Instruction manual

Page 52 Appendix B
Eagle 250 Upgrade Instructions, Rev. A00
FIXLOG
ENTER
FIXLOG.LIT Version x.x(xxx)
1. Change the number of logicals.
2. Create a sub-system driver.
Enter choice: 2
ENTER
Enter name of generic driver to be used: SCZ138
ENTER
Enter number of logical units per physical unit: 10
ENTER
Enter SCSI id (0-15): 0
ENTER
Enter number of read-ahead blocks (0-7): 5
ENTER
Enter new driver name: DSK
ENTER
New driver is now in memory.
To save the driver you have created, type:
SAVE DSK.DVR
ENTER
If you wish to disable or change the number of read-ahead blocks, simply use FIXLOG to generate a new
disk driver. If the disk driver is for the DSK: device, don't forget to use MONGEN and embed the new
driver into the system monitor.
The generic AM-138 SCSI disk driver (SCZ138.DVR) is set up for seven read-ahead blocks.
WRITE BUFFERING
AMOS (and therefore every application written for AMOS) understands only 512-byte disk blocks.
Therefore, when a disk write request is made by a program, a single block transfer is made to the disk
drive. If the program then writes the next sequential block, the system must wait the latency time of the
drive (i.e., the time it takes the drive to complete one revolution) before the next block can be written.
Latency, even on fast SCSI-2 drives, is around 7ms.
Write buffering can speed up the write process. When write buffering is enabled, all writes to the SCSI
disk are first transferred into a buffer. If the write buffer becomes at least half full, or around three
quarters of a second passes with no reads, or if a preset "guaranteed flush" time-out occurs, the
SCZ138.DVR disk driver will begin scanning through the write buffer, finding blocks that need to be
written out to the drive. The algorithm used to flush blocks out to the drive is able to find up to eight
consecutive blocks and write them to the disk drive as a single write command, therefore dramatically
improving system performance.
Another benefit of write buffering is it tends to eliminate duplicate disk writes, such as bitmap updates
during operations such as copying files and tape restores, and prevents head thrashing when reading
through random access data files and writing a sequential file out to the disk (as most report generation
programs do).