RAID Configuration question

Barry Allard ballard at
Mon Apr 30 20:16:56 CDT 2007

Hi Hank,


The Dell sales reps usually chime "If it's been < 30 days, you can simply
return the system."







Read time

Read from all drives and parity check for one stripe.

Read both drives and compare.

Improved with spindle sync and hardware raid.

Write time

Mitigated to near RAID 1 with large battery-backed write cache

Commit to both drives.

Sped with lazy battery-backed, large write-cache and command reordering

Failure rate



f = MTTF of 1 drive, r = MTTR of 1 drive, N = number of drives, G = parity
group size





Rebuild time

Long, long time: up to a week.




Read up on observed failure rates: 


"Certainly, the lesson from the FAST research is that IT budgets must
include a line item for regular replacement of hard disks, even if the MTBF
says it isn't necessary. This may cut into the expenditures of new storage
systems, something that CIOs and storage vendors prefer."


More good stuff:





From: linux-poweredge-bounces at
[mailto:linux-poweredge-bounces at] On Behalf Of Hank
Sent: Monday, April 30, 2007 3:12 PM
To: linux-poweredge
Subject: RAID Configuration question



I have a new PE 2950 (perc 5i) with 2x73 GB SAS drives, and due to a
purchasing error, 2x146GB drives (instead of 2x73GB).


I WAS going to build a 3x73GB RAID 5 array with one 73GB hot spare.


What are the cons of building the following:


146GB RAID 5 Array with three drives:


        1x146GB (use 50% of this drive = 73GB)

        1x146GB (use 50% of this drive = 73GB)


73GB RAID 1 Array

        1x146GB (use other 50% of this drive)

        1x146GB (use other 50% of this drive)


73GB Hotspare for the RAID 5 Array 

        1x73GB drive


So, If I lose the 73GB drive, the RAID 5 hot spare will kick in without
affecting the RAID 1 array at all.


If I lose either of the 146GB drives, the hot spare will kick in for the
RAID 5 array and the RAID 1 will be degraded.


But what happens when I replace the failed 146GB drive?  Will the controller
be smart enough to start rebuilding both arrays on the new drive?  (Somehow
I doubt it).


Or should I just abandon this idea and install my original plan and let
146GB go wasted?








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