This chapter discusses the peripheral device options available on high-end (Challenge and Onyx) and desktop (Indigo, Indigo2, and Indy) platforms. The controller and I/O requirements for the various options are reviewed, and specific SCSI interface considerations are discussed. A brief discussion about optimizing disk drive performance is included at the end of this chapter. This chapter also contains some specific configuration instructions and warnings.
The SCSI devices discussed in this guide include devices such as disk drives, tape drives, floptical drives, and the CD-ROM drive. Storage devices released to manufacturing after this guide was published are not listed.
The term Onyx generically refers to a graphics model, while Challenge denotes a server model. Unless there is a specific reason for differentiation, the information applies to POWER Onyx and POWER Challenge systems. The graphics and server machines use the same basic chassis but completely different backplanes. There is a rackmount and a deskside chassis and these terms are used in the manual to differentiate the product device configurations. The Indigo, Indigo2, and Indy products are referred to generically as desktop systems unless there is reason to differentiate them.
The figures shown in this manual are intended to help resolve drive configuration questions that may arise in the field when installing the listed peripherals.
Several basic peripheral containment formats are used on Challenge and Onyx platforms:
Internal front-loading devices (FLDs) can be mounted in the single seven-bay drive rack provided in the deskside units, or the two eight-bay stubby SCSIBox 2s available in the rackmount units.
The CHALLENGE Vault rack SCSI expansion rack holds a maximum of seven or twelve (depending on the model) eight-bay SCSIBoxes for rackmount external peripherals. You may also attach deskside systems to the drives in a vault rack. All SCSI connections to the CHALLENGE Vault rack must be differential SCSI lines.
A six-disk SCSI expansion box (CHALLENGEvault M) is available with deskside and desktop systems only. For installation information, see the CHALLENGEvault M Owner's Guide (P/N 007-2155-00x). Disks mounted in the vault M do not use the Challenge and Onyx system drive sled hardware for mounting drives.
An eight-device SCSI expansion box (CHALLENGE Vault L) is available for use with specific Silicon Graphics systems. Devices installed in the Vault L use the same drive sled hardware for mounting drives as that used in deskside and rackmount systems. See the CHALLENGE Vault L Installation Instructions (P/N 108-0124-00x) for detailed information on this product.
External single-ended devices, such as the desktop 8-mm SCSI Jukebox, connect to a dedicated, external, single-ended SCSI connector on the system I/O panel.
Desktop products such as Indy, Indigo2, and Indigo can use both internal and external drives. Note that with the exception of Challenge S (a server variant of the Indy product) none of the desktop models uses differential SCSI devices. The desktop models hold internally mounted drives in the following number and size:
Indigo2 has one half-height 5.25-inch and two half-height 3.5-inch drive bays
Indigo has three half-height 3.5-inch drive bays
Indy has two inch-high 3.5-inch drive bays
The main purpose in using differential SCSI controllers is to improve the signal integrity of SCSI signals so they can be transmitted farther and faster than is possible with single-ended connections.
The maximum single-ended SCSI cable length is 19.6 feet (6 meters) from the SCSI controller board to the end of the SCSI bus. The recommended length is 9.8 feet (3 meters) when the bus is transferring in fast mode (10 M transfers per second). This means an external SCSI bus must be set for slow operation (5 M transfers per second). A single-ended SCSI bus should always be as short as possible, with as few connections as possible.
Length issues also have a significant impact on the success of a SCSI drive implementation. The first issue is the maximum allowable cable length. Additional key issues regarding SCSI implementation are listed below.
Differential signals can be reliably driven greater distances than single-ended signals. The cable lengths required to connect two rackmount chassis exceed the length allowed for the single-ended SCSI interface.
The reliable cable length for differential cables using a VME SCSI differential controller permits running SCSI signals up to 81 feet (25 meters).
Pay special attention to cable lengths, particularly when external devices are used with a single-ended SCSI bus.
 | Note: Usually, SCSI devices fail because of insufficient noise margins due to exceeding the maximum cable length, cable impedance mismatches, improper cable termination, or a combination of these. If you are having trouble with certain devices, particularly external devices, verify that you have not exceeded the maximum SCSI cable length. See Chapter 2 for additional important SCSI guidelines before you begin an installation. |
Another consideration for SCSI devices is the connection method. You cannot use a T-connector on a SCSI bus. However, devices can be daisy-chained together as long as the total cable length from the I/O board to the last device on a single-ended bus does not exceed 19.6 feet (6 meters). In addition, the physical end of the bus must be properly terminated.
The storage area on a disk drive is usually divided into partitions. Several partitions (usually on different disks) can be built up into one logical disk drive using logical volumes. A logical volume can include partitions from several physical disk drives and thus can be larger than any one of your physical disks. Logical volumes allow you to
use larger filesystems
grow existing filesystems
improve performance using multiple disk drives
Logical volumes are created by either spanning or striping multiple disks. Spanning joins partitions end to end. It steps between partitions on multiple physical drives, and is the default for creating logical volumes. Storage covers multiple partitions of varying sizes, joined one after another. The resulting performance is the same as the component disk devices. Striping is more difficult to set up but can result in a significant performance gain on very large files. Disk performance can be increased by as much as three times using striping with like devices.
Consider the following guidelines before implementing logical volumes and disk striping:
Carefully determine your strategy for backups before implementing disk striping. At times, it is easier to back up smaller filesystems.
You can stripe different manufacturers' drives, but the performance improvement is relative to the slowest transfer speed of all drives used.
You can have two partitions of similar or identical capacities with varying geometries.
You can stripe across various disk interfaces; however, the resulting performance gain is difficult to characterize.
For best performance, drives in a stripe group should be on separate controller ports or buses. SCSI devices can be striped if they are on the same channel, especially if you are optimizing a system for load averaging; however, the performance gains are greater if separate channels are used.
Striping improves performance only if the drive or controller is not already being heavily used.
In a four-way logical volume using an IO4 with two devices per channel, make sure that the devices are specified so that access to the two channels is interleaved (such as disk 1 channel 0, disk 1 channel 1, disk 2 channel 0, disk 2 channel 1).
| Note: This type of application may not be advisable in a SCSI bus environment where channel 0 is single-ended and channel 1 is differential. |
 | Caution: If a problem occurs on any disk in a logical volume, all of the data in the logical volume may be lost. |