RAID configuration

[rpafort]

Hello,

In the old days ;-) you configured your RAID based on the (number of) disks. But now you choose whatever RAID configuration you like, and I don't understand this.

To be more precise, we have a DL360G5 with a P400i RAID controller. On the older DL's I was used to create a Array and choose a RAID level for redundancy, also depending on the available disks.

Now with this machine I can create an Array, without the option to choose a RAID level. Instead I have to choose a RAID level when I create a logical disk. And to make it more confusing I can choose any RAID level, no matter the available disks. I don't understand how to translate this to the real redundancy at the hardware level.

In my case I have 4 equal disks and I created 1 array and 2 logical drives. With this 4 disks I'm able to create one mirror and 1 RAID5 logical disk?! With only 4 disks?!

Please explain to me how this is possible?

Thank you for your response.

PS: I cannot find any documentation on this from HP.

[Patrick Terlisten]

Hello,

take a look into ORCA, the rom based raid setup utility, there you will find you "old school" setup of disks and raid levels.

An array is only a set of disks. In this set of disks you can create logical drives. These logical drives have a raid level. In the HP world is a mirror and a raid 1+0 the same. With two disks, it's a mirror, with at least 4 disks it's a raid 1+0. In ACU you will only find the notation "RAID 1+0".

For a raid 1+0 you need a even number of disks. You have four disks, so you are able to create a raid 1+0 and a raid 5. With only three disks, you won't be able to create a raid 1+0, but a raid 0 and a raid 5. With four disks and a BBWC module you are able to create also a RAID ADG (RAID 6, RAID 5DP, a RAID 5 with two parity blocks).

Independent of your raid level: The Smart Array controller will distribute the blocks on the hard disks in the right order. It makes no difference if your data lies on a whole disks in a raid array, or just using a part of a disk, sharing it with other blocks.

Take a look into the ACU User Guide. There you will find lots of informations.

Best regards,
Patrick

[rpafort]

Hello Patrick,

Thank you for your reply.

I read the ACU User Guide (again), and together with your explanation this is what I understand from it;

With my setup of 4 physical disks forming 1 array containing 2 logical drives, from which 1 logical drive is a RAID 1+0 and 1 logical drive is a RAID 5, the array controller will just spread the data over all 4 disks. Correct?

If so;

1. How does this effect performance? Because you choose a RAID level for redundancy and performance. And in this case the performance gain of a RAID10 is effected by the RAID5 logical drive. In short if I would like to prevent this I need to setup 2 arrays. Correct?

2. In this case a disk failure will result a degration of both logical drives. Correct? Again the solution here is to create 2 arrays. Correct?

So in conclusion you can say that if you want an "old school" RAID level with corresponding redundancy and performance, you need to setup an array for each wanted RAID level. Correct?

[Patrick Terlisten]

Hello,

--
1. How does this effect performance? Because you choose a RAID level for redundancy and performance. And in this case the performance gain of a RAID10 is effected by the RAID5 logical drive. In short if I would like to prevent this I need to setup 2 arrays. Correct?
--

Yes and no. Surely you can build up two arrays with two drives, but with such an array, you can't create a logical drive with a raid 5. The benefit of such a configuration: More IOPS for each logical drive, the load is distributed over all four disks, and not only two disks.

--
2. In this case a disk failure will result a degration of both logical drives. Correct? Again the solution here is to create 2 arrays. Correct?
--

That's right. But remember: Two disks = no raid 5. Two disks = fewer IOPS.

Best regards,
Patrick

[rpafort]

Patrick, thank you for your responses. It's all clear to me now.

[rpafort]

The responses are clear.