Drive Interfaces

So far, we've discussed drive performance factors at a low level and disk configuration at a high level, without looking at how drives connect up to the Mac in the first place. The interfaces between drives and computers put limits on performance, and the technology behind the interface sometimes has greater impact than the raw interface speed might suggest. Even the fastest drive, if connected through a CPU-intensive interface like USB 2.0, won't reach its maximum potential, and you're more likely to see dropped frames, stuttering playback, and the like. Likewise, using FireWire both for disks and DV cameras may be simple, but it's not always the interface that yields the best performance.

ATA

ATA (Advanced Technology Attachment) disk drives are used in G3s, G4s, Xserve G4s, and Xserve RAIDs. They're commonly available and very affordable. They're also called IDE (Integrated Drive Electronics) or PATA (Parallel ATA) disks.

The moniker "ATA" covers a staggering variety of interfaces, running from a pokey 3.3 MB/second up to 100 MB/second. In Mac parlance, the interface speed in MB/second usually follows the ATA name, such as "ATA/33" or "Ultra ATA/100." Faster, of course, is betteras long as the Mac supports the faster interface. Connecting an ATA/100 drive to an ATA/33 controller means the drive's communications run at a maximum of 33 MB/second. The Mac's operating instructions list the interface speed, or check the specifications online at www.info.apple.com/support/applespec.html.

More Info

If you're missing the manual for any recent Mac, you can find it online at www.info.apple.com/support/manuals.html.

Until recently, ATA drives were limited to a maximum size of around 137 GB. An extension to the ATA spec, Large Block Addressing, broke through that barrier, but most ATA Macs are limited to seeing the first 128 GB: only the G4 Mirrored Drive Door and G4 Xserve computers can use larger ATA disks on their built-in controllers.

ATA drives have been the commodity drive of the past decade and are available in all sizes and interfaces, from 2.5-inch laptop drives through 3.5-inch and the now rare 5.25-inch desktop drives. Laptop drives (as of spring 2005) top out at 80 GB capacity and 5400 RPM, with 4200 RPM drives being the most common. Desktop drives get as big as 250 GB and commonly run at 5400 or 7200 RPM. Transfer rates vary considerably, but common desktop drives will sustain 35 to 40 MB/second and peak at 60 MB/second.

An ATA interface uses a 40-pin connector with a 40- or 80-conductor, 2-inchwide ribbon cable. (An ATA interface faster than 33 MB/second uses an 80-conductor cable with added ground wires, but the same 40-pin connector as with slower versions.) Two drives can be connected to their controller on a single cable.

ATA drives have four-pin power connectors and configuration jumpers. Check your Mac's manual and drive manual for the correct setting of the jumpers: drives can be set for master, slave, or cable select modes; recent Macs usually want cable select, but depending on the Mac and the cable used, a different setting may be required.

Separate ATA interface cards can be had, but they're not especially useful for Macs: there's not normally enough room inside the box to install more drives than the Mac's built-in controllers support, and an 18-inch cable length limit and the hard-to-route ribbon cables restrict ATA drives to be internally mounted. You can add ATA drives outside the box using FireWire or SCSI cases that use ATA for the drive connection, though.

If you're intent on adding more internal ATA drives than your Mac supports, Promax (www.promax.com) offers ATA cards and mounting bracket kits. Bear in mind that G4 towers don't always have the best internal airflow, and overheating is a possibility.

SATA

Serial ATA interfaces are used in G5s, G5 Xserves, and increasing numbers of third-party drive enclosures. SATA is widely seen as the successor to parallel ATA, and SATA's higher interface speeds have led some vendors to include their highest-performance drive mechanisms in their SATA product lines.

SATA replaces the bulky 40-pin parallel ATA connector and cable with a svelte seven-pin connector and thin (1/4-inch) cable, doubles the cable length to 1 meter, and eliminates those pesky configuration jumpers: SATA cables are point-to-point, connecting a single drive to its controller.

Current SATA interfaces run at 150 MB/second, with future designs expected to run at two and four times that rate.

Advancements in the control protocol are coming, too: "command queuing" allows more efficient stacking of pending commands, so the drive and computer spend more time exchanging data and less time waiting on each other. As of summer 2004, tagged command queuing and native command queuing drives and controllers are just becoming available, but they promise, on average, a 30 percent improvement in performance over nonqueued interfaces.

Serial ATA drives, though relatively new, are already as affordable as parallel ATA drives of comparable speeds and capacities. Standard desktop drives hold from 120 GB to 250 GB or more, and typically run at 7200 RPM.

Many early SATA drives are really parallel ATA/100 drives with parallel-to-serial "bridge chips," but native SATA drives are starting to appear. (In practice, it's not a big deal; few current drives can deliver data fast enough to saturate ATA/100 interfaces to begin with.)

Some vendors sell high-performance "enterprise class" drives, optimized for speed. Western Digital's Raptor series has 36 GB and 72 GB drives, both running at 10,000 RPM and sustaining a whopping 72 MB/second transfer rate.

Drives like this give you the performance normally reserved for the "premium" SCSI and Fibre Channel interfaces at commodity prices. Note, however, that you've traded capacity for speed: you can capture video pretty darned quickly, but not much of it. Aside from the lower capacity, you'll also want to make sure these drives are well cooled; fast-spinning disks tend to run hot.

Separate SATA interface cards are available, as are mounting kits (not Apple approved!) to stick several more drives into Apple's G5 chassis. Promax, WiebeTech, and Swift Data sell the kits complete with cards; Sonnet, Acard, FirmTek, and others sell the cards alone. Netcell's RAID XL cards include hardware-based RAID 3 mode, and several vendors, including Promax, build SATA RAIDs based on it.

SATA cables can be run outside the chassis to separate drive enclosures, but SATA wasn't initially designed for this sort of use. Early adopters sometimes came to grief due to insufficient cable shielding, the somewhat fragile SATA connector, and unreliable firmware on first-generation SATA controllers when the drives were used in a RAID. These issues are being addressed over time, and external SATA storage systems from reputable vendors like Promax are becoming robust and reliable. Conservative system builders still prefer SCSI for external arrays; only time will tell if the newer SATA arrays develop an equally good reputation.

You can also find SATA-to-ATA "bridge kits" for converting parallel ATA drives to SATA interfaces. Apple does not support the use of such boards with its internal SATA controllers, but third-party SATA card suppliers may have different attitudes. Although you may be tempted to use existing ATA drives this way, you're probably just as well off sticking them into FireWire enclosures, SCSI-connected RAIDs and JBODs, or keeping them on-hand as spares for parallel ATA Macs. (RAIDs and JBODS are defined in the "Storage Architectures"section later in this chapter.)

SCSI

The Small Computer Systems Interface was the hard disk interface in Macs until ATA was adopted in the late '90s. It's currently used for high-performance interconnects, mostly to and within separate drive enclosures and hardware RAID systems. There's still a lot of SCSI equipment of all types in the Mac world, both drives inside the box and devices outside the box. SCSI is a systems-level interface, and connects tape drives, scanners, removable media drives, and the like, in addition to plain-old hard disks.

SCSI is a high-level, high-performance interface, allowing devices to go off and handle data requests without making the host computer sit around and wait for the results, so the computer can get more work done while waiting for the data to arrive.

Of course, this performance comes at a cost: SCSI interfaces are more complex and expensive than ATA or SATA interfaces. SCSI drives these days tend to be aimed at higher-performance markets instead of commodity desktop applications, so the drives themselves are often higher-performing devices with prices to match, typically costing between 2 and 10 times as much as ATA or SATA drives of comparable sizes.

SCSI has been around since the early '80s and has undergone considerable evolution. There are "narrow" and "wide" flavors (8- and 16-bit wide data paths); normal, "fast," "ultra," and "ultra2" speeds (5, 10, 20, and 40 MHz signaling, with 80 MHz transmission using double-clocking in ultra3 and later versions), and three different signal types on the wires: single-ended, high-voltage differential (HVD), and low-voltage differential (LVD).

These can be combined in various permutations to yield such mouthfuls as "fast wide SCSI," "wide ultra2 SCSI," and the like. In the latest SCSI generations, a total-throughput nomenclature has been adopted, hence "Ultra/160 LVD" for an interface with 160 MB/second data rate using LVD signaling.

The earliest SCSI buses allowed a grand total of 8 devices to be connected; more recent versions support 16 devices in a daisy-chain configuration. (In both cases, one of those devices must be the host computer.) Each SCSI device on a bus needs to be assigned a unique ID number. On bare drives, you'll find jumpers for this purpose; on SCSI enclosures, there's usually a set of switches or pushbuttons you can use to change the ID.

Both ends of the SCSI chain need to be terminated for the bus to operate properly. Some devices have internal terminations; some have jumpers or resistor packs to be installed or removed; some require the use of external terminators, which usually plug into the last open connector in the chain. (External SCSI devices that do not automatically terminate the bus internally typically have two connectors, and the terminator plugs into one of these.) To add to the complication, some terminators are passive, and some are powered. Hey, SCSI has been around for over two decades and has seen a lot of changesyou get that.

If you have two devices with the same ID on the same bus or if you get the bus termination wrong, utter confusion results, usually in the mind of the poor user. SCSI setups are very sensitive to misconfiguration; if the system misbehaves, suspect cabling, ID, and termination issues.

As long as the signal types match up, you can often interconnect different types of SCSI devices with adapter plugs and cables: SCSI is carried on 25-pin D-shell connectors, 50-pin and 68-pin dual-row headers, 68-pin flat connectors in two different sizes, and even 80-pin connectors. The bus will run at the speed of the slowest device connected and powered up.

However, interconnecting single-ended, HVD, and LVD devices will not work: you'll blow up the interface circuitry and let out all the magic smoke. There are "multimode" LVD interfaces that will work with single-ended SCSI, but they'll only work at single-ended interface speeds.

Note

Electronic devices are widely assumed to work because of "magic smoke" contained inside integrated circuits. The proof of this concept is found in the fact that when incompatible devices are hooked up or improper voltages are applied, chips often emit a burning smell and all the magic smoke comes out, after which the devices cease to function.

SCSI hardware is supposed to have different identifying icons depending on the signaling interface used:

Icons for single-ended (SE), HVD (DIFF), LVD, and multimode LVD/SE SCSI, respectively; image courtesy Information Technology Industry Council

When in doubt, assume incompatible signal types on SCSI devices until you can check spec sheets or instruction manuals.

Older Macs with 25-pin D-shell SCSI ports usually support the original 5 MB/second SCSI interface. Current Mac SCSI cards speak Ultra/160 LVD (with an interface speed of 160 MB/second) or Ultra/320 LVD (320 MB/second).

SCSI's most common use these days is connecting external drive enclosures to Macs. Many RAIDs connect this way, taking advantage of SCSI's high speed and long cable lengths. SCSI cables can run a minimum of 1.5 meters for single-ended interfaces, and up to 12 meters for LVD interfaces. (HVD can run to 25 meters, but HVD interfaces aren't being made any longer.)

The drives inside SCSI enclosures may use SCSI; in such cases (pun intended) the SCSI interface may simply chain from drive to drive, with the case providing little more than power supplies, ID setting wheels, and mounting rails. Higher-end enclosures with active electronics often use SCSI externally but may use a lower-cost interface like ATA internally. Medéa's VideoRAID series started this trend, and several other vendors have picked up on it.

More Info

All you ever wanted to know about SCSI, including a comprehensive list of the various flavors available, is at www.pcguide.com/ref/hdd/if/scsi/.

Fibre Channel

Fibre Channel is a very high performance bus interconnection, with prices to match. Fibre Channel is used as the interface to Xserve RAID systems, and is the connection of choice in building Storage Area Networks (SANs).

Fibre Channel uses serial communications at 1, 2, or 4 Gb/second, for throughputs of 100, 200, and 400 MB/second. 2 Gb FC is the most common at present; both Apple's dual-channel FC PCI card and the Xserve RAID run their ports at 2 Gb/second. You can run Fibre Channel over copper cables with up to 30 meters between devices, and over fiber optics for up to 50 kilometers. This means you can move noisy storage systems out of the edit suite and into soundproofed machine rooms.

Fibre Channel allows up to 126 devices on a bus. In its simplest form, FC is used for a point-to-point connection, as between a G5 and an Xserve RAID. If you want to add more devices, you need to configure them in a loop using the FC-AL (arbitrated loop) protocol, or a fabric architecture using Fibre Channel switches. We'll discuss this more in the section on SANs.

Point-to-point connections work with Xserve RAID because the entire RAID (make that each half of it: Xserve RAID is really two sets of drives, each with its own FC interface) is only one FC device. Like the Medéa boxes, Xserve RAID uses low-cost ATA drives internally with a dedicated RAID controller, and appears to the outside world as one Fibre Channel device.

Fibre Channel controllers, called Host Bus Adapters (HBAs), tend to be fairly expensive (Apple's own dual 2Gbit FCHBA, at $499, is one of the cheaper ones), and building comparable circuitry directly on a drive is similarly pricey, so Fiber Channel-native drives tend to be reserved for the highest-performance, least price-critical segments of the marketplace. FC drives can cost anywhere from 3 to 10 times as much as ATA drives of the same capacity (if not speed). Capacities tend to be small and data rates high; sizes run from 18 to 300 GB with speeds of 10,000 and 15,000 RPM and sustained transfer rates of up to 94 MB/second on the outer tracks, 62 on the inner tracks. (At these speeds and prices, it's common to see rates specified as ranges: buyers of such drives want to know all the gritty details.)

Although native Fibre Channel disk arrays are available, Fibre Channel is commonly used in the Mac world for high-speed connections to dedicated RAIDs comprised of ATA or SATA drives.

Fibre Channel is hot-pluggable, meaning you can connect and disconnect Fibre Channel devices with the system power on. (Be sure to dismount any drives mounted on your desktop before pulling out their cables, of course!) With a point-to-point connection that's no big deal: you either have the device connected, or not. With FC-AL, hot-plugging a device triggers the Loop Initialization Protocol (LIP) event, while hot-plugging a fabric generates a Registered State Change Notification (RSCN) sequence. Both the LIP and RSCN are mechanisms whereby all the devices on the channel reset themselves to the presence or absence of the device triggering the event, and both events interrupt any data transmissions in progress: you will drop frames if this happens. We'll discuss RSCNs more in the section on SANs.

Fibre Channel cables are delicate: at the data rates they run, even slight deformations of the conductors may cause spurious errors, intermittent faults, or complete failure. Coiling a cable too tightly, pulling it around a sharp corner, or simply stepping on it are enough to damage it beyond repair. The damage need not be visible to be real. If your Fibre Channel system isn't delivering the throughput you expect, try swapping out the cables for known good ones.

FireWire

FireWire is commonly used to connect Macs to DV devices or external disk drivesoften both at the same time, when editing with PowerBooksas well as a variety of other devices like DVD burners and iSight cameras.

FireWire, a.k.a. IEEE-1394 and i.Link, is a high-speed serial bus used for connection to storage devices as well as media devices. Macs connect to DV, DVCAM, DVCPRO, DVCPRO 50, DVCPRO HD, and HDV video recorders (see the previous lesson) using FireWire's isochronous mode, and hook up to disk drives using FireWire's asynchronous mode. Both modes can be active at the same time: bandwidth not reserved for isochronous traffic is available to any asynchronous communications that want it, and even then, 20 percent of the channels' bandwidth is reserved for asynch traffic.

FireWire is a bus architecture supporting up to 63 devices. The FireWire ports found on G3s, G4s, iBooks, and PowerBooks are 1394a or FW400 ports, capable of running at speeds of 100, 200, or 400 Mb/second (S100, S200, and S400 speeds). At S400, the maximum data rate is 50 MB/second, though 40 MB/second is a more realistic sustained number. G5s, high-end PowerBooks, and third-party PCI cards (and CardBus cards for PowerBooks with PC Card slots) offer FW800 (1394b) with an S800 data rate: about 80 MB/second sustained.

Like the other buses described, FireWire runs at the speed of the slowest device plugged in. DV and DVCAM devices typically support only S100 speed, cutting bus throughput to about 8 MB/second, just barely adequate for DV work. DVCPRO decks are switchable between S100 and S200 speeds, and higher-bit rate decks support faster speeds. Observe that all the FireWire ports on a Mac are on the same buseven the FW400 and FW800 ports on the G5sso plugging a Handycam into your Mac will drag your FW800 drive down to one-eighth of its normal bus speed. Many people wishing to use DV decks and FireWire drives at the same time purchase a PCI card providing a separate FireWire bus, so that devices can be segregated by speed.

FireWire drives are frequently used for low-bit rate editing, as with DV25. You can use them with higher bit rates, but this tends to be problematic; although FireWire is reasonably efficient, it adds another protocol layer between computer and disk, so communications are slower than with an internal disk.

FireWire is hot-pluggable, but note that doing so interrupts all data transfer on the bus for over a second as the devices on the bus vote on who gets to drive the bus. If you plug in or unplug a FireWire device while capturing to a FireWire drive (or moving DV data to or from a DV device), you will drop frames.

FireWire cables may run up to 4.5 meters. There are special, equalized cables that allow for longer runs, from Markertek and Focus Enhancements, among others.

FireWire is not a native drive interface; FireWire drives are ATA or SATA drives with a "FireWire bridge" interface attached. Most current bridges work perfectly well, but earlier ones can cause problems. First-generation FireWire bridges may not accommodate drives larger than 137 GB (just like older ATA interfaces in Macs); some perform poorly in terms of overall data transfer (the Oxford 900 chipset is often implicated, although your author has a 900-based FireWire enclosure that's never dropped a frame); and some may need updating or replacing to work with recent firmware revisions in the G5's FireWire interface.

Additionally, the first releases of OS X 10.3 changed the sequencing and usage of certain FireWire disk commands, leading to data loss with some FireWire bridges. Fingers were pointed at Oxford 911 chipsets, but the problem appears to have been more widespread. Most Oxford-using drive enclosure manufacturers offered Oxford firmware updaters on their Web sites, and by OS X 10.3.3 (if not 10.3.2) the problem seemed to have been corrected in OS X's FireWire control.

Although the drive-eating problem existed, the workaround appeared to be ensuring that all FireWire drives were dismounted before shutting down the Mac, and then not plugging them in until the Mac had booted up. Many Mac users have latched onto these workarounds as permanent safety measures even though the problem seems to have been cured.

Some video devices don't work well with FireWire drives. Some combinations of DV devices and FireWire drives simply refuse to play together at all. Single chip Canon camcorders are frequently implicated, but plenty of folks use those same camcorders with FireWire drives without problems. Similarly, iSights are often accused of corrupting FireWire disk data transfers (your author has never had a FireWire disk corrupted by his iSight, but your author may simply be unusually lucky), and making Aurora Igniter PCI cards refuse to work. In general, it's best to unplug devices you don't need to have on the bus when you're not using them. If you find that a camera or deck doesn't cooperate with your FireWire drive, try a different brand of FireWire drive or a different DV device.

Six-pin FW400 and FW800 connectors include power and return lines, so that devices can draw power from the bus. Be careful when plugging cables in: even though the connectors are polarized to prevent it, you can insert the plugs upside down if you're enthusiastic enough. Crossing the power and data lines this way is enough to let all the magic smoke out of your Mac or your FireWire drive, or both.

USB

The Universal Serial Bus is in many ways the successor to the older Apple Desktop Bus: a serial bus for the connections of various accessories, like mice, keyboards, and even storage devices. USB drives, like FireWire drives, aren't USB-native, but use bridges between ATA or SATA drives and the USB connection running back to the Mac.

USB 1.0 and 1.1, the original versions, top out at 12 Mb/second, for a piddling 1.5 MB/second transfer speed. Hi-Speed USB 2.0 runs at a more respectable 480 Mb/second, giving it 20 percent more bandwidth than FW400, at least in theory. (I have to mention the "Hi-Speed" part because there are USB 2.0 devices out there that run at pokey old USB 1.1 speeds.)

In practice, USB suffers from being a CPU-driven protocol; the controlling computer has to be much more involved in data transfers over USB than over FireWire. Thus, sustained transfer rates tend to be slightly lower on Hi-Speed USB 2.0 than on FW400. And that's in the best case, when a fast computer is being used and isn't doing anything else; if the Mac is busy doing other things (like rendering a real-time effect and displaying the output in the Canvas), performance falls off even more.

Hi-Speed USB suffers just like other multiple-speed buses do: plug in one 12 Mb device, and the entire bus slows down to 12 Mb. Fortunately, each USB 2.0 port on the G5 has its own internal hub, so you can segregate 12 Mb and 480 Mb devices from each other just by plugging them into different ports.

USB is fine for offline data backups and for moving projects from system to system, but it's simply not efficient enough to be the interface of choice for media drives. Although some brave souls have made Hi-Speed USB 2.0 drives work for DV editing without seeing too many dropped frames, it's just not robust enough for video work, especially not when FireWire is available.

When a drive provides both FireWire and USB, use the FireWire connection.