1 file system, 2 drives?

[David Rawling]

  On 27/07/2010 6:54 AM, John Almberg wrote:
> John Almberg wrote:
>>> If you have hardware controller with RAID capabilities, using native 
>>> RAID is better, otherwise look towards gvinum or maybe ccd; see also: 
>> I've just been reading up on RAID in my Absolute FreeBSD book, and it 
>> occurs to me that my client has a SCSI RAID drive chassis that he is 
>> using stupidly...
>>
>> It's a 14 bay drive, and he's currently got seven 32G drives stuck in 
>> it, configured with RAID-0. This is the original 200G drive I was 
>> talking about. It's a few years old.
>>
>> Over the next few years, this guy is going to need lots of storage 
>> for his videos.
>>
>> After a bit of reading, I'm wondering if the best idea might be to 
>> toss out those 32G drives and replace them with 3 big (say, 300G) 
>> drives configured with RAID-5. It sounds to me like a RAID-5 array 
>> can be expanded by adding new drives.
>>
>> QUESTION: is expansion normally a matter of just plugging in a new 
>> drive? Is the new drive automatically grafted onto the old drives? Or 
>> do you have to go through a process like, backing up the data, 
>> plugging in the new drive, reformatting the expanded array of drives, 
>> and restoring the data.
>>
>> I don't know the brand/model of the RAID drive chassis, but the 
>> client thinks it can be switched to use RAID 5. I'm waiting for the 
>> technical details, but assuming it can handle RAID-5 for now.
> Answering my own question...
>
> So its a HP 6402 / 128 RAID controller. From a quick skim of the 
> manual, it looks like the controller has to go through an 'expansion' 
> process when adding a new drive. This sounds time consuming, but more 
> or less automatic -- i.e., handled by the controller.
>
> Sounds like this might be the best way to go.
It's been a while since I dealt with HP SCSI RAID, but ISTR that you'd 
need to install and configure the 3 disks as a RAID 5 set, copy the data 
from the 7x36GB array to the new array, (using a temporary mount point, 
generally, and dump | restore) switch the mount points across so that 
the /videos tree is the new copy, then remove the RAID0 set from the 
controller.

You may or may not find that the RAID controller changes LUN IDs after a 
cold start too, so LUN 1 (new RAID 5) suddenly becomes LUN 0 on the cold 
start after the old RAID set is decommissioned and pulled. This is often 
accompanied by a heart attack on the part of the person restarting the 
server.

After that, though, expansion is a cinch - but it will be quite slow 
since it needs to read and write the entire content of all disks. I'd 
therefore go as many spindles as you can - 3 disks, 5 disks and 9 disks 
are what I recall as being optimal groups for RAID 5.

Also consider that you can supplement the RAIDs with the BSD tools 
previously mentioned. Today is 3 x 300GB. Tomorrow add another 3 x 300 
(assuming IOPS is OK) and concatenate them to be a 1.8TB "disk" - 2D+P + 
2D+P.

Dave.

-- 
David Rawling
PD Consulting And Security
Mob: +61 412 135 513
Email: djr at pdconsec.net