[CentOS] Advice on partitioning a Dell MD1200 disk array

Tue Sep 4 13:37:27 UTC 2012
Blake Hudson <blake at ispn.net>

Tony Molloy wrote the following on 9/4/2012 6:10 AM:
> Hi,
> I've just got possession of a Dell PE R720 with 2 MD1200 disk
> enclosures.
> Both MD1200 are fully populated with 12 x 3 TB disks
> The system will basically be a student file-server running CentOS 6.x
> serving various size files from small c programs to multi gigabyte
> audio and video files over GB ethernet.
> The first MD1200 will be configured as the NFS disk. The requirements
> are for 6 fixed equally sized partitions, one for each cohort of
> students. For this I was thinking of splitting the MD1200 into 2 RAID5
> arrays with a hot spare each. Then partitioning each into 3 ext4
> partitions.

I think that sounds like a balanced approach. With 12 SATA drives you're 
bound to run into failures during the life of these devices (3-5 yrs). 
By splitting the disks into 2 arrays, a failure of one disk will only 
degrade 1 array, affecting less users. Rebuild times will be reduced if 
there is a failure and performance for multi-user workloads should be 
increased by separating into multiple arrays.

> The second MD1200 will be used to backup the first, using BackupPC and
> for other storage purposes.
> As I won't know the storage requirements for the "backup partition"
> and they will probably change over time anyway. I was thinking of
> using LVM for it. So how to partition the MD1200 for LVM. I don't want
> to put all 12 disks in  a RAID5 and put a LVM volume on it. Can I
> split it into 2 RAID5 and have a LVM volume spanning both.
I've never found any value in LVM. To me, it adds another layer of 
complexity or room for bugs to creep in. Arrays can be resized, GPT or 
MSDOS partitions can be resized, file systems can be resized, your 
hardware can do the spanning that LVM provides, why would you want to 
add another layer of abstraction? For similar reasons, I would stick to 
a file system that comes stock with your OS, ext4 or XFS should be fine, 
as they are both well supported in CentOS.
> Any suggestions.
> Just remember I'm due to retire at the end of this month so this will
> be my last big job for the Dept. And due to financial constraints I
> will not be replaced. So I will be handing this machine over to a co-
> worker who is basically a Windoze admin with only a basic knowledge of
> Linux so nothing too fancy.  ;-)

At first, I was going to recommend 1 hot spare, but if you think the new 
admin may not have as much time, or you want things a little more 
simple, then making two identical arrays is probably going to be easier. 
If he's not as familiar with your hardware, then I would print out a 
cheat sheet on how to rebuild the array, check the status (MegaCLI?, 
OpenManage?), etc. As always, document what you have, how you put it 
together, and who to call or where to look if he's stuck.