"),
> e2image -d would be nice (export data blocks).

This is really easy thing to do, so I can do this :).

Thanks!

-Lukas

> 
> Amir.
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