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I have a Lian Li Ex-50 Raid Array setup to be 4x 2TB WD Black in a Raid5 formatted to HFS+ (journaled).

The raid array is a hardware raid running through a Sil3132 chipset (SATA2 via eSata connection) on a Mac Pro 2008.

I have recently noticed that my Raid is having some trouble streaming data to my projector (I use the raid box as a media server to host the content on) where the video is basically stuttering or pausing in order to cache more data to continue playback.

The connection is via a Cat-6 cable from my Mac Pro to a Netgear Router which then connects with another Cat-6 to the Popcorn Hour C-200 media server, so all bits are cabled, nothing wireless.

Regardless this raid has worked splendidly over the past 6 months but it now starts having trouble supplying the media continuously so I was hoping there was a way to defragment or do some other kind of optimization that would help optimize the read performance for example.

Since this is my first raid array I thought it best to ask your advice before I do something silly to all my data.

  1. Is it safe to defragment a raid array?
  2. What other tools to optimize the array are there?
  3. Any ideas on what could be causing the read/streaming issues?

Thanks a lot for reading, I look forward to your advice.

PS: The Raid5 is 79% full, using 4.34TB and having 1.12TB of free space left.

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What router are you using 100/1000 Mbit? Are you uploading/downloading anything from the internet on your Mac Pro while streaming? Do you have any firewall / security (gasp) software installed? – Dustin G. Aug 29 '11 at 13:13
Good questions! The router is 1000 Mbit BUT the C-200 Media Center only supports 100 Mbit so up until the router everything is Gigabit whereas the actual transfer speed to the streaming device is just 100 Mbit. As far as downloading goes, yes, in this last occurrence I was downloading to the raid as I was watching/streaming data. I don't have any Firewall installed and my Mac Pro is the DMZ in the router (static IP, internal + external). – Jannis Aug 29 '11 at 19:26
What level of RAID are you using? 0,1,5 etc? – Dustin G. Aug 29 '11 at 23:40
I missed the RAID5 part - oops... – Dustin G. Aug 29 '11 at 23:54
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2 Answers

up vote 1 down vote accepted

There is a low-likelyhood of your filesystem being fragmented to the point of causing the issues that you are experiencing. Apple even has a support document discussing the need for defragmenting in OS X - not to mention that RAID can overcome performance issues caused by fragmentation (if there really is any) by speed that is introduced when striping is implemented (RAID 0, 5, 1&0)
Apple - About Disk Optimization

As far as network traffic goes, you have the bandwidth available on the network to feed the popcorn box... but you were correct in identifying that the issue in the realm of the RAID array and not the network.

You problem is 1 of these 2 Scenarios:

  1. You mentioned that you had an eSATA connection between the RAID and the Mac Pro - it is possible that the card is a bottleneck: This could be caused by A.) The SIL3132 having performance issues either by not being able to keep up or poor OS X support or B.) You are using a PCI-E 1x - 4X card that is having its bandwidth saturated and not being able to keep up with the downloading to the array and streaming. (most-likely B in this instance)

  2. The RAID controller in the Lan Li box has a weak XOR engine. Typically, RAID 5 on motherboards receive assistance from your CPU in handling XOR calculations to generate parity information that is stored on each drive to provide redundancy. Since the Lan Li box is a self-contained system, RAID 5 performance is determined by the muscle the XOR engine it has. If you are hitting this for multiple data streams, the bottleneck could be the controller.

I'm guessing that the issue is the eSata card on your Mac Pro - the first test would be to see if you experience the buffering issue when you are not downloading on the array and only streaming to the Popcorn Hour.

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Thanks for your very detailed answer! Much appreciated. A couple of questions though: 1) Is there any way of testing for where the bottleneck is? Something like a benchmarking tool that simulates lots of traffic to test read/write performance… 2) Can you recommend a high quality raid controller card for Mac OS? I 2 another raid cards alread a SIL3531 (for which there are no Mac drivers) and a Highpoint RocketRAID 622 card. Will my existing raid still work when the eSata cable is connected to one of those cards instead of the Sil3132? – Jannis Aug 30 '11 at 5:10
Look for an app called "Blackmagic Disk Speed Test" in the App Store - I am assuming you have Snow Leopard or Lion, so you have the App Store – Dustin G. Aug 30 '11 at 15:10
As far as switching out the card in the Mac Pro - it will see the Lan Li as a single disk or multiple, if you have partitions. The RAID is handled onboard in your Lan Li box... – Dustin G. Aug 30 '11 at 15:13
At this point, we want to know if the eSATA card in your Mac Pro is the bottleneck, or if the Lan Li system is not very good at performing RAID 5 - if it's your eSATA card, we can swap it out... if it's the RAID controller in the Lan Li, you will either have to switch to a different RAID mode or ditch the Lan Li altogether... lets hope for the eSATA card in your Mac Pro being at fault. – Dustin G. Aug 30 '11 at 15:16
Your next step is to run the Blackmagic Disk Speed Test and report your results here ;) – Dustin G. Aug 30 '11 at 15:16
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up vote 0 down vote

Yes, its perfectly safe to defrag the array. However, at 80% full you might run into issues, as defraggers like to have ~20% to move things around on the disk - you are right on the edge there.

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The 80% issue varies with the software used, some like diskkeeper will work so long as you have 1% free. – Col Aug 29 '11 at 12:23
yes, it may work, but with 1% free, it probably wont defrag efficiently – Keltari Aug 29 '11 at 18:49
Thanks for that, I will have a look around for Mac Os X defrag software and read their instructions as to available disk space etc. Hopefully that will make a bit of difference too. Is it possible to just add another 2TB drive to the existing Raid array? – Jannis Aug 29 '11 at 19:29
I dont know anything about your RAID controller, but most likely the answer is no. Some proprietary formats, such as XRAID will let you add disks to an existing RAID, but I doubt you have that; and even if you did, odds are you didnt set it for XRAID. – Keltari Aug 29 '11 at 19:35
Thanks for the clarification. Mine is definitely not setup to be 'extendable' in any way, so since I now know this isn't an inherent feature of Raid technology this option is out for me. – Jannis Aug 30 '11 at 5:01
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