advice wanted on RAID config

I'd like to get your suggestions on some alternatives for a RAID configuration.

    Environment:
	Linux server
	external array: 8x73GB, can be one or two independant SCSI channels
	dual port controller card (it's a Sun D1000). The array is JBOD--any
	RAID happens in software only.

    Purpose:
	on-line backup device for about 10 servers, totalling ~160GB now,
	anticipated growth of 4 servers and 30~40% storage in 12 months

	Total storage need for multiple epoch backups and incrementals,
	assuming compression and smart linking of duplicate files, is
	225GB initially, with anticipated growth to ~265GB in 12 months.

	The expected I/O pattern will be almost entirely writes during
	server backups, almost entirely reads during dumps of data from
	the array to an archival tape device.

	There are relatively low throughput requirements (~5GB/hr minimum),
	as the backup window for the servers is large. Backups can be
	scheduled so that multiple epochs aren't happening simultaneously.

    Software:
	The program BackupPC (http://www.backuppc.com/) will run from the
	server, doing backups via rsync.

    Constraints:
	Budget.

	Budget.

	Budget.

	Changing the type of hardware is _not_ an option at this point. No
	ability to add more disks or higher capacity disks to the array at
	this time.

OK, that's the background. Now, here's my question: What form of RAID do you
recommend on the array? As I see it, the choices are:

Possible RAID layouts:
	RAID 0+1 (striped mirrors):
		advantages: write speed, read speed, redundancy
		disadvantages: provides least usable space, likely to be
			very tight in 9~12 months, even with minimal numbers
			of backups retained.
		usable space: 292GB

	RAID 5:
		advantages: good use of disk space, good read speed
		disadvantages: poor write speed (5~6x slower than raw disk)
		usable space: 511GB

	RAID 4 custom:
		This is what I really want opinions on... I'm thinking of
		building a RAID 4 array as the following:
		   mirror 1 disk from each of the two (independant) SCSI channels
		   stripe the remaining 6 disks

		   use the mirrored volume for the parity disk

		advantages: better write performance than RAID5, better use
			of space than RAID 0+1, more redundancy than regular
			RAID4
		
		disadvantages: badly breaks the "Keep It Simple, Stupid" motto
		usable space: 438GB


                                                                                
                                                                                
                            +----------- RAID 1 mirror ---------+       
                            |                                   |       
                            |                                   |       
   ----    ----    ----    ----        ----    ----    ----    ----     
   |1 |    |2 |    |3 |    |4 |        |5 |    |6 |    |7 |    |8 |
   |  |    |  |    |  |    |  |        |  |    |  |    |  |    |  |
   ----    ----    ----    ----        ----    ----    ----    ----     
    |       |       |                   |       |       |       |       
    |       |       |                   |       |       |       |<==parity
    |       |       |                   |       |       |       |   "disk"
    +-------+-------+-- RAID4 stripe ---+-------+-------+-------+       
                                                                                
                                                                                
    ^                        ^      ^                            ^              
    |                        |      |                            |              
    +------ SCSI channel 1 --+      + ------ SCSI channel 2 -----+              


I'm hoping that using a dedicated parity volume that's actually doing load 
balancing of mirrored writes, I'll get an overall higher write performance than 
RAID5.

So, does this sound wacky? Any suggestions?
	
----
Mark Bergman		
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