UFS2 Snapshot limitiations

[Bill Moran]

In response to "Pietro Cerutti" <pietro.cerutti at gmail.com>:

> On 3/13/07, Bill Moran <wmoran at potentialtech.com> wrote:
> > In response to "Pietro Cerutti" <pietro.cerutti at gmail.com>:
> >
> > > On 3/13/07, Doug Poland <doug at polands.org> wrote:
> > > >
> > > > On Tue, March 13, 2007 10:10, Bill Moran wrote:
> > > > > In response to "Doug Poland" <doug at polands.org>:
> > > > >
> > > > >> Hello,
> > > > >>
> > > > >> I seem to recall, but cannot find, the limitation on the number of
> > > > >> snapshots allowed on UFS2.  Could someone point me in the right
> > > > >> direction please?
> > > > >
> > > > > The handbook has it:
> > > > > http://www.freebsd.org/doc/en_US.ISO8859-1/books/handbook/snapshots.html
> > > > >
> > > > Thank you very much.
> > > >
> > >
> > > Sorry for joining the thread, but it's raised my curiosity: what's the
> > > reason for this limit?
> >
> > I'm not 100% sure, but I believe it's because snapshot information is stored
> > in the filesystem superblock, and that's all that can fit in the space
> > available in the superblock.
> 
> This is the same thing that came to my mind, but in the handbook page I found:
> "Active snapshots are recorded in the superblock so they are
> persistent across unmount and remount operations along with system
> reboots."
> 
> 1) what an "active" snapshot is?

I think you're trying too hard to read into this.  Active snapshot, as opposed
to a snapshot that has been deleted/unmounted and existed in the past, is what
I took it to mean.

Keep in mind that the existence of a snapshot mandates certain activity on
the part of the FFS drivers in order to keep that snapshot's data valid.
Thus, any snapshot is "active" as long as it exists.  In other words, if
you delete a file from the filesystem while a snapshot is active, the FFS
drivers can not reclaim that space, as it would result in data loss within
the snapshot.

> 2) is that limit limited (...) to active snapshots?

Sure.  I don't know what an "inactive" snapshot would be.

-- 
Bill Moran
http://www.potentialtech.com