From: Robert Miller (rmiller@SMUD.ORG)
Date: Wed Mar 05 2003 - 16:22:28 EST
Err, um, CAN you mirror a RAID5 array? *blinkblink*
The reason you mirror disks is for redundancy... RAID5 already has
redundancy built-in. Basically the data is striped across the disks (RAID0)
and then a parity is calculated and stored. What this means is that if you
lose a disk, the array will normally still function, in a degraded (slowed)
state. Once you replace the failed disk, the array will rebuild itself,
including the new disk in the rebuild.
RAID 0+1, AKA RAID10 or RAID-10, gives you mirroring, while striping the
data across the disks that are mirrored. This means that if one of the
disks dies in the mirror set, the corresponding disk will take over, and
once the failed drive is replaced, the disk that has the data that should be
on the new disk should copy that data back over to the new disk.
Either way, they're both acceptable fault-tolerance schemes. RAID5 is
faster on reads than writes because of the parity, and if you lose a disk
performance of the array will be degraded. RAID 1+0 writes a bit faster
than RAID5 would because it's not calculating parity, although it is still
striping, and I *believe* that there should be no performance hit if a RAID
1+0 disk dies, because the mirrored drive should pick up where the failed
disk left off.
Anyway, a long answer to a short question. As far as whether it's better to
mirror a RAID5 array or a RAID0 array, umm... mirroring RAID5 doesn't make
sense to me, although mirroring a RAID0 array will make it a RAID 1+0 array
(if your controller supports it), which is indeed better than a simple RAID0
(striped) array. However, keep in mind that with a RAID1 or RAID 1+0 array
you need to add drives in pairs so the mirroring is kept happy.
--rm
-----Original Message-----
From: IBM AIX Discussion List [mailto:aix-l@Princeton.EDU]On Behalf Of
Jolet, John
Sent: Wednesday, March 05, 2003 11:43 AM
To: aix-l@Princeton.EDU
Subject: Re: mirroring raid arrays
what that means is raid 10 is a stripe array (for speed) that is mirrored to
another stripe array (for redundancy). Raid 5 incurs a penalty on write
because the parity information has to be calculated. In practice, I don't
see much performance difference between my raid 5 and my raid 10 arrays.
Raid 5 has a lower disk count requirement than raid 10.
-----Original Message-----
From: Bill Verzal [mailto:Bill_Verzal@BCBSIL.COM]
Sent: Wednesday, March 05, 2003 1:15 PM
To: aix-l@Princeton.EDU
Subject: Re: mirroring raid arrays
RAID-10 is RAID 0 + 1. Generally accepted as the best performed.
BV
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Bill Verzal
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"Adam Hanel" <hanela@BILLINGS.K12.MT.US>
Sent by: "IBM AIX Discussion List" <aix-l@Princeton.EDU>
03/05/2003 12:08 PM
Please respond to "IBM AIX Discussion List"
To: aix-l@Princeton.EDU
cc:
Subject: mirroring raid arrays
Can some one tell me ?
Would it be better (for redundancy and speed) to mirror a raid 5 array? Or
to mirror a raid 0 array?
We are running a 7025-F50 AIX 4.3.3
-Adam
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