sharing a single disk....

[Doug Hughes]

>Standard Volume Manager (I haven't used any clustering or HA add ons) is
>supposed to not allow sharing of disk groups (therefore not disks, or
>partitions).   Is there an undocumented method of accomplishing this?
>

It's documented, but rather covertly.. Check out the vxdisk command.
It will let you, in addition to adding an entire disk into volume manager
control, take a single slice of a disk and add it is a VM disk. You use
the type=simple option. Then you can take this virtual disk and add
it into whatever disk group you want.

I have done this in a couple of special circumstances to achieve interesting
effects. I used a spare internal  disk to create a log disk
for VM on one system. I used vxdisk to actually split the disk into
two parts. On One I did the normal disk initialization with vxdisksetup
except I told it to change the puboffset so that I could use the
beginning of the disk for something else. Then I used vxdisk with
a type of simple, made sure the cylinders were aligned properly, and
added the simple device as a vm disk in a separate disk group for logging.

I wrote a short article about it last year sometime.. probably around
the Jul-Aug '98 timeframe.

The trick is, you're not really sharing. You're manually splitting up the
disk and then initializing them as separate VM devices which can then be
added to different diskgroups.

I wouldn't recommend this for easy recovery situations, but it's a fun
little hack for special circumstances. If you do it, make sure it's well
documented.