Hi Everyone,


Short story:
I'm trying to use software raid-5 in R2 on 4x WD15EARS and I'm getting max 45mb/sec write to the array, and while writing (even as low as 20mb/sec) I get stutter when playing video files (tried multiple files, sources, codecs). Is it normal?
(system spec at the bottom)

Long Story:
I have set up my HTPC with 2008 R2 using the guide and I must say, it is working wonderfully.

I have been wondering if anyone else tried using the R2 RAID-5...

I got my hand on 2 more drives (WD15EARS) on the cheap from a friend, so I wanted to used the raid 5 instead of making a second raid-1 array wit the 2 new drives.

I was using the software raid because of 1 simple reason:
Since this machine also holds my file server, I wanted to make sure to be able to transfer the array in case I decide to put the file server in another box, or if the motherboard goes belly up.

Raid-1 was working perfectly and I confirmed that I was able to move the array between OS and architecture. ( I transferred the array from a windows 7 x86 install to the present R2 x64. )

I got the data off the existing array and got the Raid-5 built...

Those that used it or know about it are laughing right now....

4 days later... the array is FINALLY initialized... (by now I'm already shopping for raid cards on ebay...)

I was happy for about 5 minutes...

I started to copy files on the array, and started back mediaportal.

Apparently, when I get "heavy IO" (writing to the array), I can't play anything.

I have tried multiple sources (including different codecs) and multiple players.

Yes the CPU usage is up from idle, which is normal for software raid, but not enough to cause such stuttering.

I also get very low write speed to the array, around 45mb/sec max.

Just wondering if anyone have any ideas on what is going on...

My best guess at present are either this is caused by software raid itself, not being to calculate parity fast enough, even though it could be doing it faster if I judge by the CPU load

or

The controller is getting bottlenecked by the array and can't manage to process the data fast enough

Wow... sorry for the long post...

***EDIT***
Added the short version, and this to help:

I'm using a 4850e (2.5ghz X2) on a Gigabyte GA-MA78GM-S2H, Radeon 5450, 2gb DDR2-800, boot disk is a 60gb Corsair Nova

Hello deskwizard,

Hope you had a happy thanksgiving man.

A RAID 5 array uses block-level striping with distributed parity. A 4-disk RAID array allows you to use 3/4 for striping (data) and the 4th drive for parity. So your fault tolerance is only 1 drive. If more than 1 drive fails, the RAID array fails. Unlike raid 4, the parity is built across all 4 drives in a distributed manner, as shown:





You are exactly right.

The problem with this method is that in order for the raid array to place the parity on the correct drive (because not all your data size is the same) RAID 5 will use a mathematical XOR operation to play a game of elimination to find out where the parity belongs. This algorithmic method can exhaust your CPU and RAM due to the operating system performing the functions of RAID. All of these constant parity calculations that the OS has to peform makes its very difficult for achieving high write speeds. In a distributed parity the RAID array has to read the blocks from the other drives as well to determine where it should place the parity so you have constant reading and writing at all times which can spike your CPU. You can combat this by adjusting the striping size when you created the spanned volume in Disk Management. Try changing the allocation size from "default" to 64KB, 128KB. If it will even let you. I'm not for sure. I've never bothered with software RAID in windows. So don't hold me to it.

The advantage of a hardware based RAID controller is that the integrated processor on the RAID controller card performs the parity calculations for you, therefore drastically reducing latency and improved throughput as a result.

Software based RAID 5 in Windows is very limiting and taxing due to the constant XOR parity calculations that need to determine where the parity should be built. There is not much you can do to improve the performance of your software RAID 5 configuration, but you could try the following tips:

Enable write-caching on all 4 of your drives in device manager:



Also, in Disk Management, try setting the striping size (allocation size) to 64KB. Try copying some files. Now try 128KB. Then maybe 256KB. It might help a little bit with small files but larger files might still exhaust your machine. Changing the striping size might not tax the CPU as much and as a result you might able to sustain a nice software-based RAID configuration.

The problem with cheapo hardware RAID cards is their lack of cache and below average processing to handle the XOR parity calculations and they really don't provide a "significant" advantage of software RAID. If you want to use a hardware RAID configuration, be prepared to fork out some $$$, in 3 figure sums my friend.

If you don't have much data to lose (or can copy all the existing data to another drive separate from the RAID) I would recommend unraiding your 4 drives and then aligning your disk partitions so they are offset right. The problem is if you have one drive with an offset o 1024KB and another with 512KB it will hurt software RAID performance because it is not storing the blocks in an organized manner. To fix this..unraid the drives so they are all 4 separate drives and then when format each one with an allocation size of "64KB". Software-based RAID 5 seems to function best at that striping size from what I've heard.

Hi halladayrules,

Thanks for the great lenght explanation on all this! It was pretty much what I was thinking of, just with the right words

Yeah I saw the price of the hardware raid cards.... I was about to bite the bullet when I thought about something....

The ATI installer for the SB drivers wont let me installed, but what If I did it manually...

Yeah, it was the god damn drivers creating this issue.

Since then, no more stuttering at all....

Speed stayed mostly the same, and will be slower if the server is doing other tasks,
but overall, I'm satisfied for now...

Wow, just saved 100$ ... thank god I checked that before I bought the damn thing, 'cause believe me, I was CLOSE.... lol


Thanks again,

Topic closed!

DW