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Poor RAID 5 write speeds - do I have a problem?


Captain Reynolds
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I have recently upgrade to WHS2011 and have 3 x 2TB WD EARS drives in a RAID 5 using the onboard raid controller of the ASUS P5K-WS motherboard. The drive is used primarily to stream videos and recorded TV to my HTPC. The read performance from the array is fine - file transfers are around 80-100 mbps and streaming is perfect. Writing to the drive however is awful. I knew that write performance in raid 5 was bad, but file transfers to the drive start out at 90mbps but then slow to a crawl between 20-30mbps after a few seconds. I would just like some feedback from anyone else using raid 5 to know if this is normal? Also, would I gain any I/O performance by adding a fourth drive? Thanks!

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RAID-5 is written in two different ways. If the writes to the array are short,

(less than half-stripe length) each write will be converted to two disk reads

and two disk writes. Performance will be 20-30% of what you would get if writting

to a stand-alone drive. If the writes to the array are large, (longer than stripe

length) the write speed can be faster than the stand-alone drive.

 

If write-back cache is enabled, The short (two disk reads and two disk writes) are

combined in cache to nearly equal the "large" write speed. A good UPS or a controller

card with battery backup would be wise protection for the data in cache waiting to

be written.

 

Any "card" with onboard XOR calculation (even if it uses the system CPU) will be

faster than motherboard Raid 5.

 

Raid 5 is a "stripe" so, four or five disk arrays will be faster than a three disk

array. This is limited by the capability of the controller and would likely reach a

point of diminishing return at a five disk raid 5 array, on a motherboard controller.

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Many thanks analog for your prompt and detailed reply! I have been searching online stores here in Australia for another EARS drive but it seems the newer EARX with SATA3 is now the norm...

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Also remember that the read speed of your source drive is also important, particularly if the data is located in the inside of the spindle where read/write speeds are the slowest. If possible, you should test read/write speeds with the source drive being an SSD so that way you know that it's not being factored into the equation.

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I'm not too concerned with the write speeds on my RAID 5 setup as they are all movie files and I want need read speeds which are fine.

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I have recently upgrade to WHS2011 and have 3 x 2TB WD EARS drives in a RAID 5 using the onboard raid controller of the ASUS P5K-WS motherboard. The drive is used primarily to stream videos and recorded TV to my HTPC. The read performance from the array is fine - file transfers are around 80-100 mbps and streaming is perfect. Writing to the drive however is awful. I knew that write performance in raid 5 was bad, but file transfers to the drive start out at 90mbps but then slow to a crawl between 20-30mbps after a few seconds. I would just like some feedback from anyone else using raid 5 to know if this is normal? Also, would I gain any I/O performance by adding a fourth drive? Thanks!

 

 

I totally agree with dagamer. As you know I am using a Highpoint and writing across the network is constant at about 115 mbs. Writing from one array to another starts at 250 mbs and taps out about 145 mbs. The performance you are getting is not normal even for motherboard controllers. The short time I could tolerate using my motherboard it was still about 100. How did you configure the EARS? 512 or 4K, jumper or no jumper? Definitely worth trying to use a controller card to see if you can improve that.

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I totally agree with dagamer. As you know I am using a Highpoint and writing across the network is constant at about 115 mbs. Writing from one array to another starts at 250 mbs and taps out about 145 mbs. The performance you are getting is not normal even for motherboard controllers. The short time I could tolerate using my motherboard it was still about 100. How did you configure the EARS? 512 or 4K, jumper or no jumper? Definitely worth trying to use a controller card to see if you can improve that.

 

The transfer speeds to the RAID5 are consistent from all of my PC's - even my main rig with a brand new Vertex 3 SSD! I formatted the drives at 4k and not using jumpers... Could I have a bad disk? Is there a utility someone can recommend?

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I mainly use ATTO but also use Crystal Diskmark at times. I have not run a benchmark on each of my pc's - but it is obvious that I am getting the same poor transfer rate when writing to the RAID 5 on the server from any of them.

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