2008/353 [zpool autoexpand property]

[Andrew Gabriel]

Glenn Skinner wrote:
>     Date: Fri, 30 May 2008 15:19:45 -0700 (PDT)
>     From: Matthew Ahrens <ahrens at sac.sfbay.sun.com>
>     Subject: zpool autoexpand property [PSARC/2008/353 Self Review]
> 
>     ...
>     B. PROBLEM
> 
>     With the addition of Dynamic LUN Expansion (PSARC 2006/373),
>     Solaris now publishes a sysevent when an underlying device is
>     expanded.  ZFS has been enhanced to take advantage of these events
>     and will adjust the pool based on the new size of the expanded
>     LUN.  However, administrators need the ability to control this
>     behavior on individual pools.
> 
>     C. PROPOSED SOLUTION
> 
>     The introduction of the 'autoexpand' will allow administrators the
>     ability to enable or disable automatic pool expansion when a
>     Dynamic LUN Expansion event is received.  The syntax for setting
>     the pool property utilizes the "set" subcommand defined in PSARC
>     2006/577:
> 
>        # zpool set autoexpand=on <pool>
>        # zpool create -o autoexpand=on <pool> <device> ..
> 
> I'm nearly certain I'm failing to see the obvious, but could you
> explain why an administrator would ever want this property to be set
> to "off"?

If you have a mirror with non-identical disks, one will be bigger
than the other, even just by a few sectors. If you temporarily
detach the smaller one, you unexpectedly find can't attach it again
if the pool auto-expands to a larger size. This has hit a number
of people. Similarly, SVM does not expand metadevices until
explicitly told to do so, which I believe is a sensible default
and avoids this problem.

-- 
Andrew