RAID 5, first write insanely fast, slow after +- 1,5GB


Apr 15, 2012
Hi All,

I just bought me some new storage (3X3TB WD disks) because i wanted 1 central place for all my data (for all the devices in my network).

I'm currently running a 5 RAID array on my pc
(later the RAID5 setup wil be moved or recreated on my Windows home server 2011, but for the moment the WHS is still running on an old motherboard that can only use 750GB of each 3TB drive :(

So for now, i'm testing it on my desktop.

Desktop specs:
Intel Extreme DP67BG Motherboard
I7 2600k sandy bridge @ 3,6GhZ
16GB of DDR memory
1 x OCZ-VERTEX 3 SSD 120GB on sata 6gb/s ports
3 X 3000GB Western Digital WD30EZRX (in RAID 5, configured at boot with intel raid tool) on sata 3gb/s ports

At first everything was working really great, amazing read/write speeds.
And i mean really insane write speeds (copying 1GB from my SSD to the rAID5 array beneath a second)

But then I realized only the first +-1,5GB writes insanely fast to the RAID5 array, afterwards the speed drops to... +-40MB/s
I have no idea what's causing this?

Can somebody explain to me?
I've also made a screen capture so you can exactly see what happens.
Movie can be found here

Please help me out! All information is welcome!
It would be really great to keep those insane write speeds for "unlimited" amounts of data...

Thanks in advance
+1 write cache. If you go into device manager, hard drive, and find the fake volume that corresponds to your raid 0 array (mine with intel driver is called volume_0000) you can open properties on the raid array and see if write caching is set. If on, turn it off and see if the 40MB/sec starts sooner.

Typically you are OK with write caching if you have a UPS backing up power for the PC, and are living dangerously if every power hit could loose MBs of data.... (a good backup plan and not caring about the last few days of data and being willing to risk doing a restore takes all the danger out).

P.S. you probably know that going to raid 10 by buying an extra drive would net you some fairly cheap performance. But you might not need it since you have the SSD.

edit: Also, if you have not yet used 'resource monitor' try it. Type 'resource monitor' in teh start --> search box. Click it to launch. The MEMORY tab should show you the 1.5GB of memory used as write pending cache when you first do teh 1GB/sec burst. The DISK tab will show you how nice the response time on the SSD is (single digit MS vs. double). You can also see the data rates going to the disk, etc.