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# RAID 3 mode, what happens if the parity disc fails?

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 RAID 3 mode, what happens if the parity disc fails? by David Phillips on Jun 15, 2005 at 7:02:45 pm

I am thinking about using a 4210 4Gb Fibre RAID in a SAN environment, but I am concerned about data loss with RAID 3 storage. How do the HUGE units handle a parity disk failure?

thanks,
david

 Re: RAID 3 mode, what happens if the parity disc fails?by Lance R. Gropper on Jun 15, 2005 at 7:56:36 pm

Hello David:

In our flavor of RAID 3, there are four data drives and one parity drives. The parity is generated via an exclusive-OR calculation. If any data drive dies, the missing data is able to be sent to the computer by reversing the calculation between the parity drives and the remaining data drives. If the parity drive itself dies, then the rest of the array effectively performs as a RAID 0 array - it isn't necessary to fill in missing data for the parity drive if it is down. Here's some examples (not using our exact technique) of how the parity works with single bits:

D1 D2 D3 D4 PR
1 0 0 1 1 - suppose we use odd parity - you can add the 1's together, and the resulting number is (2) even, therefore to make the number
odd (3) you have to use a 1 for the parity.

Now suppose D4 goes down:

D1 D2 D3 D4 PR
1 0 0 ? 1 - We used odd parity generation to generate the parity - if we add the remaining bits (including the parity drive together),
the resulting number is 2 (even), therefore the missing data from D4 is 1 (to make it odd).

Here's the scenario with the parity drive down:

D1 D2 D3 D4 PR
1 0 0 1 ? - All of the user data is readable, so it isn't necessary to calculate parity, but it is necessary to calculate parity when the
replacement drive is put int (a process known as reconstructing). In this case, we add the bits together, and they add up to
2 (even), so the parity must be 1 in order to make the total odd.

Just because there are skeptics out there who don't believe this will work for a 0 drive:

D1 D2 D3 D4 PR
1 0 ? 1 1 - If you add the bits together, the number is odd, therefore the data on the missing drive D3 must be 0 to keep the parity odd.

Just for your information, we are working on the first RAID 6 array, which we demonstrated at NAB this year (And won an award for). In this array, there are two sets of 6 drives with 2 parity drives. In this case, any two drives (of each set of 6), including both parity drives can go down and your data is still intact. It's a little too complicated to explain here how the parity is calculated in RAID 6, because if anyone were to read the description of how it is done, their head would explode -- but we do have it working at our office.

Lance Gropper
HUGE Systems
(A Ciprico Company)

 Re: RAID 3 mode, what happens if the parity disc fails?by David Phillips on Jun 15, 2005 at 8:20:06 pm

Thanks for explanation, Lance. In real-world terms, if the parity drive fails, I suppose it is a case of running with some risk (or, for the risk-averse, taking the storage offline) until the parity drive can be replaced. Next question would be: for a 4TB RAID like the 4210, how long would it take to reconstruct the parity drive(s) and could you reasonably use the storage during reconstruction?

re: the RAID 6 4212; no need to convince me, if it were available today I would wire you the money tomorrow (is that two 'p's in Gropper?) My problem is that I am looking to deploy a SAN before it will be ready, and while I would like to go with a HUGE 4Gb Fibre RAID, I am kind of concerned about the risk exposure of RAID 3.

-david

 Re: RAID 3 mode, what happens if the parity disc fails?by Lance R. Gropper on Jun 15, 2005 at 8:56:37 pm

Hello David:

It is 2 p's (I am indeed the grandson of the famous artist as well as the son of the guy who shot the superman cartoons). The only problem with RAID 3 is once a drive goes down, you are at risk until the drive is replaced. That's why we recommend getting a spare or spares just in case. On the 4TB RAID, reconstruction is between 2-3 hours. You can use the unit while it is reconstructing, however reconstruction will pause while the unit is being used. A bad scenario is if you didn't have a spare, a drive goes down, then you continue to use the unit until a 2nd drive has gone down.

On the SAN, what kind of SAN software were you planning?

Lance Gropper
HUGE Systems
(A Ciprico Company)

 Re: RAID 3 mode, what happens if the parity disc fails?by Lance R. Gropper on Jun 15, 2005 at 8:57:50 pm

Oh David:

It's the same amount of time to reconstruct any drive, regardless of whether or not it is parity...

Lance Gropper
HUGE Systems
(A Ciprico Company)

 Re: RAID 3 mode, what happens if the parity disc fails?by David Phillips on Jun 16, 2005 at 1:43:55 am

I was planning on using Apple Xsan, but I like the cross-platform friendliness of MetaSan (will be about 4 Mac clients and 2 Windows clients to start.) Not many integrators have a lot of experience with MetaSan though. Have you tested either in the Huge labs?

-david

 Re: RAID 3 mode, what happens if the parity disc fails?by Michael Anderson on Jun 16, 2005 at 2:20:59 am

Hi David,

We do have MetaSan in our lab, and so far, it's running quite well. Robert is becoming an expert. I've also met Alex, the guy who is behind the technology, and I'm very impressed with him. We have more testing to do, but I would recommend it based on what I've seen so far.

My contact there is David Pollack, 514 428 4137. Feel free to drop my name if you'd like.

Best Regards,

Michael H. Anderson
Chief Engineer
Huge Systems

 Re: RAID 3 mode, what happens if the parity disc fails?by Joaquin (Kino) Gil on Jun 15, 2005 at 8:24:38 pm

[Lance R. Gropper] "if anyone were to read the description of how it is done, their head would explode -- but we do have it working at our office.
"

... which has costed them three engineers, two trained techs, a programmer and a Fed Ex guy taking a course in IT who delivered to the wrong room. Sad losses in the name of science.

;)

 Re: RAID 3 mode, what happens if the parity disc fails?by Lance R. Gropper on Jun 15, 2005 at 9:00:04 pm

Hello Kino:

That's what made it so hard to get the award at NAB -- we went through quite a few people, before one managed to make it alive to the people giving out the awards, then with his last breath, he spoke the word "HUGE..."

Lance Gropper
HUGE Systems
(A Ciprico Company)

 Re: RAID 3 mode, what happens if the parity disc fails?by Bob on Jun 15, 2005 at 9:43:15 pm

Must've been the same development staff that wrote "The Funniest Joke in the World" that the Brits used to defeat Germany in WWII.

 Re: RAID 3 mode, what happens if the parity disc fails?by Lance R. Gropper on Jun 16, 2005 at 2:28:54 pm

Yup - Andre and I are very familiar with that Monty Python skit...