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POWER Series I/O Subsystem

Technical Specifications


High performance

Silicon Graphics disk subsystems are based on the highest performance commercially available disk drives and controllers integrated with carefully tuned drivers, resulting in consistently oustanding I/O performance.

Extent File System

Silicon Graphics proprietary file system implementation ensures that applications realize the full benefit of our high performance hardware subsystems.


A choice of subsystems based on the SCSI, ESDI, SMD and IP12 interfaces allows users to optimally match their disk subsystem with their I/O requirements and budget.


A careful product reliability qualification program and extensive burn-in and testing of every subsystem ensure excellent reliability.

IRIS POWER Series systems cover a broad range of CPU performance points. Silicon Graphics offers a wide variety of disk options that allow the user to create an optimal match between disk performance and CPU performance for every system in the POWER Series. Disk subsystems can be optimized for low cost, high throughput, high capacity, support of large numbers of users, or combinations of these objectives.

Figure 1 illustrates the POWER Series disk configuration options. The POWER Channel I/O processor is the interface between the MPLink system bus and the I/O subsystem. The POWER Channel board includes two independent, synchronous SCSI buses, a high performance VME bus, and an Ethernet interface.

SCSI disk and tape devices are suported directly by POWER Channel. ESDI, SMD, and IP12 disks are supported through the addition of controllers on the VME bus. ESDI and SMD controllers provide one control channel per board; IP12 controllers provide two independent control channels per board.

Figure 1: POWER Series I/O Subsystem and Disk
Configuration Options

It is important to note that all IP12 disk drives on the market do not offer comparable performance. Silicon Graphics offers the highest performance and largest capacity IP12 disk drives available. These drives are coupled with a highly integrated VME-based controller board that includes 1MB of on-board cache. Silicon Graphics uses a unique algorithm for dynamically managing this cache to optimize I/O performance based on patterns of recent disk activity. When moving data to and from CPU memory, the IP12 controller uses block-mode VME transfers at data rates up to 32MB/sec.

IRIX, Silicon Graphics' fully symmetric multiprocessing (SMP) version of UNIX, supports logical volumes and disk striping. Logical volumes allow a single UNIX file system to extend across multiple disk drives. Disk striping distributes a file system across multiple drives on a track-by-track or cylinder-by-cylinder basis in order to increase the data transfer rate. Striping of two drives will typically provide a data transfer speedup of 1.5X, and striping of four drives will provide a speedup in the range of 2X to 3X. Striping is supported on the SCSI, ESDI, SMD and IP12 interfaces. Drives that are striped together must all use the same interface and be of the same geometry. When using the ESDI and SMD interfaces, drives that are striped together must be configured on separate control/data channels. Multiple IP12 or synchronous SCSI drives on the same control/data channel can be striped together, though the performance improvement will be greater if separate channels are used.

IRIX includes other advanced features that maximize the disk performance user applications realize when working with the file system. For instance, IRIX's Extent File System employs many of the concepts underlying the BSD Fast File System (FFS), but achieves larger contiguous blocks (less disk fragmentation) with less software overhead than FFS.

IRIX's Integrated Data Cache (IDC) dynamically adjusts the size of the file system's buffer cache as needed to maximize system performance. IDC also reduces the copying of data between user and kernel memory space, thus increasing the sustainable I/O transfer rate and reducing the demands on the CPU, memory and system bus.

In multiprocessor systems with multiple disk controllers, disk interrupts can be directed to specific processors to balance each processor's work load and ensure fast response. Interrupt asignment is specified by the system adminstrator at system generation ("sysgen") time.

Before adding a disk product to our peripheral offering, Silicon Graphics performs an extensive design and quality evaluation of the disk subsystem. Controller firmware and the IRIX driver are tuned as needed to optimize the performance of every disk drive offered. once in production, every disk drive is extensively tested at our factory prior to shipment. All disk drives shipped from the Silicon Graphics factory are formatted, and the IRIX operating system is installed on all system disks. The results of this process are exceptionally high performance and high reliability implementations of industry standard disk subsystems.

POWER Series Disk Subsystem Specifications

                                SCSI      ESDI      SMD        IP12
Drive Capacity
  Unformatted (MB)              380,780   380,780   1230       1153
  Formatted (MB)                340,645   320,642   985        833
Drives per controller           7         4         4          16
Controllers per system          2 [+]     2         4          2
VME Slots per controller        0         1         1          1
Form Factor                     5.25"     5.25"     8"         8"

Disk Performance (rates in MB/sec)

Peak Transfer rate              2.4       2.4       3          6
Sustained transfer rate [*]     1.8       1.8       2.3        3.6
Rotation Speed (rpm)            3600      3600      3600       3600
Average seek (ms)               16        16        16         15

Disk Striping

System chassis [**]             Rack      TT        TT.Rack    TT.Rack
Drives striped                  2         2         2, 3, 4    2, 3, 4

Controller Features

Cache size (KB)                 n/a       128       128        1024
Bandwidth (MB/sec)              4         3         3          10
Command grouping                no        yes       yes        yes
Command queuing                 no        yes       yes        yes
Seek optimization               no        yes       yes        yes
Overlapped seeks                yes       yes       yes        yes
Zero latency reads              no        yes       no         yes
Read-ahead                      yes       yes       yes        yes
Dynamic read-ahead control      no        no        no         yes

 [+] Second SCSI controller requires POWER Channel option
 [*] Sequential reads through the file system from a single drive. SCSI
     and ESDI rates are for 780MB drives.
[**] TT-Twin Tower deskside enclosure - requires additional peripheral
     rack for SMD or IP12; Rack-POWER Center rack enclosure

Corporate Office
2011 N. Shoreline Boulevard
Mountain View, CA 94043
(415) 960-1980

Copyright 1992 by Silicon Graphics, Inc. All rights reserved. Silicon Graphics and the Silicon Graphiucs logo are registered trademarks, and IRIS POWER Series, POWER Channel, Extent File System, IRIX and Integrated Data Cache are trademarks, of Silicon Graphics, Inc. UNIX is a registered trademark of AT&T. Specifications are subject to change without notice.

Typed up by Ian Mapleson (mapesdhs@yahoo.com) on 01/Aug/2002
Original document code: 1000030 (2/92) (February 1992)

Ian's SGI Depot: FOR SALE! SGI Systems, Parts, Spares and Upgrades

(check my current auctions!)
[WhatsNew] [P.I.] [Indigo] [Indy] [O2] [Indigo2] [Crimson] [Challenge] [Onyx] [Octane] [Origin] [Onyx2]
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