rc.d script for memory based zfs intent log

[John]

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

As you refer to, my understanding is that sync=disabled is at the dataset
layer. The zil approach is at the zpool layer - sync=disabled would be
nice at the zpool layer. 

-John

> -- Hiroki