RAID 5 performance

Question

I have 4 320gb disks. I have put them in RAID 5(Intel ICH10). I have used RAID 0 and RAID 1 before. This is the first time I am using RAID 5. Read performance is excellent, but write is very slow at 15MB/sec. Is it normal for RAID 5?Single disk write for my disks is around 70MB/sec.

Answer 1

Those write speeds are normal for a RAID5 array using an even number of disks. To get excellent write speeds on software RAID5 controllers like the ICH8R-ICH10R, you need an odd number of disks. The only options are a 3 or 5 disk RAID5 array since there are only a maximum of 6 ports.
See this thread to get an idea of what I'm talking about. The thread is about the nForce onboard SATA RAID, but the concepts also apply to ICHR chipsets as well:
http://forums.storagereview.net/index.php?showtopic=25786
To summarize the thread, when dealing with RAID0 or RAID5, to get optimal performance you need to use aligned partitions, ideally created at offsets of 1024KB x (number of usable drives in array). The overall best stripe size to use for the best read, write performance for all filesizes is 32KB. The best cluster size to use when formatting the partition with NTFS is 32KB when storing a variety of filesizes on the array. If you are only dealing with very large files on the array (256MB+), you can get the best performance by using a 128KB stripe size for the RAID array and a 64KB cluster size for the NTFS partition. I store a variety of files on my RAID array, so I use the 32KB stripe/32KB cluster option.
The next thing you need to do is create an aligned first partition on the array. If you use Windows Vista or later to create a partition it will create an aligned first partition by default. If you are using Windows XP, then you need to use a utility called diskpar.exe (not diskpart.exe since the XP version does not have partition alignment capability, but Windows Server 2003's diskpart.exe, and Vista and later's diskpart.exe do). You can gain slightly more performance by manually aligning the partition yourself using diskpar/diskpart. If you have a 5 disk RAID5 array, you would align the first partition on the array to 4096KB, or 1024KB for every non-parity (i.e. usable) drive in the array. For a 3 disk RAID5 array, you would align on 2048KB.
Yes, you can get awesome RAID5 write speeds on an Intel onboard RAID controller using the information above. My 5 disk RAID5 array with Samsung F1 500GB drives has a maximum read speed of 350MB/s and write speeds of nearly 300MB/s. They trail off linearly as you get further into the array just as any mechanical HDD's performance does when looking at HDtach/HDTune benchmarks. You're never going to get read and write speeds that match a RAID0 array with the same number of drives, but with proper stripe/cluster/alignment you can get close. For comparison, the same 5 drives in RAID0 have a max read speed of 450MB/s and write speeds over 400MB/s. On ICHR chipsets, RAID0 arrays are not severely hampered by non-aligned partitions, but alignment does help quite a bit. For RAID5, partition alignment is essential for good write performance.
Diskpart.exe usage (done on a clean drive/array with no partitions):
1) Open a command prompt window and type diskpart then hit Enter.
2.) Type: list disk then hit enter. Look for the disk number that corresponds to your RAID array
3.) Type: select disk 1 (if disk 1 is your RAID array)
4.) Type: create partition primary align 4096 (if you have a 5 disk RAID5 array, use 2048 if you have a 3 disk array)
That's it. Format the drive making sure to select the correct cluster size (at least 32KB). If you created a first partition that didn't fill the drive, any subsequent partitions you create on the drive will be aligned because the first one is aligned.