Tape Backup

Installing and testing a tape backup system is one of those tasks that is either a breeze or a nightmare. If you don't run into any surprises , the process will take a few stress-free hoursnot including, of course, the time spent carefully planning the system before even ordering the hardware. Installations don't always go smoothly, however. At the first device unavailable message, you might as well call home to say you'll be working late.

At first glance, you wouldn't expect a tape drive installation to be any more fraught with pitfalls than installation of a hard drive. However, because the tape drive is often added to a fully configured systemusually as something of an afterthoughtyou're more likely to bring any hidden problems to the forefront. Also, given the different nature of tape drives and the fact that you're more likely to disconnect and reconnect them, the devices can strain a SCSI chain that's already marginal.

In this tutorial, we'll look at some of the dangers lurking in the back of the SCSI bus (or channel), and in the process, get an overview of SCSI fundamentals.

The Scuzzy Spec

The Small Computer Systems Interface (SCSI, pronounced SKUH-zee) specification started as an attempt to provide a device-independent way to connect multiple devices to a computer, without requiring a separate device driver customized for each type of hard disk or tape device. While the goal of eliminating separate drivers remains elusive , SCSI does promote the use of much more intelligent devices that handle many of the details of data writing and reading and defect management. Thus, the software on the computer doesn't have to include separate logic to handle each different type of drive.

A SCSI bus can address up to eight devices, each of which can be classified as initiator (a device giving commands to other devices) or target (a device carrying out those commands). Most often, the host adapter will take the role of initiator and the attached drives will be the targets. But, some SCSI devices can alternate between being initiators and targets.

Total SCSI cable length should be less than 6 meters, or about 19.7 feet. When measuring cable length, include cables inside the computer and within external devices, such as autoloaders. Cables with different impedance values should not be used in the same chain or the signal may tend to be reflected at the junction. You'll save yourself trouble by using high-quality cables and keeping the total length of the SCSI chain as short as possiblemuch shorter than the 6 meters specified in the standard.

Terminators

Each end of the SCSI chain should be terminated. Terminating resistors keep the signal from reflecting back into the cable. Some of the hardest-to-find problems spring from improper termination; having too many, too few, or incorrect placement of terminators can cause unreliable operation. Also, bear in mind that improperly terminated SCSI chains may work fine for a time, but then fail when you add another device to the chain or use applications that stress the limits of the SCSI data transfer rate.

A SCSI bus with no termination isn't likely to work at all. A bus that's missing just one terminator will work in most cases, but it won't be as reliable with long cables or in an environment with a lot of electrical noise. The same holds true for a SCSI bus in which the terminators aren't at the very ends of the chain.

Remember that for proper termination, only the devices (drives or host adapter) at the ends of the daisy chain must have terminators installed. Note that many drives come with terminating resistor packs installed. If you use drives in external cabinets, make sure that the drives installed in those cabinets are properly configured. Usually, it's easiest to remove the termination from the drive itself and install a plug-type terminator on the external box.

If the host adapter controls one SCSI channel and both internal and external SCSI devices are installed, the devices at each end of the chain need to be terminated and the host adapter itself should not be terminated. Some SCSI host adapters simplify configuration by including separate internal and external SCSI channels. With these cards, the adapter and the last drive in each chain should be terminated.

The original SCSI specification called for passive terminatorsa pair of resistors for each signal line on the SCSI bus. With its faster data transfer rates and tighter timings, the SCSI-2 specification introduced an alternate, more reliable form of termination called active termination. Incorporating one or more voltage regulators, active terminators provide much more consistent termination and are less susceptible to noise. Active terminators will work with SCSI or SCSI-2 channels. So, if you are experiencing hard-to-trace problems with SCSI devices, you may want to try replacing your passive terminators with active ones.

Improper termination isn't always the culprit when SCSI channels exhibit intermittent problems. The power supplies of some external SCSI cabinets can occasionally malfunction, causing unreliable operation of the drive and mimicking termination or SCSI ID problems with the SCSI channel. Other problems can arise from incorrect settings on the drives themselves .

Sometimes, adding a tape drive to a SCSI bus can suddenly cause the entire chain to operate more slowly. If you have a tape unit, CD-ROM, or other device attached to the SCSI bus, you should enable the disconnect feature on those devices. This allows the host-based adapter (HBA) to issue a command or series of commands to that device and then disconnect and return to servicing disk requests. If the disconnect feature is not enabled, the SCSI bus must wait for the device to return with a completion code, which may make the bus unable to fulfill other requests to talk to the drive.

SCSI IDS

Up to eight devices, including the host adapter, can attach to one SCSI channel. The SCSI ID must be different for each device on the same channel. The controller itself usually has ID 7. A bootable hard drive is usually set to ID 0. Other devices on the chain should have IDs that correspond to their priority; 0 has the lowest priority, and 7 has the highest.

This makes intuitive sense in the case of the controller, because it would obviously need the highest priority. However, with bootable hard drives, device priority may be confusing. The BIOS on many controllers will only make a drive with SCSI ID 0 bootable; this seems problematic because the device needing the highest priority is assigned the lowest. But, it turns out that SCSI priority isn't very important in normal operation. Most devices cooperate to keep the bus as free as possible, so arbitration priority is needed only as a last resort during device contention . In addition, arbitration is needed only at the beginning of a transaction between two devices, usually the host adapter and another SCSI device. While the two devices communicate, no other devices can access the SCSI bus.

SCSI ID is very important, even if arbitration priority isn't always a serious issue. Remember that each device, even internal drives, must have a unique ID. The ID also serves as the device address, a three-bit sequence that's included with each command issued to a device. Duplicate device IDs are a very common source of errors, and the problems they cause can be intermittent and hard to track down. This is especially true if you install drive mechanisms into do-it-yourself external SCSI cabinets and have incomplete information about the drive-select jumpers .

Installing a SCSI host adapter with multiple channels may ease the process of selecting unique IDs. Some SCSI cards use different channels, each supporting up to seven devices, for the internal and external connectors. Thus, you wouldn't need to worry about the IDs of internal devices when you install external SCSI devices. Separate channels can also help you segregate fast hard drives from slower devices, such as CD-ROMs and tape drives.

It's a good idea to keep tape drives on a different SCSI channel than your hard drives. This keeps fast hard drives from becoming burdened by slower devices. It's also a much safer configuration for emergency surgery on a tape drive. Although disconnecting a tape drive from the SCSI chain with the server power on is never recommended, in an emergency, you may have to. Having the tape drive segregated from the hard drives will at least ensure that you don't bring the whole file system down when you replace a jammed tape drive. Better yet, get a server that actually supports hot-swapping of SCSI devices.

Last Words

Some 8mm tape drives default to asynchronous transfer mode, while many host adapters default to synchronous transfer mode. If you power down the tape drive while the server is still running and then power it back on, the device will attempt to use its default asynchronous mode without renegotiating the transfer mode with the host controller. The next tape operation is then likely to lock up the workstation or server.

If there's no bootable hard disk on the SCSI bus, disable the SCSI adapter's BIOS and let the software driver control the SCSI devices.

Make sure that you are loading the latest version of the drivers for your host adapter. Tape backup programs will generally require ASPI (Advanced SCSI Programming Interface) drivers. The interface consists of two parts : a low-level, hardware-specific ASPI Manager that accepts ASPI commands and translates them into commands the adapter hardware understands; and an adapter-independent ASPI module. Although the ASPI module is adapter independent, it is generally customized to support a particular type of SCSI device, such as tape drives, CD-ROMs, or hard drives.

Feeling Kind Of SCSI: SCSI's New Generation

In the face of increasing speeds offered by hard drives, the SCSI specification has been updated. The SCSI-2 specification included methods for faster and wider data transfer and for more robust termination. The SCSI-3 specification, which is currently being developed, is intended to solidify some of the improvements needed to handle even faster data transfer, such as support for fiber optic interfaces. It will also add support for other types of peripherals.

Here are some of the SCSI- related terms you're likely to hear:

Differential SCSI vs. single-ended SCSI. These terms were defined by the original SCSI specification to support different types of SCSI interface cables. The most commonly encountered type is single-ended SCSI, which has one wire for each signal to be transmitted. Differential SCSI, in contrast, uses a pair of wires for each signal, providing greater immunity to electrical noise and supporting greater cable lengths.

A single SCSI bus supports either single-ended or differential devices; the host determines which type. You can't combine both types on the same chain. Most generic SCSI drives are single-ended SCSI devices.

Synchronous vs. asynchronous transfer. Defined in the original SCSI specification, synchronous transfers allow data rates of up to 5MBps on SCSI devices by allowing devices to transmit without waiting for acknowledgment from the other devicethe acknowledgments should eventually catch up, but the transmitting device doesn't have to wait for each one. SCSI-2 expanded this concept to support rates of up to 10MBps. This 10MBps rate is generally called Fast SCSI.

Fast SCSI. By reducing some of the timing margins in the SCSI specification, SCSI-2 devices can transfer data faster than SCSI devices 3MBps for asynchronous SCSI-2 transfers vs. 1.5MBps for SCSI, and up to 10MBps for SCSI-2 synchronous transfers. Fast SCSI generally refers to SCSI-2 devices capable of performing synchronous transfers faster than 5MBps.

Wide SCSI. The original SCSI specification called for a single cable serving as the backbone of the SCSI bus. This cable supported a data width of eight bits, or one byte. SCSI-2 also allows for transfers using wider data paths of 16 bits or 32 bits. However, this wider path entails the use of an additional 68-pin cable called the B-Cable. Not surprisingly, the original SCSI cable is called the A-Cable. Because wide transfers are automatically negotiated between devices, you can mix devices using different data widths on the same SCSI bus.

Because the B-Cable carries only data signals, Wide SCSI devices conforming to the SCSI-2 standard still use only eight bits for command, status, message, and arbitration lines. SCSI-3 will add a specification for a new type of cable, the P-Cable, that supports 16-bit arbitration, as well as 16-bit data transfers.

This tutorial, number 84, by Dave Fogle, was originally published in the August 1995 issue of LAN Magazine/Network Magazine.