Some questions about vfs.zfs.prefetch_disable=1 and ZFS filesystem versions

[Dan Naumov]

After looking at the arc_summary.pl script (found at
http://jhell.googlecode.com/files/arc_summary.pl), I have realized
that my system has set vfs.zfs.prefetch_disable=1 by default, looking
at dmesg, I see:

=========================
ZFS NOTICE: Prefetch is disabled by default if less than 4GB of RAM is present;
            to enable, add "vfs.zfs.prefetch_disable=0" to /boot/loader.conf.
=========================

...except I do have 4gb of RAM. Is this caused by integrated GPU
snatching some of my memory at boot? From dmesg:

=========================
real memory  = 4294967296 (4096 MB)
avail memory = 4088082432 (3898 MB)
=========================

What kind of things does this tunable affect and how much of a
performance impact does enabling / disabling it have? Should I
manually enable it?

I've also noticed a really weird inconsistency, my dmesg says the following:

=========================
ZFS filesystem version 13
ZFS storage pool version 13
=========================

Yet:

=========================
zfs get version
NAME                          PROPERTY  VALUE                         SOURCE
cerberus                      version   3                             -
cerberus/DATA                 version   3                             -
cerberus/ROOT                 version   3                             -
cerberus/ROOT/cerberus        version   3                             -
cerberus/home                 version   3                             -
cerberus/home/atombsd         version   3                             -
cerberus/home/friction        version   3                             -
cerberus/home/jago            version   3                             -
cerberus/home/karni           version   3                             -
cerberus/tmp                  version   3                             -
cerberus/usr-local            version   3                             -
cerberus/usr-obj              version   3                             -
cerberus/usr-ports            version   3                             -
cerberus/usr-ports-distfiles  version   3                             -
cerberus/usr-src              version   3                             -
cerberus/var                  version   3                             -
cerberus/var-db               version   3                             -
cerberus/var-log              version   3                             -
cerberus/var-tmp              version   3                             -
=========================

Is this normal or should "zfs get version" also show version 13? This
is on a system with the pool and filesystems created with 8.0-RELEASE,
by the way.

Thanks!

- Sincerely,
Dan Naumov