Re: [Xen-devel] A snapshot is not (really) a cow

[Christian Limpach <Christian.Limpach@xxxxxxxxxxxx>]

On Mon, Sep 27, 2004 at 02:35:49PM +0100, Peri Hankey wrote:
> Although I did get quite a lot wrong in my analysis, I come back to the 
> point that as lvm2 creates additional snapshots of a the same original, 
> it adds them to a list of snapshots of that original. If something goes 
> wrong while a new snapshot is being added, the effect in my experience 
> is pretty bad: all of the other snapshots of the same original become 
> unusable and you lose control of xenU domains that have their root 
> filesystems in those snapshots.
> 
> There may be a bug in lvm2 in the handling of error conditions when 
> adding a new snapshot. But  if there are linkages between snapshots of 
> the same original it is going to be pretty hard to guarantee that the 
> snapshots are properly independent of each other and proof against this 
> kind of snarlup.
> 
> You can see what I mean if you look at the code:

As said before, this is needed to allow writes to the original and it
has to be a bug in error handling if snapshot creation failing renders
all snapshots of the same original lv unusable.

It seems to me that you have 2 options:
- fix the error handling
- create a new snapshot module which doesn't allow writes to the
  original

I don't see the benefits of the 2nd option, but nobody's stopping you ;-)

    christian



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