zfs discuss - Jul 2006 - ZFS compression best-practice?

Hi there,

Customer has some questions regarding ZFS compression.

- What is the compression algorithm used?

- How much space saving, ballpark?
(I said it depends on the data but customer still wants to know if we 
have stats.)

- Is there a ZFS feature that will output the real uncompressed size of 
the data?  The scenario is if they had to move a compressed ZFS 
filesystem back to UFS, say.  ''ls'' will give the
file''s real
uncompressed size, but customer had rather not write a script to sum 
everything up.

- Is there a Best-Practice for when and when not to turn on compression? 
  I read somewhere, I forget where, that compression should be used 
primarily to save space, but is there a good balance to be struck 
between giving up a few CPU cycles to gain more disk space?

- Compressed files have a -T bit set, I looked this up, docs say this 
has no meaning, but does it have special meaning to ZFS?

- Customer wants to do a diff between snapshots.  Is there an RFE 
already filed?

_ Customer would like benchmarking numbers.  I think there is a blog 
item but do we have something more "official"?

Thanks very much.

CT

On Thu, Jul 27, 2006 at 03:54:02PM -0400, Christine Tran
wrote:
> - What is the compression algorithm used?
It is based on the Lempel-Ziv algorithm.
> - Is there a ZFS feature that will output the real uncompressed size of 
> the data?  The scenario is if they had to move a compressed ZFS 
> filesystem back to UFS, say.  ''ls'' will give the
file''s real
> uncompressed size, but customer had rather not write a script to sum 
> everything up.
You can multiply the ''referenced'' and
''compressratio'' property of a
filesystem to find out how much space it would use if uncompressed.
> - Customer wants to do a diff between snapshots.  Is there an RFE 
> already filed?
Two, in fact:

6370738 zfs diffs filesystems
6425091 want ''zfs diff'' to list files that have changed
between snapshots
> _ Customer would like benchmarking numbers.  I think there is a blog 
> item but do we have something more "official"?
No; we''re working on some more unofficial benchmark numbers, though :-)

--matt