Chapter 4 PA-RISC Computer Systems - OpenPA.net

Stratus Continuum Hardware Details
(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.
Expansion chassis is available to house additional secondary I/O cards or disk shelves (for large disk
farms, etc).
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” (that is, 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 DMA engines for the big I/O boards is designed such that the main memory content is protected
from an errant card from scribbling over addresses that it was not supposed to write to. This of course
is programmed by the OS device drivers.
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.
Other communications interfaces employ software-driven failover schemes.
All disks are mirrored. Early FTX 3.x releases used an in-house virtual disk layer (VDL) driver, but
later releases switched to a modified version of the Veritas VxVM product. In HP-UX, HP’s own LVM
(logical volume manager) is used. VOS, of course, has its own disk mirroring scheme.
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. The 400 is 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.
Again, all disks are mirrored as they are on the 600/1200 series, and communications interfaces use
software-controlled failover mechanisms.
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