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Date:      Sun, 5 Feb 95 13:33:41 MST
From:      terry@cs.weber.edu (Terry Lambert)
To:        rjs@clark.net (Ron Steele)
Cc:        freebsd-questions@freefall.cdrom.com
Subject:   Re: Disk Mirrors
Message-ID:  <9502052033.AA02973@cs.weber.edu>
In-Reply-To: <Pine.SOL.3.91.950203204212.24479A-100000@explorer> from "Ron Steele" at Feb 3, 95 09:07:08 pm

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> So, I would be interested in developing some sort of mirroring/logical 
> volume capability for FreeBSD.  I feel that this would lend a lot of credit
> to FreeBSD as a "commercial" product, and perhaps make it a more
> acceptable replacement for commercial Unix systems to people
> that may be reluctant to use a freeware product.
> 
> I am an experienced C/Unix person, but have never gotten my feet wet
> in the kernel, so I would need some mentoring.  Still I think I could
> pull this off with a little help.
> 
> If anyone else is interested in pursuing this please email me.  Some
> words of encouragement from an experienced file system person would be
> especially welcome.

A "correct" implementation would require file system changes to insure
cross volume staggered synchronization and two drive recovery using
fsck to pick "more correct" information for best recovery.


A quick and dirty implementation would require the minimum work, that of
abstracting geometry and offset information for non-identical drives to
provide the lessor of two sector extents in the case that the disks are
not identical.

It would also require "perfect media" -- or pseudo-perfect.  For other
than SCSI, this means getting BAD144 working reliably.  If you didn't
do this, indexing would be a nightmare, as would identically selecting
inodes and disk blocks for use without some kind of master/slave
relationship between the drives (symmetry is the key).

This probaly means VM changes for dirty page handling.

I would suggest  checking out the papers on Zebra and other high
availability work in the past Proceedings of Usenix (ftp.sage.usenix.org).
Not that I recommend Zebra.


					Terry Lambert
					terry@cs.weber.edu
---
Any opinions in this posting are my own and not those of my present
or previous employers.



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