mksnap_ffs woes

[Kris Kennaway]

On Wed, Mar 30, 2005 at 05:08:24PM +0100, Andrea Venturoli wrote:
> Kris Kennaway wrote:
> 
> >mksnap_ffs may sometimes take a long time (order of tens of minutes)
> >to generate the snapshot.  During this time, other writes to the
> >filesystem may be suspended.  Are you sure this isn't what you're
> >seeing?
> 
> I am not sure, but I don't think so.
> 
> First of all, while I would not expect an exactly deterministic 
> behaviour, I still find it strange that most of the times it works 
> within seconds, and occasionally it might need so long! Then again, I do 
> not have enough insight to explain why it would or why it shouldn't.

It's possible you're seeing deadlock bugs in the snapshot code; you'd
need to compile your system with DDB support and obtain traceback
information as described in the developers' handbook chapter on kernel
debugging.

> Lastly, if what you say is true, that "writes to the filesytem may be 
> suspended" for that long, then I don't see much point in using this 
> feature. At least, it is not one that suits *my* needs: our clients must 
> be up 24/7; if that happens they will time out and someone will need to 
> go there and powercycle them.

The snapshot code was intended to support background fsck and that
alone.  It's also optionally used by the dump code, but it was not
written as a general-purpose live filesystem snapshot service.

> Am I going in the wrong direction then?
> Are there other alternatives for live backups?

Plenty, if you don't demand an instantaneous image of the backup
image.

> Is there some sort of more detailed documentation (tutorials, howtos, 
> ...) on this topic?

The pros and cons of the snapshot code have been discussed on the
mailing lists, and there is a (slightly outdated) information file
here: /usr/src/sys/ufs/ffs/README.snapshot

Kris
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