Making UFS snapshots

[Daniel Feenberg]

On Thu, 18 Aug 2005, Giorgos Keramidas wrote:

> On 2005-08-17 16:32, Daniel Feenberg <feenberg at nber.org> wrote:
> >
> > I notice on this list that Garance Drosehn
> > <http://docs.FreeBSD.org/cgi/mid.cgi?p06230924bf1c752ccf7f> reports making
> > a snapshot of a 4 gigabyte filesystem in less than one second. We have a
> > 859 gigabyte filesystem and snapshots take about 75 minutes to complete.
> 
> Making a snapshot is not very slow if the disk is relatively idle at the
> time.  Perhaps this is what's biting you?

The computer and the disk system is otherwise idle - no activity other
than taking the snapshot.

Since the original posting I found Dr McKusik's 1999 Usenix paper
describing snapshots which suggests the time for taking a snapshot should
be "brief", and that file system activity should resume after a time no
longer than that required for an unmount. This suggests to me that
something is wrong with our setup, but I still have no idea what.

However, I have found some messages from users with experience similar to
ours e.g.

http://www.mail-archive.com/freebsd-stable@freebsd.org/msg67320.html


Dan Feenberg
feenberg isat nber dotte org
617-588-0343


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