Impossible compression ratio on ZFS

[Jeremy Chadwick]

On Mon, Jun 13, 2011 at 10:13:33AM +0100, Steven Hartland wrote:
> I've just done an optimisation of a mysql table which is located
> on a compressed zfs partition and spotted that the size reported
> by du is impossibly small for the file:-
> 
> du -h detail*
> 1.5K    detail.frm
> 7.5K    detail.ibd
> 
> ls -l detail*
> -rw-rw----  1 mysql  mysql       8660 Jun 13 10:00 detail.frm
> -rw-rw----  1 mysql  mysql  650117120 Jun 13 10:04 detail.ibd
> 
> The table format for those interested is 3 int's, 3 indexes and
> contains 8million rows.
> 
> I highly doubt that my 620MB table is taking up just 7.5K on disk
> any ideas?

Well-known "quirk"; welcome to ZFS.  :-) The following article is long,
but if you grab a coffee and read it in full, it'll shed some light on
the ordeal:

http://www.cuddletech.com/blog/pivot/entry.php?id=983

There's also this:

http://blog.buttermountain.co.uk/2008/05/10/zfs-compression-when-du-and-ls-appear-to-disagree/

This is one of the many reasons I do not use ZFS compression.  Not
spreading FUD, just saying stuff like this throws users for a loop, case
in point.

-- 
| Jeremy Chadwick                                   jdc at parodius.com |
| Parodius Networking                       http://www.parodius.com/ |
| UNIX Systems Administrator                   Mountain View, CA, US |
| Making life hard for others since 1977.               PGP 4BD6C0CB |