lebisol: back in the exchange 5.5 days, when read/write ratios were 8:1, RAID 5 was fine. In a modern Exchange 2003 environment, the read write ration is 3:1 (uncached clients) or 2:1 (outlook clients in cached mode). The RAID 5 write penalty of 4 is too high in either case.
rws70: The dell install program is a culprit, as it the latest version of IBM ServerGuide. There are two methods to check your allocation unit size (also referred to as cluster size).
1. From the command line run chkdsk with no options. This runs chkdsk in read only mode. Toward the bottom of the output, you'll see a line like "512 bytes in each allocation unit."
2. In the disk defragmentation aplet in the MMC, select the disk and click ?analyze". After the disk is analyzed, click the "veiw report" button. There is no need to actually defragment the disk. In the report, you'll see a line like "cluster size = 512 bytes"
When an application sends an IO request larger than the allocation unit size to the filesystem, NTFS splits the request into multiple IO request of the allocation unit size. You can see this with the perfmon physical disk counter split IOs/sec. If the allocation unit size is 512 bytes, and the application sends a 4K request, NTFS splits it into 8 512byte IOs and sends 8 IOs to the disk.
The same vendors that kill performance with a non-optimal allocation unit size tend to ship with all the disks in one big RAID 5 array as well. With it's write penalty of 4, RAID 5 is suited for applications that read a lot and don't write much. The OS pagefile and system temp directory (both located on the boot volume by default) are just the opposite; they write as much or more than they read. I'm convinced that these vendors intentionally ship the worst possible configuration in terms of disk performance in order to sell unnecessary spindles. One of my customers, a public utility in Arizona, did a study on Dell 28XX as shipped vs. configuring the drives RAID 10 and manually creating the system partition with windows setup; the result was a 4X performance improvement.
This has nothing to do with disk alignment. Disk alignment has been around for a long time. In strict terms it applies only to disks wth a fixed geometry, which have not existed since the days of 9GB SCSI drives. These days, a disk has a variable number of sectors per track ranging between 128 and 250ish. the geometry you see from bios is a fiction created by the disk firmware. Presently, aligning disk tracks really has a lot more to do with aligning cache slots than anything else, and is highly dependent on which specific disk and controller you are using. You don't get the kind of mileage performance you used to three or four years ago due to the advances in disk technology.
