Bull DPX/20, Installing And Maintaining

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1–3
Chapter 1. About the storage system
The SP consists of a printed-circuit board with two memory modules (SIMMs) and a
bezel with the two SCSI bus connectors, SP SCSI ID switch, status light,
asynchronous console connector, a slot for the write-cache memory card, and latches
to secure the SP in place. The SP is visible from the rear of the storage system.
The memory modules provide either 8 or 32 Mbytes of SP memory. The
storage-system buffers use a minimum of 4 Mbytes of this memory
.
The rest of the
memory is available for read caching if the SP does not have a write-cache memory
card, and for both read and write caching if the SP does have this card.
The status light indicates the following:
Replace light (amber) — Lights when the SP is defective or during powerup.
The SP processes data written to or read from the disk-drive modules, and controls
the disk modules in the storage system through a synchronous SCSI-2 bus. It has two
internal SCSI-2 buses, bus A and bus B. Bus A supports as many as four disk-drive
modules, and bus B supports as many as three disk-drive modules, for a total of seven
disk-drive modules.
If you use the ArrayGUIde  utility to manage the storage system, this utility
communicates with the storage system using the SCSI bus. If you do not use this
utility, then the SP can use an RS-232 connection to a terminal or to the server.
Write-cache memory card
Write protect
switch
Battery
holder
The write-cache memory card is required for storage-system write caching. It is a
battery-backed-up PCMCIA card with memory that mirrors the storage-system
write cache on the SP. The replaceable battery maintains the card’s memory during
a power or SP failure to provide a safe copy of the write cache.
When power returns after a power failure, the storage system restores the SP’s write
cache from the data on the card. If the SP fails, you can use the card to transfer the
write cache to a replacement SP. When you insert the card into a replacement SP in
the
same storage system, the storage system copies the data from the card into the
SP’s write cache. If you insert the card into an SP in a different storage system, the
storage system detects that the card contains data from another storage system’s
write cache and does not access the data.
The battery will maintain the card’s memory continuously without power for 13
months. When the SP detects that the card has been removed or that its battery is
low, it flushes the write cache data to disk and disables storage-system write
caching.