Max out your memory. Having lots of free memory will improve your virtual-memory performance (and Unix takes advantage of extra memory more effectively than Windows does). Fortunately, with RAM as cheap as it is now, a gigabyte or three is unlikely to bust your budget even if you're economizing.
Most people think of the processor as the most important choice in specifying any kind of personal-computer system. But for typical job loads under Linux, the processor type is nearly a red herring — it's far more important to specify a capable system bus and disk I/O subsystem. If you don't believe this, you may find it enlightening to keep top(1) running for a while as you use your machine. Notice how seldom the CPU idle percentage drops below 90%!
It's true that after people upgrade their motherboards they often do report a throughput increase. But this is often more due to other changes that go with the processor upgrade, such as improved cache memory or an increase in the clocking speed of the system's front-side bus (enabling data to get in and out of the processor faster).
If you're buying for Linux on a fixed budget, it makes sense to trade away some excess processor clocks to get a faster bus and disk subsystem. If you're building a monster hot-rod, go ahead and buy that fastest available processor — but once you've gotten past that gearhead desire for big numbers, pay careful attention to bus speeds and your disk subsystem, because that's where you'll get the serious performance wins. The gap between processor speed and I/O subsystem throughput has only widened in the last five years.
How does it translate into a recipe in 2004? Like this; if you're building a hot rod,
Do buy a machine with the fastest available "front-side" (e.g. processor-to-memory) bus. (In February 2004 the maximum is still 266 MHz.)
Do get a high-speed SCSI controller and the fastest SCSI disks you can get your hands on.
If you're economizing, you can back down on these. But in trading away SCSI for EIDE your reliability (expected time before failure) will drop. We'll cover this in more detail in the next section.
For the fastest disks you can find, pay close attention to average seek and latency time. The former is an average time required to seek to any track; the latter is the maximum time required for any sector on a track to come under the heads, and is a function of the disk's rotation speed.
Of these, average seek time is much more important. When you're running Linux or any other virtual-memory operating system, a one millisecond faster seek time can make a really substantial difference in system throughput. Back when PC processors were slow enough for the comparison to be possible (and I was running System V Unix), it was easily worth as much as a 30MHz increment in processor speed. Today the corresponding figure would probably be as much as 300MHz!
The manufacturers themselves avoid running up seek time on the larger-capacity drives by stacking platters vertically rather than increasing the platter size. Thus, seek time (which is proportional to the platter radius and head-motion speed) tends to be constant across different capacities in the same product line. This is good because it means you don't have to worry about a capacity-vs.-speed tradeoff.
Disks of less than 40GB capacity simply aren't being manufactured anymore; there's no margin in them. Our spies tell us that all major disk makers retooled their lines a while back to produce 9GB unit platters, which are simply being stacked 2N per spindle to produce ranges of drives with roughly 18GB increments of capacity.
Average drive latency is inversely proportional to the disk's rotational speed. For years, most disks spun at 3600 rpm; most disks now spin at 7,200 or 10,000rpm, and high-end disks at 15,000 rpm. These fast-spin disks run extremely hot; cooling is becoming a critical constraint in drive design.
Another basic decision is IDE vs. SCSI. Either kind of disk costs about the same, but the premium for a SCSI card varies all over the lot, partly because of price differences between VLB and PCI SCSI cards and especially because many motherboard vendors bundle an IDE chipset right on the system board. SCSI gives you better speed and throughput and loads the processor less, a win for larger disks and an especially significant consideration in a multi-user environment; also it's more expandable. You can have at most two IDE devices; four for EIDE. SCSI permits up to 7 (15 for wide SCSI).
Admittedly, the case for SCSI has eroded a bit since 2001; the new generation of IDE drives is very fast, and controller cards now normally feature a channel per drive and DMA (Direct Memory Access), so that some of of the multi-user contention problems that used to dog IDE have diminished. At 10KRPM and below, IDE is as good as SCSI now (a painful admission for an old-time IDE-hater like me), but at the 15KRPM high end SCSI still rules.
Of course, IDE is cheaper. Many motherboards have IDE right on board now; if not, you'll pay maybe $15 for an IDE adapter board, as opposed to $200+ for the leading SCSI controller. Also, the cheap SCSI cabling most vendors ship can be flaky. You have to use expensive high-class cables for consistently good results. See Mark Sutton's horror story.
These days you seldom see plain IDE; souped-up variants are more usual. These are "Enhanced IDE" (E-IDE) and "Fast AT Attachment" (usually ATA for short). ATA is Seagate's subset of E-IDE, excluding some features designed to permit chaining with CD-ROMs and tape drives using the "ATAPI" interface (an E-IDE extension); in practice, ATA and E-IDE are identical.
You'll need to be careful about chaining in CD-ROMs and tape drives when using IDE/ATA. The IDE bus sends all commands to all disks; they're supposed to latch, and each drive then checks to see whether it is the intended target. The problem is that badly-written drivers for CD-ROMs and tapes can collide with the disk command set. It takes expertise to match these peripherals.
Neither ATA nor E-IDE has the sustained throughput capacity of SCSI (they're not designed to) but they are 60-90% faster than plain old IDE. E-IDE's new "mode 3" boosts the IDE transfer rate from IDE's 3.3MB/sec to 13.3MB/sec. The new interface supports up to 4 drives of up to 8.4 gigabytes capacity.
E-IDE and ATA are advertised as being completely compatible with old IDE. Theoretically, you can mix IDE, E-IDE and ATA drives and controllers any way you like, and the worst result you'll get is conventional IDE performance if the enhancements don't match up (the controller picks the lowest latch speed). In practice, some IDE controllers (notably the BusLogic) choke on enhanced IDE.
Accordingly, I recommend against trying to mix device types an an E-IDE/ATA bus. Unfortunately, this removes much of E-IDE/ATA's usefulness!
E-IDE on drives above 540MB does automatic block mapping to fool the BIOS about the drive geometry (avoiding limits in the BIOS type tables). They don't require special Unix drivers.
Many motherboards now support "dual EIDE" channels, i.e. two separate [E]IDE interfaces each of which can, theoretically, support two IDE disks or ATA-style devices.
For starters, SCSI is still at least 10%-15% faster than IDE/ATAPI running flat out. Like Windows, IDE is layered over an ancestral design (ST-506) that's antiquated and prone to failure under stress. For example, on the Tyan K7 motherboards, there are known data-corruption problems with the ATA controller in the presence of various DMA-using bus-mastering cards.
SCSI, on the other hand, was designed from the beginning to scale up well to high-speed, high-throughput systems. Because it's perceived as a "professional" choice, SCSI peripherals are generally better engineered than IDE/ATAPI equivalents, and new high-performing drive technologies tend to become available in SCSI first. You'll pay a few dollars more, but for Linux the cost is well repaid in increased throughput and reliability.
The one aspect of SCSI that often gets overlooked is that it is a true multitasking interface, thanks to the "disconnect/reconnect" sequence that almost all SCSI hardware implements. With disconnect/reconnect, if a target device has to perform some kind of time-consuming mechanical operation (e.g., a seek in the case of a disk or a medium position operation in the case of a tape drive) the device will release control of the SCSI bus and allow it to be used for some other operation.IDE/ATAPI has no such capability and is often responsible for a system stall while a disk, CD-drive or tape drive seeks to the desired medium position.
(Incidentally, SCSI performance can sometimes be improved by setting the ID of the most frequently used disk drive as high as possible. The SCSI priority pecking order is such that devices with higher ID's get first crack at the bus when arbitration occurs during the selection phase.)
Rick's comments from 2001 are still apposite: "They call me a SCSI bigot. So be it — but my hardware keeps being future-proof, I don't have to run bizarre emulation layers to address CDRs, I never run low on IRQs or resort to IRQ-sharing (on account of 3-4 ATA controllers each needing one, plus special adapters for scanners, etc.), all my hard drives have hardware-level hot-fix, all my hard disk/CD/tape/etc. devices support a stable standard rather than this month's cheap extension kludge, and I don't have to worry about adverse interactions at the hardware or driver levels from mixing ATA and SCSI."
The cutting edge in SCSI devices is ultra wide LVD (low-voltage-differential) SCSI drives with 320MB/sec transfer speed, running over a 68-pin cable (this is twice as fast as the LVD-160 drives we used last time around). Vendors often call LVD drives "SCSI-3", which is incorrect as most of these devices don't have built-in support for the entire SCSI-3 protocol, and it would be overdesign if they did (the extra commands are designed for use with CD and multimedia devices).
Fast ultra LVD is a bit more expensive to support than the older versions of SCSI (for which key words are "single-ended", describing the electrical interface, and "narrow", describing the width of data transfers over the older-style 50-pin connector). Thus, you're likely to find it only on hard drives that are physically capable of doing high-speed data access off their media; slower devices such as tapes and CD drives are normally still built with the narrow single-ended variant.
The LVD-160 standard defines the SCSI bus, not the drive itself. Therefore, when used with a single hard drive in a lightly loaded system (e.g., a Linux machine supporting only one user) LVD-160 will have only a marginal effect on system performance. This is because a single hard drive running flat out will use only about 15-20 percent of the available bandwidth, as current drive technology can manage no more than about 28-30 MB/sec off the platters, less if a time consuming seek is involved. This rate could be higher, of course, if a read request was pending and the drive had cached the desired data. Where the LVD-160 bandwidth really becomes advantageous is in implementations of multiple drives (e.g., RAID 5) and/or when activities produce the frequent issue of drive access commands. The latter condition would be common in any environment that supports a lot of users.
Current SCSI drives are not quite fast enough to flood more than half the SCSI bus bandwidth, so you can have at least two drives on a single bus pumping full speed without using it up. In reality, you don't keep drives running full speed all the time, so you should be able to have 3-4 drives on a bus before you really start feeling bandwidth crunch.
The following, by Ashok Singhal <ashoks@duckjibe.eng.sun.com> of Sun Microsystems with additions by your humble editor, is a valiant attempt to demystify SCSI terminology.
The terms "SCSI", "SCSI-2", and "SCSI-3" refer to three different specifications. Each specification has a number of options. Many of these options are independent of each other. I like to think of the main options (there are others that I'll skip over because I don't know enough about them to talk about them on the net) by classifying them into five categories:
This refers to the commands that the controllers understand. You'll no longer see SCSI-1 in new hardware. SCSI-3 is a superset of SCSI-2 including commands intended for CD-R and streaming multimedia devices.
single-ended (max cable length 6 meters)
differential (max cable length 25 meters)
This option is independent of command set, speed, and path width. Differential is less common but allows higher transfer speeds, better noise immunity and longer cables. It's rare in SCSI-1 controllers.
You will normally see single-ended SCSI controllers on low-speed devices such as tapes and CD drives, and differential SCSI on hard drives (look for the specification LVD which means "low-voltage differential").
Nowadays most controllers support both electrical interfaces, but if you mix LVD with single-ended on the same chain, the whole chain will fall back to single-ended (and possibly halve the speed of the faster devices).
Asynchronous (acknowledge each word (8, 16 or 32 bits) transferred.
Synchronous (multiple-word transfers permitted between ACKS).
Synchronous is faster. This mode is negotiated between controller and device; modes may be mixed on the same bus. This is independent of command set, data width, and electrical interface.
Normal transfer speed is 5 megabytes/sec. The "fast" option (10 mb/sec) is defined only in SCSI-2 and SCSI-3. Fast-20 (or "Ultra") is 20 mb/sec; Fast-40 (or "Ultra-2") is 40MB/sec. The fast options basically defines shorter timing parameters such as the assertion period and hold time.
The parameters of the synchronous transfer are negotiated between each target and initiator so different speed transfers can occur over the same bus.
The standard SCSI data path is 8 bits wide. The "wide" option exploits a 16- or 32-bit data path (uses 68-pin rather than 50-pin data cables). You also get 4-bit rather than 3-bit device IDs, so you can have up to 16 devices. The wide option doubles or quadruples your transfer rate, so for example a fast-20/wide SCSI link using 16 bits transfers 40mb/sec.
What are those "LUN" numbers you see when you boot up? Think of them as sub-addresses on the SCSI bus. Most SCSI devices have only one "logical" device inside them, thus they're LUN zero. Some SCSI devices can, however, present more than one separate logical unit to the bus master, with different LUNs (0 through 7). The only context in which you'll normally use LUNs is with CD-ROM juke boxes. Some have been marketed that offer up to 7 CD-ROMS with one read head. These use the LUN to differentiate which disk to select.
(There's history behind this. Back in the days of EISA, drives were supposed to be under the control of a separate SCSI controller, which could handle up to 7 such devices (15 for wide SCSI). These drives were to be the Logical Units; hence the LUN, or Logical Unit Number. Then, up to 7 of these SCSI controllers would be run by the controller that we today consider the SCSI controller. In practice, hardware cost dropped so rapidly, and capability increased so rapidly, it became more logical to embed the controller on the drive.)
Here are a couple of rules and heuristics to follow:
Rule 1: Total SCSI cable length (both external and internal devices) must not exceed six meters. For modern Ultra SCSI (with its higher speed) cut that to three feet!
It's probably not a good idea to cable 20MB/s or faster SCSI devices externally at all. If you must, one of our informants advises using a Granite Digital "perfect impedance" teflon cable (or equivalent); these cables basically provide a near-perfect electrical environment for a decent price, and can be ordered in custom configurations if needed.
A common error is to forget the length of the ribbon cable used for internal devices when adding external ones (that is, devices chained to the SCSI board's external connector).
Rule 2: Both ends of the bus have to be electrically terminated.
On older devices this is done with removable resistor packs — typically 8-pin-inline widgets, yellow or blue, that are plugged into a plastic connector somewhere near the edge of the PCB board on your device. Peripherals commonly come with resistor packs plugged in; you must remove the packs on all devices except the two end ones in the physical chain.
Newer devices advertised as having "internal termination" have a jumper or switch on the PCB board that enables termination. These devices are preferable, because the resistor packs are easy to lose or damage.
Rule 3: No more than seven devices per chain (fifteen for Wide SCSI).
There are eight SCSI IDs per controller. The controller reserves ID 7 or 15, so your devices can use IDs 0 through 6 (or 0 through 14, wide). No two devices can share an ID; if this happens by accident, neither will work.
The conventional ID assignments are: Primary hard disk = ID 0, Secondary hard disk = ID 1, Tape = ID 2. Some Unixes (notably SCO) have these wired in. You select a device's ID with jumpers on the PCB or a thumbwheel.
SCSI IDs are completely independent of physical device chain position.
Heuristic 1: If you're buying narrow SCSI, stick with controllers and devices that use the Centronics-style 50-pin connector. Internally these connectors are physically identical to diskette cables. Externally they use a D50 shell. This "standard" connector is common in the desktop/tower/rackmount-PC world, but you'll find lots of funky DIN and mini-DIN plugs on devices designed for Macintosh boxes and some laptops. Ask in advance and don't get burned.
Heuristic 2: For now, when buying a controller, go with an Adaptec xx42 or one of its clones such as the BusLogic 542. (I like the BusLogic 946 and 956, two particularly fast Adaptec clones well-supported under Linux.) The Adaptec is the card everybody supports and the de-facto standard. Occasional integration problems have been reported with Unix under Future Domain and UltraStor cards, apparently due to command-set incompatibilities. At least, before you buy these, make sure your OS explicitly supports them.
However: Beware the combination of an Adaptec 1542 with a PCI Mach32 video card. Older (1.1) Linux kernels handled it OK, but all current ones choke. Your editor had to replace his 1542 because of this, swearing sulphurously the while.
Heuristic 3: You'll have fewer hassles if all your cables are made by the same outfit. (This is due to impedence reflections from minor mismatches. You can get situations where cable A will work with B, cable B will work with C, but A and C aren't happy together. It's also non-commutative. The fact that `computer to A to B' works doesn't mean that `computer to B to A' will work.
Heuristic 4. Beware Cheap SCSI Cables!
Mark Sutton tells the following instructive horror story in a note dated 5 Apr 1997:
I recently added an additional SCSI hard drive to my home machine. I bought an OEM packaged Quantum Fireball 2 gig SCSI drive (meaning, I bought a drive in shrinkwrap, without so much as mounting hardware or a manual. Thank God for Quantum's web page or I would have had no idea how to disable termination or set the SCSI ID on this sucker. Anyway, I digress...). I stuck the drive in an external mounting kit that I found in a pile of discarded computer parts at work and my that boss said I could have. (All 5 of my internal bays were full of devices.)
Anyway, I had my drive, and my external SCSI mounting kit, I needed a cable.
I went into my friendly local CompUSA in search of a SCSI cable, and side-by-side, on two hooks, were two "identical" SCSI cables. Both were 3 feet. Both had Centronics to Centronics connectors, both were made by the same manufacturer. They had slightly different model numbers. One was $16.00, one was $30.00. Of course, I bought the $16 cable.
Bad, I say, bad bad mistake. I hooked this sucker up like so:
+--------+ +-------+ +-----------+ +-------+
|Internal|---|Adaptec|-----|New Quantum|-----|UMAX |
|Devices | |1542CF | ^ | Disk | ^ |Scanner|
+--------+ +-------+ | +-----------+ | +-------+
| |
New $16 cable Cable that came
with scanner.
|
Shortly after booting, I found that data all over my old internal hard drive was being hosed. This was happening in DOS as well as in Linux. Any disk access on either disk was hosing data on both disks, attempts to scan were resulting in corrupted scans *and* hosing files on the hard disks. By the time I finished swapping cables around, and checking terminations and settings, I had to restore both Linux and DOS from backups.
I went back to CompUSA, exchanged the $16 cable for the $30 one, hooked it up and had no more problems.
I carefully examined the cables and discovered that the $30 cable contained 24 individual twisted pairs. Each data line was twisted with a ground line. The $16 cable was 24 data wires with one overall grounded shield. Yet, both of these cables (from the same manufacturer) were being sold as SCSI cables!
You get what you pay for.
(Another correspondent guesses that the cheap cable probably said "Macintosh" on it. The Mac connector is missing most of its ground pins.)
There's a USENET SCSI FAQ. Also see the home page of the T10 committee that writes SCSI standards.
There is a large searchable database of hard disk and controller information at the PC DISK Hardware Database.
(This section comes to us courtesy of Perry The Cynic, <perry@sutr.cynic.org>; it was written in 1998. My own experience agrees pretty completely with his. I have revised the numbers in it since to reflect more recent developments.)
Building a good I/O subsystem boils down to two major points: pick matched components so you don't over-build any piece without benefit, and construct the whole pipe such that it can feed what your OS/application combo needs.
It's important to recognize that "balance" is with respect to not only a particular processor/memory subsystem, but also to a particular OS and application mix. A Unix server machine running the whole TCP/IP server suite has radically different I/O requirements than a video-editing workstation. For the "big boys" a good consultant will sample the I/O mix (by reading existing system performance logs or taking new measurements) and figure out how big the I/O system needs to be to satisfy that app mix. This is not something your typical Linux buyer will want to do; for one, the application mix is not static and will change over time. So what you'll do instead is design an I/O subsystem that is internally matched and provides maximum potential I/O performance for the money you're willing to spend. Then you look at the price points and compare them with those for the memory subsystem. That's the most important trade-off inside the box.
So the job now is to design and buy an I/O subsystem that is well matched to provide the best bang for your buck. The two major performance numbers for disk I/O are latency and bandwidth. Latency is how long a program has to wait to get a little piece of random data it asked for. Bandwidth is how much contiguous data can be sent to/from the disk once you've done the first piece. Latency is measured in milliseconds (ms); bandwidth in megabytes per second (MB/s). Obviously, a third number of interest is how big all of your disks are together (how much storage you've got), in Gigabytes (GB).
Within a rather big envelope, minimizing latency is the cat's meow. Every millisecond you shave off effective latency will make your system feel significantly faster. Bandwidth, on the other hand, only helps you if you suck a big chunk of contiguous data off the disk, which happens rarely to most programs. You have to keep bandwidth in mind to avoid mis-matching pieces, because (obviously) the lowest usable bandwidth in a pipe constrains everything else.
I'm going to ignore IDE. IDE is no good for multi-processing systems, period. You may use an IDE CD-ROM if you don't care about its performance, but if you care about your I/O performance, go SCSI. (Beware that if you mix an IDE CD-ROM with SCSI drives under Linux, you'll have to run a SCSI emulation layer that is a bit flaky.)
Let's look at the disks first. Whenever you seriously look at a disk, get its data sheet. Every reputable manufacturer has them on their website; just read off the product code and follow the bouncing lights. Beware of numbers (`<12ms fast!') you may see in ads; these folks often look for the lowest/highest numbers on the data sheet and stick them into the ad copy. Not dishonest (usually), but ignorant.
What you need to find out for a disk is:
What kind of SCSI interface does it have? Look for "fast", "ultra", and "wide". Ignore disks that say "fiber" (this is a specialty physical layer not appropriate for the insides of small computers). Note that you'll often find the same disk with different interfaces.
What is the "typical seek" time (ms)? Make sure you get "typical", not "track-to-track" or "maximum" or some other measure (these don't relate in obvious ways, due to things like head-settling time).
What is the rotational speed? This is typically 4500, 5400, 7200, 10000, or 15000 rpm (rotations per minute). Also look for "rotational latency" (in ms). (In a pinch, average rotational latency is approx. 30000/rpm in milliseconds.)
What is the ‘media transfer rate’ or speed (in MB/s)? Many disks will have a range of numbers (say, 7.2-10.8MB/s). Don't confuse this with the "interface transfer rate" which is always a round number (10 or 20 or 40MB/s) and is the speed of the SCSI bus itself.
These numbers will let you do apple-with-apples comparisons of disks. Beware that they will differ on different-size models of the same disk; typically, bigger disks have slower seek times.
Now what does it all mean? Bandwidth first: the `media transfer rate' is how much data you can, under ideal conditions, get off the disk per second. This is a function mostly of rotation speed; the faster the disk rotates, the more data passes under the heads per time unit. This constrains the sustained bandwidth of this disk.
More interestingly, your effective latency is the sum of typical seek time and rotational latency. So for a disk with 8.5ms seek time and 4ms rotational latency, you can expect to spend about 12.5ms between the moment the disk `wants' to read your data and the moment when it actually starts reading it. This is the one number you are trying to make small. Thus, you're looking for a disk with low seek times and high rotation (RPM) rates.
For comparison purposes, the first hard drive I ever bought was a 20MB drive with 65ms seek time and about 3000RPM rotation. A floppy drive has about 100-200ms seek time. A CD-ROM drive can be anywhere between 120ms (fast) and 400ms (slow). The best IDE harddrives have about 10-12ms and 5400 rpm. The best SCSI harddrive I know (the Fujitsu MAM) runs 3.9ms/15000rpm.
Fast, big drives are expensive. Really big drives are very expensive. Really fast drives are pretty expensive. On the other end, really slow, small drives are cheap but not cost effective, because it doesn't cost any less to make the cases, ship the drives, and sell them.
In between is a ‘sweet spot’ where moving in either direction (cheaper or more expensive) will cost you more than you get out of it. The sweet spot moves (towards better value) with time. Right now (early 2004), it's about at 36GB drives, 6ms, 10000rpm, ultra2 SCSI. If you can make the effort, go to your local computer superstore and write down a dozen or so drives they sell ‘naked’. (If they don't sell at least a dozen hard drives naked, find yourself a better store. Use the Web, Luke!) Plot cost against size, seek and rotational speed, and it will usually become pretty obvious which ones to get for your budget.
Do look for specials in stores; many superstores buy overstock from manufacturers. If this is near the sweet spot, it's often surprisingly cheaper than comparable drives. Just make sure you understand the warranty procedures.
Note that if you need a lot of capacity, you may be better off with two (or more) drives than a single, bigger one. Not only can it be cheaper but you end up with two separate head assemblies that move independently, which can cut down on latency quite a bit (see below).
Once you've decided which kind of drive(s) you want, you must decide how to distribute them over one or more SCSI buses. Yes, you may want more than one SCSI bus. (My current desktop machine has three.) Essentially, the trick is to make sure that all the disks on one bus, talking at the same time, don't exceed the capacity of that bus. At this time, I can't recommend anything but an Ultra/Wide SCSI controller. This means that the attached SCSI bus can transfer data at up to 40MB/s for an Ultra/Wide disk, 20MB/s for an Ultra/narrow disk, and 10MB/s for a `fast SCSI' disk. These numbers allow you do do your math: an 8MB/s disk will eat an entire bus on its own if it's ‘fast’ (10MB/s). Three 6MB/s ultra/narrow disks fit onto one bus (3x6=18MB/s<20MB/s), but just barely. Two ultra/wide Cheetahs (12.8MB/s) will share an (ultra/wide) bus (25.6<40), but they would collide on an ultra/narrow bus, and any one Cheetah would be bandwidth constrained on a (non-ultra) `fast' bus (12.8 > 10).
If you find that you need two SCSI buses, you can go with ‘dual channel’ versions of many popular SCSI controller cards (including the Adaptec). These are simply two controllers on one card (thus taking only one PCI slot). This is cheaper and more compact than two cards; however, on some motherboards with more than 3 PCI slots, using two cards may be somewhat faster (ask me what a PCI bridge is :-).
SCSI performance can sometimes be improved by setting the ID of the most frequently used disk drive as high as possible. The SCSI priority pecking order is such that devices with higher ID's get first crack at the bus when arbitration occurs during the selection phase.
How do you deal with slow SCSI devices — CD-ROMS, scanners, tape drives, etc.? If you stick these onto a SCSI bus with fast disks, they will slow down things a bit. You can either accept that (as in "I hardly ever use my scanner anyway"), or stick them onto a separate SCSI bus off a cheap controller card. Or you can (try to) get an ATA version to stick onto that inevitable IDE interface on your motherboard. The same logic applies to disks you won't normally use, such as removables for data exchange.
If you find yourself at the high end of the bandwidth game, be aware that the theoretical maximum of the PCI bus itself is 132MB/s. That means that a dual ultra/wide SCSI controller (2x40MB/s) can fill more than half of the PCI bus's bandwidth, and it is not advised to add another fast controller to that mix. As it is, your device driver better be well written, or your entire system will melt down (figuratively speaking).
Incidentally, all of the numbers I used are ‘optimal’ bandwidth numbers. The real scoop is usually somewhere between 50-70% of nominal, but things tend to cancel out — the drives don't quite transfer as fast as they might, but the SCSI bus has overhead too, as does the controller card.
Whether you have a single disk or multiple ones, on one or several SCSI buses, you should give careful thought to their partition layout. Given a set of disks and controllers, this is the most crucial performance decision you'll make.
A partition is a contiguous group of sectors on the disk. Partitioning typically starts at the outside and proceeds inwards. All partitions on one disk share a single head assembly. That means that if you try to overlap I/O on the first and last partition of a disk, the heads must move full stroke back and forth over the disk, which can radically increase seek time delays. A partition that is in the middle of a partition stack is likely to have best seek performance, since at worst the heads only have to move half-way to get there (and they're likely to be around the area anyway).
Whenever possible, split partitions that compete onto different disks. For example, /usr and the swap should be on different disks if at all possible (unless you have outrageous amounts of RAM).
Another wrinkle is that most modern disks use `zone sectoring'. The upshot is that outside partitions will have higher bandwidth than inner ones (there is more data under the heads per revolution). So if you need a work area for data streaming (say, a CD-R pre-image to record), it should go on an outside (early numbered) partition of a fast-rotating disk. Conversely, it's a good convention to put rarely-used, performance-uncritical partitions on the inside (last).
Another note concerns SCSI mode pages. Each (modern) SCSI disk has a small part of its disk (or a dedicated EEPROM) reserved for persistent configuration information. These parameters are called ‘mode pages’, for the mechanism (in the SCSI protocol) for accessing them. Mode page parameters determine, among others, how the disk will write-cache, what forms of error recovery it uses, how its RAM cache is organized, etc. Very few configuration utilities allow access to mode page parameters (I use FWB Toolkit on a Mac — it's simply the best tool I know for that task), and the settings are usually factory preset for, uh, Windows 95 environments with marginal hardware and single-user operation. Particularly the cache organization and disconnect/reconnect pages can make a tremendous difference in actual performance. Unfortunately there's really no easy lunch here - if you set mode page parameters wrong, you can screw up your data in ways you won't notice until months later, so this is definitely `no playing with the pushebuttons' territory.
Ah yes, caches. There are three major points where you could cache I/O buffers: the OS, the SCSI controller, and the on-disk controller. Intelligent OS caching is by far the biggest win, for many reasons. RAM caches on SCSI controller cards are pretty pointless these days; you shouldn't pay extra for them, and experiment with disabling them if you're into tinkering.
RAM caches on the drives themselves are a mixed bag. At moderate size (1-2MB), they are a potential big win for Windows 95/98, because Windows has stupid VM and I/O drivers. If you run a true multi-tasking OS like Linux, having unified RAM caches on the disk is a significant loss, since the overlapping I/O threads kick each other out of the cache, and the disk ends up performing work for nothing.
Most high-performance disks can be reconfigured (using mode page parameters, see above) to have `segmented' caches (sort of like a set-associative memory cache). With that configured properly, the RAM caches can be a moderate win, not because caching is so great on the disk (it's much better in the OS), but because it allows the disk controller more flexibility to reschedule its I/O request queue. You won't really notice it unless you routinely have >2 I/O requests pending at the SCSI level. The conventional wisdom (try it both ways) applies.
And finally I do have to make a disclaimer. Much of the stuff above is shameless simplification. In reality, high-performance SCSI disks are very complicated beasties. They run little mini-operating systems that are most comfortable if they have 10-20 I/O requests pending at the same time. Under those circumstances, the amortized global latencies are much reduced, though any single request may experience longer latencies than if it were the only one pending. The only really valid analysis are stochastic-process models, which we really don't want to get into here. :-)