Hi there,

I recently purchased my first server for colocation and I just wanted to ask those with experience with Dell PowerEdge servers for a bit of RAID configuration advice.

The server specs are as follows:

Dell PowerEdge 2850
Dual Core Dual Xeon 2.8 with 8GB RAM
5 x 73GB 15k SCSI drives
Embedded PERC 4e/Di RAID Controller

My Dell rep recommend that I set up 2 RAID-1 arrays and use the 5th drive as a global hot spare, which is this server's current configuration.

I have CentOS 4.3 and cPanel installed on the first RAID-1 array.
The second RAID-1 array is currently mounted as /backup and used to store cPanel's archived backups.

This server is currently in production, but I anticipate that I will run out of disk space on the primary RAID-1 array before the server is at its capacity with clients hosted on it.

Is it possible to add a 6th hard drive and create a third RAID-1 array using the embedded RAID controller for use as /home2?

If not, is it possible to recreate the second RAID-1 array as /home2 and add a 6th drive that is not in a RAID config for use as /backup (since it doesn't need to be in a RAID array), while keeping the global hot spare?

If the above two options are not possible, will I have to explore using RAID-5 or RAID-10 for the last 4 drives, keep the initial RAID-1 array?

Thanks for your help!

So it's a 1x6 backplane?

I'd do a Raid 10 on the last four if you don't intend to change the primary raid 1 either. Raid 5 would give you another 60GB of space but wouldn't do anything really for performance vs raid 10. I'm curious as to why he recommended you split up the drives like that when it wasn't a split 2x4 backplane.

dell 2850 should have 6x hot-swap SCA bays, so adding the 6th HDD shouldn't be a problem.

why not just add a 146G/10k (big enough to cover 2x RAID1, 10k drive is good enough for "backup") and mounted as "/backup", convert the 2nd RAID1 as "/home2", then keep the global hot-spare which is always a good idea for multi array sets.

146G/10k SCA should be about the same price with 73G/15k.

My order quote shows that it's a 2+4 Split Backplane Daughtercard with Embedded RAID. (My RAID config menu shows 3 drives per channel, I'm not sure if that matters though or would tell what kind of backplane I have.)

Will C.W. Lee's suggestion work in this case, as that would likely be the best and most logical for me?

I guess I could see if I can disable the drive that's currently acting as the global hot spare and see if I can use it outside of a RAID config.

it shouldn't matter if it is 1x6 or 2+4 as long as they are connected to the same RAID controller, otherwise the hot-spare shouldn't be working at all....

you can always set single drive as "volume" or RAID0 in RAID BIOS.

Great, that will be perfect then!

Thanks for your advice.

Not my preferred way of setting up a 2850's with a 2+4 backplane, but if you're not after performance that'll do what you want regardless.

If you ever do change your mind and want to setup Raid 10. The Perc4/EI is a little wierd about setting up Raid10. You need to create two Raid 1 sets, then span them together.

I've read that Dell's RAID-10 setup on these controllers isn't true RAID1+0 but rather just concatenated RAID-1 without striping.
(http://docs.us.dell.com/support/edocs/software/smarrman/marb32/ch3_stor.htm)

While we are at it what is the difference/benefit of a 1x6 and a 2x4 backplane? I might want to do things different on the next server I order as I just went with what they suggested for this first one (which may not have been the absolute best for my needs).

Like I said, not standard but the performance benefits are still there. I've tested my Dell PE 1750's, 1850's, and 2850's all with Perc4/DI using Raid 1, Raid 5, and Raid 10.

The 2+4 lets you split up the I/O, so if you're concerned about IO you can keep your MySQL databases on a raid 10 drive, and the rest on the 2 drive Raid 1. The only way to improve upon that would be adding a 2nd SCSI card. Just depends on your requirements, and potential bottlenecks (if any) from your disks.

Raid 1 (146GB 10k) did about 80 MB/s

Raid 5 (3 Drive 146GB 10k on a PE1750) 70 MB/s

Raid 10 on a 2+4 backplane maxed out at 140 MB/s with 4x36GB 15k drives

SCSI has this annoying channel bandwidth limitation. The performance will hit this bottleneck when you have more than 5 to 6 drives on the same channel. Having split backplane means actually having two channels, to avoid this bottleneck.

The problem is, since both of the channels of the ROMB are used for internal drives, you cannot add a internal-to-external adapter and hook up external arrays.

There is another school of thought - when you set up a RAID-10, you should have the each drive of each RAID-1 on seperate channel. For example, let's say you have four drives - the first RAID-1 would be channel-0, drive-0 + channel-1, drive-0, the second RAID-1 would be channel-0, drive-1 + channel-1, drive-1. The idea is, you can lose a whole channel and you will still have your data. Of course, it would make more sense to have 3+3 backplane but it is probably complicates the server internals a lot than 2 + 4.

The PERC-4Di is pretty fast. I am sure you will be very happy about it.

Thanks for all the information, definately good stuff to know!

Hi,

I have also a Dell 2850 with 6 HD's:
2 in a RAID 1 2x36 GB with LD 0 of 36GB (changed into 1x72 & 1x144)
3 in a RAID 5 3x72 GB with LD 1 of 144GB
1 as HotSpare

The 'problem/chalange' is as follows:
The RAID 1 is getting to small (2x36GB) for my Windows 2008 server C drive. So I changed one 36GB disk for a 72GB and after a couple of days the other 36 to a 144GB disk (because I had those two and not 2x72 of 2x144). Everything is working fine. But now I want to enlarge/expand de Logical Drive (LD no. 0) from 36 to 72 GB.
I only managed to make an extra (third) Logical Drive (LD no. 2). Now I do not know how to delete this extra LD 2 on RAID 1 AND how to expand the RAID 1 LD 0 into a LD of 72 GB.

I also have a Dell 2850 with
Embedded PERC 4e/Di RAID Controller

I am using 2 drives configured with RAID 1



As an adjunct regular back-ups every friday we want to hot swap one of the drives and store it. Hopefully over the weekend the swapped drive will be fully rebuilt. We have 10 extra hot swap drives and want to rotate these.

My question is this.... if our server is hit with a nasty virus or we suffer from unrecoverable data corruption, can we pull both drives and insert one of the drives we pulled from the prior Friday ?