Adding capacity to a RAID array
Posted in General, Storage Applications, Platforms, Storage Interconnects & RAID, Storage Management, Application Environments, Advisor - Neil by Neil(I’ve been away from the keyboard for a while, and needed to get the fingers working again, so I’ve stayed away from anything new we are doing so as not to upset the marketing lads any more than usual and have gone for something simple to get the hang of this again 
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There are two ways of adding capacity … I call them horizontally and vertically. By horizontally I mean across the array … adding more drives to an array to increase it’s size.
Adding capacity vertically is what I call the ability of a RAID controller to increase the size of an array by using additional space that may exist on the disks already in the array.
Thinking horizontally …
The most common expansion request we get is whether you can just add a drive to an array and increase it’s size. The basic answer (without all the caveats) is yes. Add the drive to the system (hot swap is always good as you don’t have to bring the system down), then go into Adaptec Storage Manager, right-click on the array and “reconfigure the array”. Simple. Choose all the drives, RAID type and set your capacity.
Thinking vertically …
This one is not so obvious. It’s basically the same approach - reconfiguring the array but just choosing a new size. Lots of people just seem to miss the fact that you can use the same disks etc, just increase the size. Of course there needs to be space on the disks, but you might have deleted another array freeing up space, or, heaven forbid, changed the drives over to larger disks by using the dreaded “replace one at a time and let the array rebuild” method. Note that there is also another way that we don’t advertise to migrate an array from one set of drives to another set, but I’ll put that one in a separate blog.
Either way, if there is space available on the disks you can grow the array without any great hassle.
Note that you can do this live without shutting down the system (read the caveat about hot swap drives). The RAID card will just chug away in the background expanding the array while you are still using the system. Will there be performance degradation? Yes. Will it kill your system? No. Do it over the weekend.
Now the caveats …
HostRAID cards don’t go past 2TB. If you want >2TB from Adaptec look at our 2, 3 and 5 series Unified Serial cards.
There are a couple of major issues here which revolve around the OS, not the RAID array. XP 32 does not support greater than 2TB - end of story. Other, newer Windows OS do support greater than 2TB (check the limits with the vendor), but not for the boot volume. There is nothing you can do about these limitations, and for that matter there is nothing we can do about it either.
2TB? Sounds big, but today it’s an absolute no-brainer to break this limit so check out your OS as well as your RAID card before going big.
There, 10 minutes of typing and I feel all warm and bloggered again - that’s better.
Ciao
Neil
September 6th, 2009 at 6:29 am
I am still confused. (but a much higher level!!)
I have a 31205 configured with 6 1.0 TB WD drives. Using Raid 5 I now have 3 logical drives shown as 1.99 TB, 1.99TB and 559GB.
I just added another WD 1.0 TB to the system. I do not see how to increase the size of pool and have the 559GB increased and all 3 logical drives striped across all 7 physical drives.
Could you please explain. I have studied the users guide and used the help function but I must be missing something.
September 7th, 2009 at 5:04 pm
Al,
Glad to see that I didn’t do myself out of job in one hit … I’ll have a second go though …
I’m surmising that you created 2tb arrays because of limits within your operating system (which would point to XP 32-bit). Of course you can’t have a boot disk greater than 2tb which would explain the first array, but then that doesn’t explain much because I wouldn’t really have a 2tb boot array (that’s a whole chunk of Windows OS to go wrong).
However, for whatever reason you have done this, the result is that 1tb drives give about 930gb space. Your two 2tb arrays are taking up approx 405gb each from each drive, leaving your 559gb array with chunk of approx 120gb.
Now if you want to add a drive and increase the size of arrays, you have to think about the structure of each array. The 2tb array (I’ll explain one but they are both the same), is made up of 5 x 405gb chunks of data and 1 x 405gb chunk of parity. It’s not quite that way on the disks, but that’s the easiest way of thinking about it.
Now if you want to increase the size of the array by adding a disk, all the chunks on all the disks needs to be the same … which sort of makes sense … they are all members of the same team and all need to have the same capabilities. So when you add a drive to the system and “modify the array” the only choice you have in capacity terms is to increase the array by 405gb in size.
The same goes for the 559gb array. Each of the chunks are 120gb. So the only use for the new disk is 120gb can be added from the new disk to the array (increasing it’s size by 120gb).
You have probably picked the worst possible example of disk utilisation and capacity expansion that I can think of to highlight the point I was making. My previous blog was a very simple scenario, while you have a complicated scenario.
Without knowing anything about your system, I’d really have to ask why you have 3 arrays of the sizes you do at the moment … they are really limiting your options in this scenario.
Ciao
Neil