UFS2 snapshots on large filesystems

user user at dhp.com
Sun Nov 6 21:24:56 PST 2005

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 ?

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 ?


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