zfs discuss - Oct 2008 - Managing low free space and snapshots

Good evening!

Here at work we are considering switching our linux-based NAS systems to
OpenSolaris because of ZFS, but I have some doubts.

Imagine we have a 500Gb hard disk full of data. We do an snapshot of the data,
for backup or whatever, then we delete those files and try to save another 500
gigs of info there.

How does the copy-on-write manage the free space? What would happen in that
case? Would it look like the hard disk is empty to the network users when in
fact is full because of the snapshot?

Would the snapshot be overwritten or the system would warn there is no free
space?

Thanks in advance.
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This message posted from opensolaris.org

Hello,
 
 In the situation you have described, if i understood well, you would not have
any space. When you take a snapshot, your snapshot is referencing the blocks
older than it...
 Ex.: 
 You have a 500gb disk, and create a 5gb file, you got 495gb free space. 
 So you delete the file, you have 500gb again. But if you take a snapshot
*before* delete the file, that file is still there (in the snap), and using
space (5gb)... ;-)

 Leal
[http://www.posix.brte.com.br/blog]
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This message posted from opensolaris.org

Jose Luis Lopez Campoy wrote:
> Good evening!
> 
> Here at work we are considering switching our linux-based NAS systems to
> OpenSolaris because of ZFS, but I have some doubts.
> 
> Imagine we have a 500Gb hard disk full of data. We do an snapshot of the
> data, for backup or whatever, then we delete those files and try to save
> another 500 gigs of info there.
that won''t work - the snapshot is will cause the data to be retained
*as it
was at the time of taking the snapshot*.
> How does the copy-on-write manage the free space?
the space isn''t free as long as there''s a snapshot referencing
it.
> Would the snapshot be overwritten or the system would warn there is no
> free space?
I''d expect you''d get "no free space" or something
like that.

Michael
-- 
Michael Schuster        http://blogs.sun.com/recursion
Recursion, n.: see ''Recursion''

As others have said, ZFS works a little differently to a standard file system. 
Basically remember that you don''t get anything for free.  I think of
snapshots as an efficient way of storing backups.  You have a copy of all your
data, but it''s still a copy that''s stored on the disk.

The real benefit is when you realise how little data changes.  At work we have
around 500GB of live data, but our rate of change is only around 50GB per year. 
That means a 1TB drive can store all our live data, and potentially around ten
years worth of snapshot backups too.

The biggest downside to snapshots is that you can''t easily free up
space.  So long as you are aware of that, and plan accordingly, the benefits are
well worth it.
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This message posted from opensolaris.org