another interesting idea (zfs based upgrades)

[Dave Airlie]

On Thu, 2009-03-26 at 17:19 -0400, James Antill wrote:
> On Thu, 2009-03-26 at 20:52 +0200, Jonathan Dieter wrote:
> > On Thu, 2009-03-26 at 14:30 -0400, Seth Vidal wrote:
> > > 
> > > On Thu, 26 Mar 2009, Neal Becker wrote:
> > > 
> > > > http://tech.xerces.com/unbreakable-upgrades-zfs-and-apt-get.geek
> > > >
> > > 
> > > Here's how this would work in the yum world:
> > > 
> > > 1. if any package from @core (or any of their deps) are being upgraded 
> > > then run an lvm snapshot and store it/add it to grub
> > > 2. proceed with update
> > > 
> > > If anyone would like to write the lvm snapshot rootfs and store/add it to 
> > > grub I can write the yum plugin to do the rest.
> > > 
> > > It's not radically exciting code.
> > > 
> > > In yum you wouldn't even need to stop the yum update command and ask the 
> > > user to run a separate program.
> > 
> > My experience with lvm snapshotting was that it slowed down disk speeds
> > significantly.  I would expect that filesystem based snapshotting (which
> > btrfs will have) might be more efficient.  Or we could make sure the
> > snapshot is only left long enough to make sure the updates work.
> 
>  IMNSO the problem isn't the speed hit, it's the fact that you are using
> a file system snapshot (or LVM block layer snapshot, same difference) as
> a file system transaction.
>  This is completely insane. Imagine if mysql implemented transactions by
> doing a copy of the entire DB, and then for rollback just moved back to
> the old version (if you don't see the problem, think about more than one
> connection/transaction at once).
> 
>  snapshot + rollback can work in some situations, and the yum developers
> will be happy to provide some help to people who want to make this
> easier to achieve. However even if all the tools made this as automated
> as possible, you have to know what you are doing. So, again IMNSHO, this
> can't be a general/default solution.

Better off having yum ptrace every install process, trap any file
interactions, and back those files up into a transaction rollback, also
take a list of new files created.

Dave.