On Tue, Feb 01, 2005 at 07:39:27AM +1100, Daniel Carosone wrote:
> On Mon, Jan 31, 2005 at 11:25:52AM -0800, Jason Thorpe wrote:
> > The issue is that the FFS code in these kernels needs to be aware of 
> > snapshots that may already exist on the file systems.
> 
> Hm, so there's some new on-disk format the new kernel must recognise -
> though perhaps only if I've actually created a snapshot? What are the
> consequences if the kernel doesn't recognise this new format?  What if
> I boot an older kernel on this machine?  As we saw with the superblock
> upgrade issues, that happens more often that we might expect, though
> this time hopefully it only affects people actually making snapshots.

If you boot an older kernel and mount a file system with snapshots:

- If the kernel is at least 2.0 the snapshots will not be updated
  and therefore miss changes made to the file system.

- Accessing the snapshot file on older kernels might panic because
  snapshots use block numbers 0 and 1.

> If the consequences are nasty, should we do something like raise the
> FFS version number on disk, once snapshots have been created, if
> there's no other way to make these filesystems safe from older
> kernels?
> 
> Space issues aside, having snapshots in INSTALL kernels could be
> _very_ useful for those using snapshots and iso's to upgrade systems
> (which is the only time the compatibility question will arise, of
> course).  Not that it should be a substitute for a proper backup
> before the upgrade, but lets face it, many people won't do one.
> Teaching sysinst to offer the option of making such a snapshot before
> upgrading would be a seriously useful end-user feature.
> 
> Now, how do I actually use internal snapshots?  I know about fss(4),
> but haven't come across docs for the latter.

Use fssconfig(8) and use a backing store on the same file system.

> --
> Dan.

-- 
Juergen Hannken-Illjes - hannken@eis.cs.tu-bs.de - TU Braunschweig (Germany)