Author Topic: Any reason not to use Raid 1 for RT PPU?  (Read 2068 times)

Offline DrBlaze

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Any reason not to use Raid 1 for RT PPU?
« on: March 20, 2013, 01:21:39 pm »
So for the last few months my setup has been 10x2TB DRU and 1x2TB PPU.  I need more space and am starting to replace 2TB drives with 3TB drives, obviously starting with the PPU.

I recently installed an LSI-based 8-port adapter (perc H310), which has allowed me to retire a couple older add-in controllers and has freed up an onboard 2-port controller capable of Raid 1.

My thought is, why not mirror 2 identical 3TB drives and use this for my PPU?  This would obviously be to get around RT's limitation of 1 PPU.

I thought I remembered this being discussed many months ago, but after an exhaustive search I can find no mention anywhere for this idea.  This surprises me since, from what I have seen, the limit of 1 PPU for RT is one of the biggest reasons for users opting for Snapshot.

So has anyone tried this?  Is there any obvious downside I am missing?

Offline Brahim

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Re: Any reason not to use Raid 1 for RT PPU?
« Reply #1 on: March 20, 2013, 02:03:32 pm »
It does not have the same protection level.
Good thinking though. :)

A mirror'ed PPU only protects the PPU and not the RAID.
Dual parity on the other hand protects the RAID by allowing any two drives to fail.

Offline DrBlaze

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Re: Any reason not to use Raid 1 for RT PPU?
« Reply #2 on: March 20, 2013, 05:29:45 pm »
Yes, I didn't mean to imply that I thought it was as good as having 2 PPU.  But it would allow for 1 DRU and 1 PPU to fail simultaneously, and since the PPU is the most important drive and probably gets worked the most if we're regularly writing data to the array, I think it might be worth it.

Anyhow, if you can't foresee any negative impact on the array I think I'll test it out.


Offline Gasman3055

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Re: Any reason not to use Raid 1 for RT PPU?
« Reply #3 on: March 26, 2013, 04:05:34 pm »
Actually, the PPU is the least important drive. It is the most written-to drive so speed matters more here.

Lose any 1 drive, you should be able to recover. Lose 2 data drives, then cannot recover either one regardless of the PPU.

Lose 1 data drive and the PPU, you lost the data on the data drive. You rebuild the PPU.

I am waiting for a dual parity real-time FlexRaid myself for a somewhat different reason. This allows a data drive failure and an data error on another drive. (Read analysis stuff on why big drives really need RAID-6. Has to do with 10^15 unrecoverable error rate specs on drives that are 3 x 10^15). For now I am only using FlexRaid to hold the backups of the backups of my systems. I am using FlexRaid to guard again the unrecoverable error more than drive failure (which can usually be caught with close attention to S.M.A.R.T.)

Offline DrBlaze

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Re: Any reason not to use Raid 1 for RT PPU?
« Reply #4 on: March 26, 2013, 04:52:28 pm »
Well I guess you can look at it in different ways.  To me the parity drive is the most important because in an emergency it can stand in for any other drive.  Without it, if a drive fails (and it will) you have lost that data.  I've rebuilt a couple drives in the last 3 months and until that process was complete I can assure you that my parity drive was the most important drive on the planet to me :)

For me (until dual parity arrives) it will be nice to know that if one of those mirrored PPU drives dies I'm still protected.