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RAID 5 migration to larger array

A

asmielia

Limp Gawd
Joined
Aug 10, 2003
Messages
337
Hi,
I had a question about RAID 5. I'm thinking about getting a simple 3x400gb setup and a 4 port PCI-E raid card. The cards I've been looking at have online capacity expansion, so I could add an extra drive later on which would bring me up to 1.2TB.

I was wondering if this scenario would be possible:
-3 years from now, I'm out of space on my 4 drive array, and 1TB drives are now the sweet spot in price/performance
-I remove one of the 400gb drives, and put in a 1TB drive in its place, and rebuild the array
-repeat this process with all drives until I have nothing but 1TB drives in the system

Would it be possible to then adjust the array size without killing the data stored?

Thanks,
Adrian
 
asmielia said:
Hi,
I had a question about RAID 5. I'm thinking about getting a simple 3x400gb setup and a 4 port PCI-E raid card. The cards I've been looking at have online capacity expansion, so I could add an extra drive later on which would bring me up to 1.2TB.

I was wondering if this scenario would be possible:
-3 years from now, I'm out of space on my 4 drive array, and 1TB drives are now the sweet spot in price/performance
-I remove one of the 400gb drives, and put in a 1TB drive in its place, and rebuild the array
-repeat this process with all drives until I have nothing but 1TB drives in the system

Would it be possible to then adjust the array size without killing the data stored?

Thanks,
Adrian
Click to expand...

I have done this with SCSI drives on a HP Proliant with a Smart Array controller. yes you can.. but then you have to expand the array and the partition after you are done to use the rest of the space... So you need a controller that will do OCE (Online Capacity Expansion) and a product like Partition Magic (or Server Magic for server OS's) to do this successfully.

==>Lazn
 
That will depend on if the raid controller has that ability.. I have seen it on SCSI controllers, but I've never used it.. I think I would rather get another controller and build a new array and move the data to the new array. Or replace the drives, then restore the data from backup.
 
Except that all the drives in an array need to be the same size, so you couldnt exactly add a 1TB drive to an array of 400GB drives. To migrate to larger drives, you'd need to add at least 3, create a Raid5 array, and then move the data over from the array of 400GB drives.
 
RavenD said:
Except that all the drives in an array need to be the same size, so you couldnt exactly add a 1TB drive to an array of 400GB drives. To migrate to larger drives, you'd need to add at least 3, create a Raid5 array, and then move the data over from the array of 400GB drives.
Click to expand...


not true.. You can add a larger drive to an array, but it will only use the size of the smallest drive. There have been times when a drive failed in an array and I had to install a larger drive in place, you just loose the extra space on that drive.

You need a controller that is capable of expanding the array once all the drives are replaced, otherwise you just end up with an array of 1TB drives with only a usable space of the old 400GB drive array. This is a long process as the array needs to rebuild after each drive is swapped out. It is faster and easier to build a new array and move the data onto it.
 
Are you sure that "online array expansion" includes the ability to expand from 400GB used on 1TB drives to the full 1TB. I was under the impression it just allowed you to add drives to the array without losing data.
 
No.. Just because a controller has OCE does not mean it can do this. standard OCE is just adding new drives into an existing array, I'm not sure what you would call this. You would have to check with the mfr. of the controller you choose to see if it is capable.
 
Or just avoid the headache and buy a new controller in a few years.

That's my plan. Once I max out my 2TB array (8x 320GB, Raid5), I'll look at using 750GB or 960GB drives on an entirely new controller.
 
Thanks for the responses guys. I just wanted to know if it was possible and it seems like it is.

Adrian
 
asmielia said:
-I remove one of the 400gb drives, and put in a 1TB drive in its place, and rebuild the array
-repeat this process with all drives until I have nothing but 1TB drives in the system
Click to expand...
If you do this, the 1TB drives would be added to the array with 400GB useable. Even if you replaced all the 400GB drives with 1TB drives, in this scenario, only 400GB would be useable from each 1TB drive. Online capacity expansion wont help here, it only allows you to add drives to the array and rebuild it without losing data - it wont let you change how much of each drive is allocated to the array, which is what you'd need here.
 
RavenD said:
If you do this, the 1TB drives would be added to the array with 400GB useable. Even if you replaced all the 400GB drives with 1TB drives, in this scenario, only 400GB would be useable from each 1TB drive. Online capacity expansion wont help here, it only allows you to add drives to the array and rebuild it without losing data - it wont let you change how much of each drive is allocated to the array, which is what you'd need here.
Click to expand...

Not sure what it is called, but my SmartArray controllers from HP can do it.

The drives show up with empty space available on them untill you expand the array to use the unused space. (then once the array is expanded, you have to expand the partition)

==>Lazn
 
I have heard of a person doing an equivalent operation on a Promise SX4 RAID card. It supports OCE. After replacing his smaller drives (one by one) with larger ones, he was able to extend the partition into the free space. it did take a while thanks to the rebuilds.
 
OCE is an up-time preservation tool, not something that replaces a backup. If you had a backup, this issue wouldn't be such a concern. Moreover, even with OCE, you should have a backup, because things go wrong sometimes.

3x400 GB RAID 5 => < 800 GB of data => can probably fit in a single 750 GB drive.

/em goes offline to check his own backup
 
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