Optimizing your server's (or workstation's) disk subsysteminvolves more than just disk drives. The relevant controllers, protocols, and configurations have just as much of an effect. Let's look at some non-RAID tips for improving the performance of or administering disk controllers.
Tip 1. Use Multichannel RAID Controllers.
Your main concern when optimizing performance is the RAID controller. Of
course, you want it to have the latest and fastest electronics possible (e.g.,
PCI; a fast I/O processor such as the Intel i960 or equivalent from AMI or
Symbios), but you also want it to have multiple channels. This requirement is
like having multiple SCSI cards in the same slotthe controller uses only
one interrupt and can fully use the available bandwidth of the peripheral slot
(such as the full 132MBps on standard 32-bit PCI). Multichanneling provides the
advantages of controller duplexing without the costs usually associated with it,
because you can use all channels with common circuits for optimal performance.
Tip 2. Spread RAID Volumes Across Channels Intelligently.
Using multiple channels effectively means spreading your RAID volumes across
them in a way that maximizes your use of available bandwidth. For a simple
two-channel controller, you must put each drive in a mirror set on its own
channel or put half of a RAID 0 volume on one channel and the other half on the
second channel. For advanced controllers with three or four channels, try to use
them all. Go beyond just spreading the volume across the channelsuse them
intelligently. For example, with a four-channel card, you could build a RAID 01
volume whose mirror sets each span two channels, but the striping spans all
four. You can use even a two-channel card more efficiently with an intelligent
configuration, such as a RAID 10 volume that stripes and mirrors on both
channels simultaneously. You'll need to play around with the configuration to
see what suits your needs best.
Tip 3. Balance Drives Evenly Across Channels.
Also keep in mind how disks are arranged across your controllers. Too many
disk drives on one channel (such as 15 drives on a SCSI-2 bus) don't help system
performance, because you run out of bandwidth. In addition, just as with adding
CPUs to the system, you reach a point of diminishing returns on performance
enhancementeach added drive results in less of a percentage growth
(ideally, from one to two drives: 50 percent; two to three: 33 percent; three to
four: 25 percent). Without enough drives, you aren't using all your controller's
single-channel performance. The just-right level is when you balance drives
evenly across channels.
Tip 4. Choose High-Speed Disk Drives.
When setting up a disk subsystem (either for upgrade or a new system),
also consider the speeds of the physical devices you use. The new 4GB and 9GB
drives are faster than the old 2.1GB ones (such as the new 10K-RPM Cheetah
drives from Seagate) with higher revolutions per minute (RPMs) and greater burst
transfer rates.
Tip 5. Use an Online Configurable Controller.
Online configurable controllers improve disk subsystem uptime and
manageability by letting you dynamically change your disk layout without
powering down the server or even taking the disk volume offline. For example,
you can extend a RAID 5 volume's capacity by adding more drives while it's
actively running a database applicationa performance hit will occur as the
controller rebuilds the volume, but the system remains online.
You can use the same features for changing the RAID level running on the system (such as from 5 to 0). Some controllers are configurable through a command-line interface (a DOS-like window), while others have a fully NT-integrated GUI. Both alternatives are better than booting the server into the card's firmware, but Windows-standard administration software can make your life a little easier.
