linux-kernel - Re: RFC: Self-snapshotting in Linux

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Message-ID: <da08e304-eb95-4ace-a069-de1a57b86e9f@e39g2000hsf.googlegroups.com>
Date:	Wed, 16 Apr 2008 03:27:26 -0700 (PDT)
From:	Alan Jenkins <alan-jenkins@...fmail.co.uk>
To:	Ben Nizette <bn@...sdigital.com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: RFC: Self-snapshotting in Linux

Ben Nizette wrote:
> On Wed, 2008-04-16 at 16:44 +0800, Peter Teoh wrote:
> > Essentially, to reiterate the key idea:   able to snapshot the current
> > kernel+userspace permanent.....restore from another snapshot....and
> > then switch back again if needed etc.....will the implementation be
> > difficult...if not impossible????
> >
>
> As I see it the main thing is that VMWare doesn't have to worry about
> trying to put hardware in to (and pull out of) low power modes.  VMWare
> hardware is never left in an undefined state by poorly written drivers
> etc.
>
> I think hibernation is about what you want; snapshotting as you describe
> it should fall down for about the same reasons

I guess there is the hardware / drivers issue.  I would like to claim
I've found hibernation to be reliable but unfortunately that's not
100% true.  As you say, that's inherent to snapshotting on
unvirtualised hardware - calling it snapshotting instead of
hibernation wouldn't change anything.

I think VMware's real advantages are more about management and
convenience.
1) Disk virtualisation.  It's much more convenient to create a VMware
image than a new linux partition.   Linux will let you use disk images
on a filesystem using the loopback driver, but that tends not to be
well supported by installers.  wubi[1] is the obvious counterexample
here.
2) Linux hibernation does the in-memory snapshot OK, but the
*filesystem* obviouslly isn't a part of that snapshot.  In some
unfortunate circumstances, you can end up resuming a memory image
which is not consistent with the filesystem and corrupting it.
IMHO filesystems need to include a "last modified" timestamp and check
it on resume.

[1] http://wubi-installer.org/
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