Alpha Microsystems AM-6060 Owner's Manual download pdf (Page 86)

AM-6060 Computer Owner's Manual, Rev. 00

Appendix E - Read-Ahead and

Write Buffering

INTRODUCTION

Earlier AMOS systems achieved high levels of performance by using an intelligent disk controller (such

as the AM-520) to offload a large portion of the CPU overhead associated with disk access. A benefit of

this offloading is that extra cycles are available on the controller to perform functions such as read-ahead

and write buffering. Both of these schemes are used by the current AM-520 firmware, but cannot be used

on other non-intelligent interfaces such as the Alpha Micro SASI interface because the main processor

running AMOS has to control the SASI interface, stealing CPU cycles away from other resources, such

as the terminal service system and user jobs.

The AM-176 board uses a programmable RISC DMA controller for SCSI bus communications and for

data transfer to and from the AM-176's SCSI bus. The 68060 CPU is only involved with setup before and

cleanup after a SCSI command is sent to a device



the rest of the command, including data transfer, is

handled by the RISC processor.

Having the RISC processor take care of these details allows both read-ahead and write buffering without

the need for a separate controller. Also, fast SCSI-2 disk drives will give better performance than an

AM-520 using ESDI drives for the following reasons:

•

Physically, SCSI-2 drives are faster than all ESDI drives. They spin the platters twice as fast

(reducing latency) and have significantly faster seek times.

•

Data transfer rates are higher with fast SCSI-2 drives. ESDI drives have a maximum transfer rate

of 2.25MB/s, whereas fast SCSI-2 drives transfer data at 10MB/s (or around 4 times faster).

Wide SCSI has a maximum transfer rate of 20MB/s.

•

The data transfer path is much faster with the RISC SCSI-2 controller. It is able to read from or

write to system memory 32 bits at a time, taking 120ns per read or write. The AM-520 transfers

data 16 bits at a time, taking 210ns per 16-bit transfer (or 420ns per 32-bit transfer).

READ-AHEAD

The AM-176's SCSI disk driver, SCZR60.DVR, is able to perform read-ahead directly into the AMOS

(DCACHE) disk cache. When any program attempts to read a physical block from a disk, the SCZR60

driver will also read up to an additional seven sequential blocks from the disk drive and store these read-

ahead blocks in the cache.

This read-ahead scheme works very well when jobs on the system are doing a large number of sequential

reads. For example, data base searches and programs like REDALL may execute much faster because the

data they require is already in memory and only has to be transferred from the cache into the user

partition.

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Alpha Microsystems AM-6060 Owner's Manual Page 86