RAID 5 Questions

[dwayne001]

Hope these don't sound too stupid, but let's start here:
From "http://www.storagereview.com/guide2000/ref/hdd/perf/raid/conf/advExpansion.html"

Logical expansion comes down to the RAID level you choose to employ, and the features of your RAID controller. Some will allow a drive to be added to a striped set (with or without parity) while others will not. If your controller supports expansion you are in good shape--the controller will let you add the drive and then will reconfigure the array to use the added storage. If your controller doesn't support this advanced feature, you will have to destructively "disassemble" the smaller array and assemble the new, larger array. This will generally result in all the data on the array being lost, so you must be sure you have it properly backed up--and tested--before you do this.
Even if your controller supports logical expansion and you have the room for the drives, you should think twice about doing it for the following reasons:
· Size Matching and Capacity: With few exceptions, all RAID arrays are constrained by the size of the smallest drive(s) in the array--any drives larger than this will not utilize their additional capacity. This means that if you have an array with five 9 GB drives and you add a spankin' new 36 GB drive to it, surprise! 75% of the drive will sit there wasted.
· Performance: Older drives that are much smaller than new ones are also slower than new ones. You will lose most of the benefit of a new drive if you put it in an array with a bunch of smaller ones.
· Reliability: If the existing drives are old enough to be too small, they are probably at least nearing the end of their service life. This means they will become increasingly likely to fail.
For all of the reasons above, many RAID users never expand their arrays: they just use all the drive slots on the case, and when the drives get old, they replace them. They give up the storage of the old array, but with drive capacities doubling every year or two, this can make less difference than you might think. If you pay $X for four drives of a given capacity today, in three or four years that same $X will buy a single drive with capacity as high as the entire array! There are arrays still in use from the mid-1990s that have 4 GB drives in them; a new array with disks comparable to what these drives cost five years ago would dwarf the capacity of the old array.

According to this, once cannot expand an array past the size of the original/smallest disk. So what if you, over a period of a month or so, swap out all of the disks with larger ones? An example of what I mean: if you had three 40gb drives in RAID 5, and you added a 200gb drive, to have four drives in total, you'd only have 160gb all together. But then you, after the array is all taken care of, remove one of those 40gb drives, and then put in its place another 200gb drive. You continue this, always waiting for the array to right itself after a change until all the 40gb drives have been removed and all that is left are 4 200gb drives.

Is this even possible?

Moreover—every time you take a drive out, what happens to the data it held? I know there's some redundancy in RAID five, but I imagine some of the data would be lost….if you plugged one of those removed drives into another system, would whatever's on it even be readable?


I can't seem to find the answers to these questions anywhere, which is why I turn to the [H].

My post was lengthy, So I can understand if you don't have the time to fully answer. As such, any resources you could point me to in these repects wuold be much appreciated.

I'm hoping Ice Czar has a chance to respond to my inquiries. *_*

Thanks in advance to anyone who wants to share

 

[Jabroni31169]

Two your first question...no....that would not work. You would have to recreate the raid array to use all of the 200gig on each disk.

No you dont loose data...hence the striping with parity....if you loose one drive you fine....if you loose two drives at once...your screwed. And most of the time...when a drive dies...its dead. You could spend a large amount of money to recover the data but only large large companies do that.

 

[dwayne001]

Even if you let the array rebuild the data on the new drive? And then you take away another drive? And then you let it rebuild? And so on until all of the drives are 200? Wouldn't that make the smallest drive 200, so the array would take advantage of it all?

 

[Jabroni31169]

Neg...because when it rebuilds the first drive....it only builds the size of the rest of the drives....IE: 20, 20, 200 is seen as a 20, 20 ,20. SO when you rebuild even after the first 200 its still seen as 20, 20, 20.

 

[Leadman584]

If you put 5 36GB drives in RAID5, as I have done on numerous occassions, total available space is roughly 144GB. If I were to somehow manage gettting a 50TB drive, the array would still be 144GB Max storage capacity. Smallest drive x (total drives -1). Moral of the story is that if you are running a server, don't put a tiny drive on a connector to fill the space, you substantially reduce the capacity of a RAID 5 array.

 

[dwayne001]

Thanks, so in a year i'm gonna have to rebuild the array anyhow....

but that was quick and painless, thanks.

Now, If i have an array of 3 200gb disks in raid five....does that mean I get ~400gb in space total? Since one of the drives is used for parity?

And adding to that-- should I decide to add another 200gb, I assume the array would just grow to 4x200gb - 200gb = 600gb. Is this the case?

As much as I feel like i'll be laughing at myself 8months from now, I think i'm gonna try and pull off a raid five array starting with either 3 200gb or 3 250gb disks. The controller i'm interested in supports up to 8 drives, so i think i'll add to it as time goes by. Any thoughts on this approach?

 

[Leadman584]

Actually sounds quite sane to me (who am I to judge, I'm nuts). With a really good controller, it will auto build onto every new drive you manage to put on it. I've seen mine rebuild arrays on 3 occasions, basically cause I'm an idiot. Experiences, obviously will vary.

 

[doormat]

Make sure if you get an 8 port raid card, that your card supports online capacity expansion - not all cards can just see and add any new HD you plug in. I believe some 3ware cards and the broadcom/raidcore cards do it, but check the individual card's capabilities.