Pumping Up Your Server

Optimizing your server's (or workstation's) disksubsysteminvolves more than just disk drives.The relevant controllers, protocols, and configurations have just as much of aneffect. Let's look at some non-RAID tips for improving the performance of oradministering disk controllers.

Tip 1. Use Multichannel RAID Controllers.
Your main concern when optimizing performance is the RAID controller. Ofcourse, 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 orSymbios), but you also want it to have multiple channels. This requirement islike having multiple SCSI cards in the same slot­the controller uses onlyone interrupt and can fully use the available bandwidth of the peripheral slot(such as the full 132MBps on standard 32-bit PCI). Multichanneling provides theadvantages 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 acrossthem in a way that maximizes your use of available bandwidth. For a simpletwo-channel controller, you must put each drive in a mirror set on its ownchannel or put half of a RAID 0 volume on one channel and the other half on thesecond channel. For advanced controllers with three or four channels, try to usethem all. Go beyond just spreading the volume across the channels­use themintelligently. For example, with a four-channel card, you could build a RAID 01volume whose mirror sets each span two channels, but the striping spans allfour. You can use even a two-channel card more efficiently with an intelligentconfiguration, such as a RAID 10 volume that stripes and mirrors on bothchannels simultaneously. You'll need to play around with the configuration tosee what suits your needs best.

Tip 3. Balance Drives Evenly Across Channels.
Also keep in mind how disks are arranged across your controllers. Too manydisk drives on one channel (such as 15 drives on a SCSI-2 bus) don't help systemperformance, because you run out of bandwidth. In addition, just as with addingCPUs to the system, you reach a point of diminishing returns on performanceenhancement­each added drive results in less of a percentage growth(ideally, from one to two drives: 50 percent; two to three: 33 percent; three tofour: 25 percent). Without enough drives, you aren't using all your controller'ssingle-channel performance. The just-right level is when you balance drivesevenly 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 9GBdrives are faster than the old 2.1GB ones (such as the new 10K-RPM Cheetahdrives from Seagate) with higher revolutions per minute (RPMs) and greater bursttransfer rates.

Tip 5. Use an Online Configurable Controller.
Online configurable controllers improve disk subsystem uptime andmanageability by letting you dynamically change your disk layout withoutpowering 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'sactively running a database application­a performance hit will occur as thecontroller rebuilds the volume, but the system remains online.

You can use the same features for changing the RAID level running on thesystem (such as from 5 to 0). Some controllers are configurable through acommand-line interface (a DOS-like window), while others have a fullyNT-integrated GUI. Both alternatives are better than booting the server into thecard's firmware, but Windows-standard administration software can make your lifea little easier.