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Stratus Continuum

Thanks to Ti Kan (2004) for the input.

Stratus Technologies produced a line of Ultra High Availability Fault Tolerant PA-RISC servers, called Continuum, in the 1990s. The Continuum were based on different PA-RISC processors and sold as the Continuum 400, 600 and 1200 series between 1995 and 2004. These systems feature a great deal of redundancy, with up to four CPUs to form one single logical processor.

PA-RISC-powered Stratus were phased out in the mid-2000s in favor of Intel-based systems, the Xeon Pentium 4 based ftServer V Series. Before PA-RISC, Stratus computers used different architectures – Stratus/32, XA400 and XA2000 from the 1980s used Motorola 68000s CPUs, XA/R from the early-1990s used Intel i860 RISC.

Selected 1999 base prices

Model	Processor	Introduced	Base price
412	L1/P2 PA-7100 96	1996	$66,000
418	L1/P4 PA-8000 180	1996-97	$140,500
428	L2/P8 PA-8000 180	1996-97	$240,500
618	L1/P4 PA-8000 180	1996-97	$274,000
628	L2/P8 PA-8000 180	1996-97	$424,000
1218	L1/P4 PA-8000 180	1996-97	$499,000
1228	L2/P8 PA-8000 180	1996-97	$649,000

Prices are only basic server configuration from HP Professional in 1999.

Continuum 400

The Continuum 400 series has the same CPU/memory architecture as the 600/1200, but the I/O bus is different. Instead of a Golf bus, it has an X bus that connects each CPU/memory module to a pair of PCI bridge boards. All I/O connectivity is via PCI cards.

There are two PCI bays of 7 slots each, connected downstream from the PCI bridge boards. Each bay has a dual channel SCSI adapter on it as standard equipment. These are also cross-wired and dual-initiated much in the same way as the SCSI ports on the 600/1200 systems.

Continuum 400s were also typically shipped with a pair of Ethernet adapter cards. The PCI bridge boards also each contains a removable PCMCIA flash memory card. This is used as the boot device. FTX puts the bootloader as well as the UNIX kernel on there, whereas HP-UX only uses it for the bootloader.

The PCI bay doors control the power the the PCI slots. Once opened, all slots in that bay are powered off to facilitate removal and insertion of cards. The system continues to run on cards in the other bay. An interlock mechanism prevents both bay doors from being opened at the same time.

Two chassis versions were available, one a short form-factor AC-powered, the other a tall CO central office version with a choice of AC or DC power.

The Continuum 400 supported mainly Stratus-modified HP-UX as operating systems, with Stratus own FTX Unix only sold exceptionally.

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Continuum 600 and 1200

The Continuum 600 and 1200 series are similar designs but with different chassis configuration. The 600 has six slots for the main Golf bus, and the rest of the space is filled with I/O card cages meant for secondary I/O boards. The 1200 has twelve slots for the main bus which occupies the entire width of the chassis. Secondary I/O boards go into a separate chassis.

Both models have space for two rows of cooling fans on the top, and two rows of disk drives on the bottom and also either a QIC or DAT tape drive or CDROM drive. The redundant power supplies with built-in UPS resides at the very bottom.

The main Golf bus is the main interconnect between the big boards:

• G7xx - CPU and memory boards
• K450 - 4-channel HVD fast wide SCSI and Ethernet adapter
• K460 - 4-channel HVD fast wide SCSI and Ethernet adapter
• K470 - A carrier board for PMC PCI-mezzanine daughter cards
• K600 - Adapter to the secondary I/O card cages

On the 600 chassis, the six slots consists of two for the pair or CPU/memory boards, and four more slots for two pairs of big boards. On the 1200 chassis, there are slots for two pairs of CPU/memory boards and four pairs of big boards.

In addition the 600/1200 main chassis also has a pair of Console Controller cards which provides the RS232 console terminal and RSN modem connectivity. This controller also has a command mode that allows the operator to type commands on the console to reset the system, power down, power up, etc. It runs on housekeeping power that is independent of the rest of the system. The Console controller also contains some environmental monitoring circuitry that checks the chassis internal temperature and will increase the cooling fan speed if necessary.

The secondary I/O chassis can be used to plug in a wide array of I/O boards, all Stratus proprietary. These boards are also used on the XA/R line. FTX supported many of the communications boards like ISDN, serial, parallel, X.25, and all sorts of other comm boards. HP-UX did not support many of those, if any. VOS also supported disk and tape I/O through this.

The Continuum 600 and 1200 were geared towards the VOS transactional operating system from Stratus, with FTX Unix offered only exceptionally.

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Processors and architecture

Each logical processor is physically two pairs of actual CPUs, that means four physical CPU chips per single logical one. Each pair is located on a separate FRU. All processors run lock-stepped, they do exactly the same thing at the same time. Comparator logic between each two physical CPU pair monitors for discrepancies.

If any physical CPU glitches or does something different, the comparator logic will detect the error and take that pair of CPUs offline, while the system continues to run on the other pair. There is no failover time. On multi-processor boards, each FRU contains multiple pairs of the logical processor halves.

The memory is self-checking and ECC corrected. If an uncorrectable error occurs, the FRU in which the memory is located will also be taken offline.

The big I/O boards are also self-checking and contain a pair of everything. However, with the exception of the K600 they do not run lock-stepped to the twin FRU. For example on the K450/K460 boards, each of the SCSI host adapters is connected via the backplane into the same SCSI bus on the partner board, but each board’s controller occupies a different SCSI target ID.

Only one controller is normally active, but when a failure occurs on the active board, all I/O is switched to the other controller. For the Ethernet ports on that board, they can be wired up to the same network or to different networks, and a software RNI redundant network interface layer provides transparent switching. All disks are mirrored.

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Operating systems

Operating system support was split between the Continuum 400 on the one hand and the Continuum 600 and 1200 on the other hand.

FTX Unix and HP-UX: Stratus Continuum 400 were marketed and offered commercially with Stratus-modified HP-UX Unix 11.00 as main choice but also supported Stratus FTX Unix, System V Unix from Stratus and only sold on an exceptional basis.

FTX (Fault Tolerant Unix) was a System V Unix operating system from Stratus designed for high availability and reliability with fault-tolerance mechanisms. It was developed by Stratus for its own fault-tolerant computer systems.

There also was a cancelled effort to port the Stratus VOS operating system to the 400s. Continuum 400 servers running the Stratus-modified HP-UX 11.00 were fully binary compatible with stock HP-UX — programs compiled for normal HP-UX ran without changes on Continuum 400.

Stratus VOS: Continuum 600 and 1200 were sold primarily with Stratus VOS for transaction processing, with releases 13.0 (1995) to 14.7.2 (2005) on PA-RISC hardware. Also offered on the 600s and 1200s on exceptional basis was Stratus FTX, hardware support was limited though.

VOS was developed by Stratus as transaction-oriented operating system for its fault-tolerant servers. Stratus added a Unix implementation (System V) called Unix System Facilities (USF) to VOS and later added POSIX compliance as well.

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References

• HP-UX Continuum 400 Series (PA-7100) Technical Service Guide (no URL)
• The Stratus Continuum Family (URL gone)
• Stratus Continuum 400 Series Fault-Tolerant HP-UX Servers, Stratus Technologies 2003
• Stratus Continuum Products 600 and 1200, Stratus Technologies 2003
• Stratus Machine History (August 2017: Paul Green. Accessed March 2021)
• StratusHP-UX Brochure, Stratus 2004 (archive.org mirror)
• Stratus VOS Operating System, Count on Stratus, Stratus 2004 (archive.org mirror) Stratus Continuum Series - VOS, Stratus Virtual Operating system, Stratus 2003 (archive.org mirror) SVOS Software Release Bulletins, Stratus 2004, FTP (archive.org mirror)
• Continuum 600/1200 Series (PA-7100) Service Announcement, Stratus 1998 (archive.org mirror)
• Stratus Shifts High-End Servers From PA-RISC to Intel, Computer World 2004
• Stratus Continuum 600/1200 Series Systems, cabinet llustration and parts, Stratus customers service
Servers in the Stratussphere, HP Professional June 1999 (archive.org mirror, page 23)

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OpenPA is an independent resource on PA-RISC computers, technology and operating systems, edited since 1999 by Paul Weissmann. He also publishes OpenKRITIS on critical infrastructures and Insel Westberlin, a Berlin archive. OpenPA is a registered serial publication, ISSN 1866-2757.

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