Best RAID setup for Home Directories

Question

Level 1

25 points

Best RAID setup for Home Directories

Hi there,

I'm looking to upgrade our existing infrastructure which is 5 or 6 years old. We have an old Xserve RAID which has 4x 250Gb disks in a RAID 5 configuration which gives about 640Gb of storage for home directories. However the peak number of users has shot up to something like 85.

I want to be able to increase the storage space for home users but also to provide a RAID storage solution that has optimised performance for the number of users we now have.

Is there any kind of ratio or guideline to the kind of configuration/system I should be looking at? Can anyone offer any advice?

It's a gigabit ethernet network, so obviously I realise that performance is limited by that also.

Any help much appreciated.

Mac OS X (10.5.6)

Posted on Oct 7, 2009 2:45 AM

Reply

Answer 1

Camelot

Level 9

54,333 points

Oct 7, 2009 8:19 AM in response to Fridgemagnet

Is there any kind of ratio or guideline to the kind of configuration/system I should be looking at? Can anyone offer any advice?

What specific issues are you running into now?

i.e. what is your primary driver here? Is it to get more space? or is performance suffering? or are you just concerned that the XServe RAID is getting old and you're concerned about reliability?

Those are going to be important questions to ask before anyone can suggest a path. Without knowing what problem you're trying to solve it's hard to find the right solution.

Also add to that the host system that the RAID Is connected to (i.e. the machine that's sharing the RAID to network clients. It's entirely possible for that machine to be the bottleneck, so it needs to be considered, too.

Reply

Answer 2

Fridgemagnet Author

Level 1

25 points

Oct 7, 2009 8:48 AM in response to Camelot

One of the issues is space, although I have space on the RAID itself for more drive modules the only sizes I'm able to source (1tb) would be expensive even just to replace the four drives that are already being used for the user homes and If I add 1tb drives to the existing setup I think they'd only use 250Gb each? Is that correct?

Users are seeing a lot of beach-balling, it comes and goes, particularly bad in MS Office 2008 for which I have some folder redirection in place and I'm also redirecting cache folders.

The system that the RAID is connected to is fairly new, Dual Quad core Xeon with 8Gb of RAM running 10.5.8. It occasionally spikes to 90-100% cpu at busy times but generally CPU activity is around 20% during the day.

I can configure link aggregation but I understand that will only help with redundancy not performance!?

I have to do something about storage anyway and I'm wondering whether if I have more physical drives in my RAID configuration it will improve performance for clients (80 users writing to four drives feels like it might be an issue) which is one of the main reasons for my hesitancy in replacing the existing drives like for like with 1tb drives.

I know there are many things that could be affecting performance and it's part of a larger Windows network where although it has some fairly new components (newish switches etc) a lot of things are poorly configured, no VLANS etc.

Thanks!

Reply

Answer 3

Fridgemagnet Author

Level 1

25 points

Oct 7, 2009 9:06 AM in response to Camelot

Just to mention, the Windows network admins want to implement a SAN over the next couple of years which would integrate storage across both our networks, I agree in principle, I'm a little worried about their ability to deliver on this but want to be co-operative which is what has jinxed me from seeking the funding for a full on replacement for the ageing Xserve RAID. But storage is low now and so is performance so I either need a stop gap with the existing RAID which will do for at least a year if not two, or I could seek funding for something like a Drobo!?

Reply

Answer 4

Camelot

Level 9

54,333 points

Oct 7, 2009 11:29 AM in response to Fridgemagnet

OK, that helps a lot.

The CPU load on the server seems a little higher than I'd expect. What's the activity level of the RAID? Are the disks continually active? Are the fiber channel activity lights (the vertical columns of LEDs in the middle) all active?

That will help track the issue to the host system or the RAID.

If the disk activity lights are always busy then it may be an issue with the number of drives for the number of users, and more physical drives would help.
This could also be indicative of a too-full volume (RAID 5 volumes will degrade in performance over about 85% full. At 95% full performance may be excruciatingly slow).

If the fiber channel activity levels are high then the problem is the amount of disk access and your best bet would be to expand to the other half of the array (this will double the available fiber channel bandwidth between the host and the RAID).

If the RAID activity levels aren't high then it could be a network bottleneck on the server, in which case link aggregation could help - it isn't just for redundancy, it will load balance client traffic over the links, effectively increasing performance for clients.

You are right that if you try to expand the current volume with drives, the drives will be sized to 250GB, regardless of their physical capacity. For this reason you would need to build a new array on the other side of the RAID and copy the data over.
The ideal situation would be 14 new drives which you could build into two 7-disk arrays which are then striped on the host system.

Reply

Answer 5

Oct 7, 2009 12:12 PM in response to Fridgemagnet

Documents such as "Deploying Mac_OS_X_for_K12Education" suggest that more than 150 concurrent users may tend to tax your Server.

I do not think your server is out of compute power. It is most certainly not obsolete. Camelot has provided some precise observations about XServe RAID performance that bear consideration long before wholesale replacement of either your XServe RAID or your Server computer.

Have you considered moving teacher-owned (or equivalent single-user) computers, fixed or portable, to Portable Home Directories? Have you re-assigned Caches from the User Homes to Caches on the workstations where they logged on?

Reply

Answer 6

Fridgemagnet Author

Level 1

25 points

Oct 8, 2009 3:29 AM in response to Camelot

Thank you both.

The disk activity lights do a very good impression of Close Encounters at peak times with all four drive lights flashing constantly. The vertical column of LEDs certainly tops out with all lights blazing but goes up and down.

I would love to populate all the bays on both sides but unfortunately the only drive modules I'm able to source are 750Gb ones which will cost £679.00 each (about $1100) and that's before tax. This was from a source recommended by Apple.

There are already two other 750Gb drive modules on the other side which are being used for less critical storage. If I were to move that data elsewhere and sacrifice their capacities down to 250Gb giving me 6x 250Gb drives, what would be the best configuration for them, can I put the two new modules on the other side of the RAID and add them to the array? Or do I need to evenly distribute them?

I haven't looked a great deal into mobile homes, a lot of the staff users move from machine to machine a lot, wouldn't this mean their home folder would be pulled across the network as they move from room to room, machine to machine?

I don't think it's worth spending a lot of money on this RAID because of the issue getting spares, nobody thought to order any replacement parts or modules 5 years ago when it was purchased so I just need to make the best of it until whatever SAN solution is sourced comes on line.

Thanks again for your help.

Reply