Shadow Disks

[Cliff Van Dyke]

I'm currently implementing a shadow disk capability for Harris' CX/UX
version of UNIX. Shadow disk is the capability to duplicate a disk
dynamically so that you have a backup copy in the event that one of
the disks fail.  We've taken Pyramid's basic approach of shadowing
individual partitions using a virtual disk driver which sits immediately
above the regular disk driver. I'd like to draw on the collective
wisdom of this group as a sanity check on a couple of issues.

Issue 1
-------
There are times when one of the shadowed copies will be deemed to be
good and other shadowed copies will be deemed to be of unknown quality.
For instance, after a system crash we'll pick one copy to fsck and
we'll need to ensure that the other copy is up to date with the
fsck'ed copy. In another instance, when a new shadow partition is
brought online (for instance, after the disk drive is fixed following
a failure), we'll need to populate the new partition with known good
data.

Our general solution to this is to copy all of the data from the known
good copy to the partition with the unknown data. We properly
coordinate this updating with other I/O currently happening to the
virtual disk so as to allow the updating to happen as a background
activity. Nevertheless, this is a lot of I/O to do.

Am I missing something?

Issue 2
-------
My current intent is to remember which disks fail in a file on the root
partition. This information will be used on future reboots to reconfigure
the system the same way after a reboot as before the reboot. The
operator can always interactively attempt to mark a partition as failed
or to bring a failed partition online.

That sure sounds like the right way to do it to me. Does it to you?

Issue 3
-------
Are there any experiences concerning shadow disk that you've had that
are worth sharing with an implementor?

Please e-mail responses to me. I'll summarize to the net if it seems
warranted.
--
Cliff Van Dyke                   cliff at ssd.harris.com
Harris Computer Systems          cliff%ssd.harris.com at eddie.mit.edu
2101 W. Cypress Creek Rd.        ...!{mit-eddie,uunet,novavax}!hcx1!cliff
Ft. Lauderdale, FL 33309-1892        
Tel: (305) 974-1700