[CentOS] RAID 6 - opinions

Thu Apr 11 19:20:11 UTC 2013
Keith Keller <kkeller at wombat.san-francisco.ca.us>

On 2013-04-11, Joseph Spenner <joseph85750 at yahoo.com> wrote:
>>From: "m.roth at 5-cent.us" <m.roth at 5-cent.us>
>>To: CentOS mailing list <centos at centos.org> 
>>Sent: Thursday, April 11, 2013 8:36 AM
>>Subject: [CentOS] RAID 6 - opinions
>>I'm setting up this huge RAID 6 box. I've always thought of hot spares,
>>but I'm reading things that are comparing RAID 5 with a hot spare to RAID
>>6, implying that the latter doesn't need one. I *certainly* have enough
>>drives to spare in this RAID box: 42 of 'em, so two questions: should I
>>assign one or more hot spares, and, if so, how many?

As another poster mentioned, I'd even break this up into multiple RAID6
arrays.  One big honking 42 drive array, if they're large disks, will
take forever to rebuild after a failure.

> As far as having a spare disk on a RAID6, I'd say it's not necessary.  As long as you have some mechanism in place to inform you if/when a disk fails, you'll not suffer any performance hit.

With this many drives, I'd designate at least one as a global spare
anyway.  Yes, you lose some capacity, but you have even more cushion if,
say, you're out of town for a week, a drive fails, and your backup
person is sick.  One possible configuration is to create three RAID6
arrays with 11 drives each (or one or two with 12 instead), and group
them using LVM.  You could also simply create one RAID6 with the capacity
you need for the next few months, then create new arrays and add them to
your volume group as you need them.  This has the added bonus that you
look like a genius for deploying new capacity so quickly.  :)  Recently
I acquired a half-empty storage array, so that I can add larger drives as
they become available instead of being tied to drive sizes of today.

> A RAID5 with a hot spare isn't really the same as a RAID6.  For those not familiar with this, a RAID5 in degraded mode (after it lost a disk) will suffer a performance hit, as well as while it rebuilds from a hot spare.  A RAID6 after losing a disk will not suffer.

I seem to remember reading on the linux RAID mailing list that, at least
for linux md RAID6 (which the OP may not be using), performance on a RAID6
with one missing drive is slightly worse than optimal RAID5.  I could be
wrong however, and perhaps a hardware RAID controller doesn't have this


kkeller at wombat.san-francisco.ca.us