zfs discuss - Aug 2008 - ZFS noob question

Hi. 
I took a snapshot of a directory in which I hold PDF files related to math. 
I then added a 50MB pdf file from a CD (Oxford Math Reference; I strongly
reccomend this to any math enthusiast) and did "zfs list" to see the
size of the snapshot (sheer curiosity). I don''t have compression turned
on for this filesystem. However, it seems that the 50MB PDF took up only 64K.
How is that possible? 
Is ZFS such a good filesystem, that it shrinks files to a mere fraction of their
size?
I viewed the file and it seems to be intact.
Is there anyone here, who can explain to me how this happened?  
I am awe-struck... in a good way, of course :D.
--
This message posted from opensolaris.org

theshadowgate at gmail.com said:
> I took a snapshot of a directory in which I hold PDF files related to math.
> I then added a 50MB pdf file from a CD (Oxford Math Reference; I strongly
> reccomend this to any math enthusiast) and did "zfs list" to see
the size of
> the snapshot (sheer curiosity). I don''t have compression turned on
for this
> filesystem. However, it seems that the 50MB PDF took up only 64K.  How is
> that possible?  Is ZFS such a good filesystem, that it shrinks files to a
> mere fraction of their size? 
If I understand correctly, you were expecting the snapshot to grow
in size because you made a change to the current filesystem, right?

Since the new file did not exist in the old snapshot, it could never
have known about the blocks of data in the new file.  The older snapshot
only needs to remember blocks that existed at the time of the snapshot
and which differ now, e.g. blocks in files which get modified or removed.

I would expect that when you add a new file, the only blocks that change
would be in the directory node (metadata blocks) which ends up containing
the new file.  That could indeed be 64k worth of changed blocks.

Regards,

Marion