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RAID 5 question

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by p.hernandez

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A co-worker of mine has a theory that if I take a drive from a RAID 5 set and put it into another server also using RAID 5, the data from the original server will propagate to the drives on the new server. I'm not sure if this is true. Can someoneconfirm or deny this? Thanks.

13 replies

by lordinfidel

When a raid controller creates a raid set, it writes a signature to the drive indicating that it is part of a unique raid set.

When you replace a failed drive in a raid set with a new one, and rebuild the raid set. What ever was on the new driveis overwritten.

It does not go the opposite way. If you only have 1 drive left in your raid set, all of your data is essentially gone or corrupt in such a way that only a restore from backup will save it.

by cb admin

I'm using RAID 5 on my server and I need to add more space to both the OS partition and my exchange partition. I have purchased another drive, same model type and size as the current ones, of this new drive I want to divide the space to both my OS partition (c: drive) and exchange partition (e: drive), is it safe to do this without losing my current data? Should I use the smartstart cd or can I do it from within w2k directly using the compaq raid utility?

by lordinfidel

You can't expand a raid set once it is created.

Your only option is to back everything up. Delete the raid set, install the new drive, recreate the raid set, and restore everything back to the drive.

EXCEPT, you have one glaring issue. Sinceyour OS is on the raid set, you can not just back it up and expect it to work.

This is why you always mirror the root drives instead of putting it in a raid5 set that holds other data.

Again, RaidSets can not be expanded once they are created. Even volume spanning at this point is tricky. And I would not even consider using partition magic on a raid set.

by cb admin

Ok, maybe I worded my question wrong. What I want to do is add more space to my server, and I believe this is possible. Again, currently this is what a have; four 18GB drives configured as RAID 5 using three of the drives and the other assigned as a spare, all my drives create one logical drive. Using the Compaq SmartArray Configuration utility I do see an option to Expand or Extend my Raid configuration, I don't know what is the difference between the two. If what you are saying is that there is no way to add more drives to an array then I think you are wrong. I've seen people add more physical drives to a RAID 5 configuration, I actually paid a consultant to add more space to one of my servers about a year ago, it was my fault that I didn't see how he did it, that's why I want to try to do it myself this time, I'm just worry about losing data and I honestly, after budget cuts, I don't have money to hire another consultant.

by xzeppelin

With the help of an IBM rep I just added an additional 18GB drive to an existing Raid 5 array containing Network file shares. It's a HW Raid setup and all files are still intact and in use. Software used was IBM ServerRAID Manager.

by clearsmashdrop

One thing Ive seen when you take a disk from a Raid 5 box ( Im talking NAS boxes ) and put it into another box of the same manufacture is you end up with the machine seeing two raids, one of which is obviously broken.

So you have to make sure youformat that new disk first and set it as a spare.

As for your original question, others already answered it.

by tomsal

Sorry if you are serious, no disrespect -- but after I saw the original post date (4/1) I was wondering if this is an April Fools joke.

But NO. It simply will not work.

by lordinfidel

Except, apparently comapq's smart Array has an extend array option. (see cb admin's posts to me above)

Now I personally have never tried it, although I do have plenty of compaq servers with SmartArray controllers. Nor do I plan on ever trying it.
"Danger Will Robinson, Danger"

This definetly can't be done on adaptec cards or dells PERC cards (which is based on adaptec anyways).

It just seems too much like partition magic to me.

by lordinfidel

After reading http://tinyurl.com/9cf3
Compaq's site, certain smart array controllers will let you do this.

Although there are caveats, and I personally would never, ever, ever even think about doing this. In my opinion it is too risky. And I'ma seasoned admin who is accustomed to taking risks.

The de facto raid controller, no you can not just add another drive, so I am not mistaken in this area. I've set up so many servers with raid that I can do it in my sleep while eating a banana and singing the german national anthem.

Now you said that you paid a consultant to add space. But yet you say you only have 4 drives.
3 are configured as Raid5 and the other as a HotSpare.

So my question is, how was it possible that he added more space. I say this because the Min Raid5 config is 3 drives. So if before the consultant came, you had to have 3 drives at the bare minimum for Raid5 to even work. Which would of have meant that you had at the most 36 gigs of storage space.Now if he added a 4th drive and that drive is in fact the HotSpare (which by definition is not used until one of the other drives in the array fails). Then you should still *only* have 36 gig's max storage space.

Now if he did use the expand utility and was able to get you up to 54 gigs of space, you do not have a HotSpare. Because in order to get 54 gigs of space from 18 gig drives, you need 4 x 18 gig drives.

The equation for Raid5 is: #of drives - 1 * gigs per drive = total avail space.

by cb admin

Again, sorry, I guess I forgot to specify that the addition of a drive that the consultant came and did for me was on a different server. That server, compaq proliant 1650R, had 4 drives (RAID5, 3+spare) in the end after the addition I ended up with 5 drives on my server. Now I'm trying to do the same to this new server, and that's why I'm looking for some sort of help, and again, working for a non-profit with limited budgets I can't afford a consultant.

Thanks your feedback.

by clarkietii

Have followed up on your reply on raid and seems you are a wizard on the same. Why dont you enlighten this chap on more about raid. Where can I get infor on raid

Thanks

Clark

by rschmid1

You need at least two of the three drives to have that happen.

One drive get half the file info, the second drive gets the other half, the third gets a parity bit set to track the two files and allows restores if either of the two fail.

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