UFS2 Snapshots in 6.1-Beta4 - Confirmed Problems

[Kris Kennaway]

On Tue, Mar 21, 2006 at 01:36:44AM -0800, John Kozubik wrote:

> Here is the behavior I have witnessed:
> 
> 
> First, I have confirmed that a filesystem with multiple snapshots that
> undergoes multiple, rapid deletions of files, will cause the system to
> hang.  I have witnessed this before, but had not confirmed it or
> documented it in a PR.  Now that I have confirmed this behavior, I have
> documented it in: kern/94769
> 
> This is a serious problem because, in addition to making it nearly
> impossible to run a system with multiple snapshots, it is conceivable that
> enough rapid file deletions could occur on an otherwise non snapshotted
> system that has a single snapshot on it due to a background fsck, to cause
> the system to hang.

> Second, kern/92292 is still a problem.  I have reproduced this error in
> 6.1-BETA4 (and have seen it happening since 5.1).  The (small) difference
> is that the cp process seems to stick in the flswai state instead of
> biowr.

Thanks, I'll try to reproduce these two.

> This next one is complicated, and I haven't submitted a PR for it yet, but
> I believe it is quite serious for reasons I will expand on below.
> 
> The problem is:  If you completely fill a filesystem (109% usage in `df`
> on most systems) that has a snapshot on it, the system becomes very
> unresponsive - all interactive and disk response lags terribly and,
> although the system is not hung, it is in many cases unusable.

Yes :-( This will probably not be fixable in time for 6.1, but
hopefully it will be addressed in the future.  FYI, the system may
also panic in this situation.

Kris
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