Alpha Microsystems Eagle 100 Specifikace stáhnout pdf (Strana 72)

Page B-2 Appendix B

B.2READ AHEAD

The Roadrunner’s SCSI disk driver, SCZRR.DVR, is able to perform read-ahead directly

into AMOS disk cache. When any program attempts to read a physical block from a

disk, the SCZRR 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 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.

Programs that do significant random disk access (such as RNDRED) tend to slow down

with this read ahead scheme. Most of the slow down is caused by "thrashing" of the

cache, where cache entries that will be used again are removed from the cache due to

the allocation requirements of the read-ahead blocks (which typically are never used).

The actual data transfer overhead is very little, as most SCSI disk drives (especially fast

SCSI-2 drives) have a track cache built into the drive allowing both the target and read

ahead blocks to be transferred over the SCSI cable without delay.

B.2.1Controlling Read-Ahead

In order for read-ahead to occur on the Roadrunner, the AMOS disk cache

(DCACHE.SYS) must be installed as normal and the full SCSI dispatcher (SCZRR.SYS)

must also be installed. The number of read-ahead blocks to be transferred into cache on

every physical disk access is controlled by the FIXLOG program.

When you use FIXLOG to create a driver for the Roadrunner hardware, a new option

will appear, which is the number of read-ahead blocks. For example, type:

FIXLOG

RETURN

FIXLOG.LIT Version x.x(xxx)

1. Change the number of logicals

2. Create a sub-system driver

Enter choice: 2

RETURN

Enter name of generic driver to be used: SCZRR

RETURN

Enter number of logical units per physical unit: 10

RETURN

Enter SCSI id (0-6): 0

RETURN

Enter number of read-ahead blocks (0-7): 5

RETURN

Enter new driver name: MAX

RETURN

New driver is now in memory.

To save the driver you have created, type:

SAVE MAX.DVR

RETURN

PDI-00172-60, Rev. A02

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