I recently configured a RAID 5 partition in Server 2008 with 4 RAID 5 disks. These disks are connected through a SATA expansion card that uses PCIe. This morning, I checked and they had finally finished synchronizing, and so I tried to do some speed tests.

Copying off the disks started pretty much fine - speeds began at > 125MB/s, then trailed down to about 70MB/s, which I found odd but not worrying. Writing TO the disks however is a completely different story. I attempted to copy some of my VM host ISOs onto the disks (~2-4 GB apiece) and this resulted in speeds of approximately 10MB/s. I tried copying both from a local disk (connected directly to the motherboard) and from another server ththe gigabit network and results were the same.

I checked the performance monitor while transferring the files and the only thing that stuck out was that my memory hard faults shot up to 6,000 per minute (spiking around 200/s) by explorer.exe. The system is running 2GB of DDR667 ECC RAM and a quad-core 2.3GHz opteron.

Is there anything I can do to fix this performance issue (buy more RAM? move the drives to a faster box?, etc) or am I just screwed so long as I stick to windows.

  • Software or hardware RAID, seems to be hardware and if so what make of controller, are you sure you have the best driver for it and how did you initialize the RAID volume? Assuming reasonably OK hardware RAID then RAID 5 read speeds with 4 disks should be 200Meg-400Meg/sec, writes will be more or less the same as a single disk so you're seeing a major impact across the board, any chance you have a failed disk in there? – Helvick Feb 28 '10 at 18:32
  • In addition to Helvick's questions... Are you using the full capacity of the 4 disks for the single RAID5 volume? If not, is there something else sharing those disks that might be competing for I/Os? – William Feb 28 '10 at 22:14

You copy them in explorer? I would say that the 6000 page faults hint on a serious issue in memory management - possibly file buffers using too much space, starting to swap. Not sure how your tempfile relates to the discs, but it could be you overloda the discs without doing real work.

Server 2008 is having this old WIndows Vista problem, IIRC, of being too aggressive with caching on disc copies - try going to a current operating system (2008 R2), that should fix it.

Software or hardware RAID 5? You know that for a large file store it makes sense to do optimized formatting as NTFS standard settings result in lots of split IO? Any reason you have so little RAM on the machine? I mean - 2gb sounds like a toy for me, so unless this is a dedicated little NAS box, what is it for? Definitely not running virtual machines ;)


It sounds like a classic case of misaligned partition in RAID 5. Server 2008 should prevent that from happening with software based RAID but if the controller card is doing the RAID and presenting a single "disk" to the OS the RAID controller determines that alignment issue.

Your simplest most straight forward bet is to backup the data, delete the raid volume, create a RAID 10 volume, and restore the data.

There are three possible results

  1. your read and write speeds noticeably increase in RAID 10 vs RAID 5. In this case you take your licks on less disk space for the array assuming you can afford that trade off and call it a win. The causes of this are multiple but know that RAID 5 is notorious for poor write performance and it may not be worth diagnosing further. If you can't deal with the space loss buy more disks and do another RAID 1 array in addition to the RAID 10 or buy higher capacity disks and do any combination you like between RAID 1 + 1 or RAID 1 + 10. It could be chalked up to a slow raid controller, partition misalignment, a driver bug, etcetera.

  2. your write speeds increase but reads don't change much. You have the same suspects as above.

  3. You get absolutely no increase at all on reads or writes. Your raid controller is seriously underpowered or there is a massive bug involved.

You could buy a better controller and fight with RAID 5 or 6, you could spend more time making RAID 5 work with the existing hardware. But to me it's just better to avoid parity based RAID altogether.

  • Actually it does not sound like misaligned partitions. Can not be under standard windows (which uses a node size of 4kb). – TomTom May 9 '10 at 1:33

Just for reference, search for "iops calculator" in your favorite search engine. Learn that more disks make more IOPS in a RAID10 configuration, for example.

IOPS is really what you need to determine when it comes to disk speed.

Also remember that a degraded RAID5 array is much slower than a healthy RAID5 array and might influence the decision of which RAID to go for.

Another thing, I've seem some SATA expansion cards can only handle SATA2 speeds for example, so it slows down the bus to SATA3 disks. Some can only handle SATA1.

I would start by checking the specs of the controller, and the specs of the exact disks you are using.

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