rc.d script for memory based zfs intent log

[Hiroki Sato]

John <jwd at FreeBSD.ORG> wrote
  in <20130109031834.GA14386 at FreeBSD.org>:

jw> ----- Hiroki Sato's Original Message -----
jw> > John <jwd at freebsd.org> wrote
jw> >   in <20130109023327.GA1888 at FreeBSD.org>:
jw> >
jw> > jw> Hi Folks,
jw> > jw>
jw> > jw>    Here's an rc.d script that provides a nice performance boost on
jw> > jw> ZFS/NFS based file servers. It also helps in other areas not specific
jw> > jw> to NFS.
jw> > jw>
jw> > jw>    It attaches the log device at system startup and removes it at
jw> > jw> system shutdown time. Example;
jw> > jw>
jw> > jw> memzil_pools="tank"
jw> > jw> memzil_bootfs="YES"
jw> > jw> service memzil onestart
jw> > jw> zpool status tank
jw> > jw> service memzil onestop
jw> > jw>
jw> > jw>    This configuration provides a nice performance boost especially to
jw> > jw> NFS, but also helps in other areas not specific to NFS.
jw> > jw>
jw> > jw>    Please DO NOT USE this script if your system is not UPS backed, preferably
jw> > jw> with dual power supplies on separate circuits. If your system crashes you
jw> > jw> may lose data. The script contains information on recovery.
jw> > jw>
jw> > jw> http://people.freebsd.org/~jwd/memzil.txt
jw> > jw>
jw> > jw>    Comments/Improvements appreciated.
jw> >
jw> >  Why is simply setting sync=disabled to the ZFS dataset not enough?
jw>
jw> As you refer to, my understanding is that sync=disabled is at the dataset
jw> layer. The zil approach is at the zpool layer - sync=disabled would be
jw> nice at the zpool layer.

 Can you elaborate why it matters?  If one wants to disable ZIL, just
 setting sync=disabled should be enough.  Using memory disk to
 effectively disable ZIL on physical HDDs looks a roundabout way to do
 the same thing to me.

-- Hiroki
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