three follow-up questions RE: UFS2 snapshots on large filesystems

Eric Anderson anderson at
Fri Nov 11 09:29:07 PST 2005

user wrote:
> thank you scott - see below:
> On Fri, 4 Nov 2005, Scott Long wrote:
>>The UFS snapshot code was written at a time when disks were typically 
>>around 4-9GB in size, not 400GB in size =-)  Unfortunately, the amount
>>of time it takes to do the initial snapshot bookkeeping scales linearly
>>with the size of the drive, and many people have reported that it takes
>>considerable amount of time (anywhere from several minutes to several 
>>dozen minutes) on large drives/arrays like you describe.  So, you should
>>test and plan accordingly if you are interested in using them.
> Testing is what I need to do.  I have a few follow up questions:
> First, are there any sysctl or kernel tunables that change any of what you
> are discussing above ?
> Second, let's say I am willing to accept the long snapshot creation period
> ... are there other drawbacks as well during the course of _running with_
> the snapshot once it is created ?  Or are all costs paid initially ?

There will be a slight performance penalty with snapshots on the 
filesystem, but it shouldn't be noticeable.

> Finally, I have read the bsdcon3 paper that mccusick wrote where he
> addressed the dual problems of not enough kernel memory (10 megabytes) to
> cache disk pages, and the system deadlocking that occurs with two
> snapshots.  Is it true that both of the fixes he elucidated in that paper
> are built into what I see as fbsd 5.4 now ?

I've seen deadlocking on 5.4, however some recent work on the filesystem 
and snapshots went into 6.0 and -CURRENT, and since then, I've seen no 
issues.  I highly recommend 6.0 for this.


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

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