Impossible compression ratio on ZFS

[Steven Hartland]

----- Original Message ----- 
From: "Jeremy Chadwick" <freebsd at jdc.parodius.com>
 
> 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.

I think your miss-understanding my question, its not the fact that its
showing different sizes from du and ls, that's 100% expected but clearly
8million rows of 3 int's can't possibly compress down to 7.5K.

Having just looked back at the machine, an hour later, the values now
seem correct with du showing:-
278M    detail.ibd

I checked this several times, over what had to be 10mins or more even
did a flush tables to ensure everything had been written out as far
as mysql was concerned.

So it seems that zfs was still processing the file for a good amount of
time, and during that time was showing incorrect disk usage for said file.

I'm wondering if the data is some how being processed in l2 arc or
something?

For reference we're running 8.2-RELEASE, on an areca backed raid6 with
two ssd drives in l2 arc.

zpool status
  pool: tank
 state: ONLINE
 scrub: none requested
config:

        NAME        STATE     READ WRITE CKSUM
        tank        ONLINE       0     0     0
          da0p3     ONLINE       0     0     0
        cache
          ada0      ONLINE       0     0     0
          ada1      ONLINE       0     0     0

errors: No known data errors

Obviously everything seems to have caught up and is now showing real
stats but confused as to why it would take quite so long to display
the real usage via du.

    Regards
    Steve

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