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Raid sets with 2 cards to create 1?

MrGuvernment

MrGuvernment

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I was curious about something to add a level of redundancy in a single server, i know SAN include this tech with dual port harddrives and such so just thinking out loud..

Not sure if nested is the right term for this, but would this be possible?

2 raid cards in 1 system

1st raid card with a raid 1 array with 4 hard drives

2nd raid card with a raid 0 with 4 hard drives

Then creating a raid 10 from the combined cards using the raid software, not using say windows raid or something.

Thus if 1 card fails the other is still active with the data, since either controller will, or should, have all of the data on it ?

Or am i missing something glaringly obvious, which after writing this i think i am, but i cant quiet put my finger on it!
 
I think more what you are trying to say is mirrors (R1) spanned on two disks over different cards, so that one drive is on each card. Then the mirrors are striped for performance. In my opinion card failure rates are pretty low, in my experience (though it does happen), so this kind of setup seems pretty rare. If you're set on doing a single system I would think RAID6 would be better since it can tolerate two disk failures simultaneously.

In my opinion you're better off having a good backup system and a good plan to get from broken server back to working, than try and get something like this working. I know certain Areca cards can work in tandem like this for RAID6 arrays that are then striped into RAID60 but I feel like thats a lot of eggs in one basket.

This style of redundancy is already available with certain SAN systems (The HP/Lefthand system I use at work does this, but over iSCSI between two or more SAN units). The units can be setup in RAID5 or RAID6, then over the network it does "RAID 10" where it can grab the requests from either SAN units. It can also do Network RAID5/6 as well, spanning across multiple SAN units.

If you really need such a high level of availability I'd look into some of the higher-end SAN systems, or something that does ZFS with block-level replication. This eliminates the whole disk bank/server/san unit as a single point of failure.
 
It will work, but your raid array layout wont.

A raid 0 with 4 drives will be larger than a raid 1 with the same drives, also most controllers will only let you create a raid 1 (mirror) with 2 drives. So, lets assume the raid controller will even let you create a raid 1 array with 4 drives.

Raid-0 with 4 2TB drives will be 8TB.
Raid-1 with 4 2TB drives will be 2TB.

What you need to do if you want this to work is create 2 identical Raid Arrays.

1st Array: Raid-0 with 4 2TB drives will be 8TB.
2nd Array: Raid-0 with 4 2TB drives will be 8TB.

Then use software Raid to mirror (Raid 1) both those Raid Arrays, this would give you a Raid 0+1, best of both world. You get the read AND write speed bonus of the Raid-0 plus the redundancy of the Raid-1. You would also get 8TB of storage. Plus software Raid-1 uses basically no CPU resources so you wont need to worry about that either.

You could do it the other way around to get pseudo Raid-10 which is better than Raid 0+1, but if you went Raid 10 you would only mirror 2 drives each and then stripe them, this way you would get 8TB of storage.

So, your final and best layout would be (with same drives):

Card 1, 1st Array: Raid-1 with 2 2TB Drives will be 2TB
Card 1, 2nd Array: Raid-1 with 2 2TB Drives will be 2TB
Card 2, 1st Array: Raid-1 with 2 2TB Drives will be 2TB
Card 2, 2nd Array: Raid-1 with 2 2TB Drives will be 2TB

Now, in software (or raid controllers if they support it) you would Stripe (Raid-0) all the Raid-1 Arrays, finally ending up with an 8TB 1+0 which would be Raid 10. Striping uses very little CPU resources also, so it would be fine to do this in software too.
 
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depends on the controller not all controllers can stripe across arrays on different cards.

i do this in ZFS across 4 controllers with caching across 2 more controllers.
 
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