backup strategies

Paul Mather paul at
Mon Oct 31 11:38:18 PST 2005

On Mon, 31 Oct 2005 18:59:21 +0100, Csaba Henk <csaba-ml at> wrote:

> On Mon, Oct 31, 2005 at 10:32:02AM -0600, Eric Schuele wrote:
> > The online manual mentions it in 16.13.  Wouldn't hurt for it to be
> in 
> > the man page as well.
> Oh, yeah, thanks.
> This makes things clear. I missed this somehow.
> > AFAIK FreeBSD 5.0+. Other *BSD as well, i believe... but someone
> else 
> > would have to answer that.
> > 
> > >(AFAIK, softupdates is supported also in other members of the BSD
> > >family, yet the NetBSD dump manpage didn't have such a -L flag...)
> OK, as I understand now, softupdates might be available for other
> BSD-s,
> but snapshotting is not a "free bonus" which comes with softupdates,
> but
> a new innovation based on that... and is a true FBSD innovation. So
> I'd
> guess it's still a unique thing.

Not quite: NetBSD also features softupdates and also supports snapshots
(though I don't know how stable it is, as I've never tried it on my
NetBSD system).  The snapshot interface under NetBSD is different from
that on FreeBSD: you can create a snapshot "device" that can be used to
snapshot a file system.  (You can then mount or dump the snapshot device
to get a consistent image/backup of the filesystem being snapshotted.)
The main difference appears to be you are not limited to snapshots
residing on the same file system of which you are taking a snapshot,
which could be handy for near-full active filesystems.  See fss(4) and
fssconfig(8) man pages under NetBSD for details.

The other thing to note about FreeBSD snapshots that I don't think has
been mentioned is that they are only supported on UFS2 filesystems,
meaning they are unavailable under FreeBSD 4.x and earlier (or on older
filesystems created by those older versions of FreeBSD).

I've been using snapshots under FreeBSD 5 onwards, pretty much since the
feature became available, and have only ever had a problem once (due to
a race condition in the snapshot code, it was surmised), and that was
with a nightly automated network backup of snapshots of all filesystems
on a system with high disk I/O.  I find it to be a really valuable


e-mail: paul at

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