preventing deadlocks in snapshot directories - unexplained

[Eric Anderson]

user wrote:
> On Fri, 13 Jan 2006, Eric Anderson wrote:
>
>   
>> What happens if you do:
>>
>> rm -f snap_directory1/snapshot
>> rmdir snap_directory1
>>
>> Does the same thing happen?
>>     
>
>
> I will try that as I try to reproduce the problem.
>
> I am aware that there are two "lags" when creating a snapshot - the lock
> that occurs in the snapshot directory for the entire course of
> creation/destruction, and also a short system-wide lag that occurs for a
> small portion of the time of creation/destruction.
>
> It is possible that on a heavily taxed system, the two could be confused
> for one another, which is possibly what happened when I noticed the
> one-deep directory "locking" when I deleted a two-deep directory snapshot.
>   

The second lag you are talking about is when writes are suspended to 
that particular filesystem, which is so brief, it's unlikely you noticed it.

> I think the main, conceptual question I am asking is this - what is the
> difference between these three strategies:
>
> #1:
>
> /.snap/snapshot1
> /.snap/snapshot2
> /.snap/snapshot3
>
> #2
>
> /.snap/snapdir1/snapshot1
> /.snap/snapdir2/snapshot2
> /.snap/snapdir3/snapshot3
>
> #3
>
> /.snap/snapshot1
> /.snap2/snapshot2
> /.snap3/snapshot3
>
>
> Now, so far in this thread, some have claimed that #2 and #3 are
> identical, which seems reasonable.  My only problem is that I _think_ I am
> seeing problems with #2.  We'll see.
>
> However, what about #1 above ?  In that case, all of the snapshots are in
> one directory, and I feel that this is dangerous, or at least poorly done
> - isn't there a very real possibility of a deadlock there, because you
> could be creating a snapshot in /.snap at the same time as you delete one
> (or create a different one ?)
>
> The man page says nothing about multiple snapshots, etc., and I want to
> know if, one way or another, it is a good instinct on my part to make sure
> multiple snapshots do not end up in the exact same directory...
>   

I'm not certain why it hangs for you - seems like I've been through 
this, and I could do some testing also next week.

Eric



-- 
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Eric Anderson        Sr. Systems Administrator        Centaur Technology
Anything that works is better than anything that doesn't.
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