Alpha Microsystems Eagle 250 Manuel d'instructions télécharger pdf (Page 58)

Read-ahead and Write Buffering Page 53

Eagle 250 Upgrade Instructions, Rev. A00

Potential Pitfalls

Obviously, there can be problems with write buffering, especially if the system either crashes or is

powered off while writes are pending in the write buffer. If that happens, all pending writes are lost.

Though this sounds like a major problem, it can also happen if write buffering is not enabled. However,

write buffering increases the number of writes at risk. The primary write buffering risks are an errant

software operation or a hardware failure that causes a system crash.

To help reduce the possibility of data loss, certain safeguards have been put in place. Writes are not

buffered indefinitely; they are performed whenever the device is not performing reads. Even if the drive

is busy with read requests, the buffer is still periodically flushed, based on a user definable "absolute

flush time." In addition, if you have a UPS installed and connected to the AM-138-10's UPS status port,

and you experience a power failure and the UPS status port senses a low battery condition, AMOS will

flush and disable the write buffer in preparation for a system shutdown. Also, the MONTST command

automatically flushes the write buffer.

Therefore, you must weigh the potential for data loss (which is always there) versus the dramatic

performance increase seen when using write buffering. If you are worried about the reliability of write

buffering, it may be worth keeping in mind that the AM-520 disk controller has always used write

buffering on a track-by-track basis (however, not quite as efficiently as the AM-138-10 write buffering

scheme). The SMARTDRV program that comes with MS-DOS does write buffering (you may have

noticed the "Waiting for system shutdown" message when rebooting a PC with CTRL-ALT-DELETE)

and UNIX-based computers have always done it.

Setting Up Write Buffering

To enable write buffering, you must be using the full SCSI dispatcher (SCZ138-10.SYS). Enable write

buffering by adding parameters to the SYSTEM statement used to load the SCSI disk driver into system

memory. Append "/N" followed by the buffer size and flush period enables write buffering for that

device. The syntax is:

SYSTEM DVR:dev/N buffer-size flush-period

The buffer-size is the size of the write buffer (you specify the size in Kilobytes). We advise a buffer size

of 100K to 200K.

The flush-period is the maximum number of seconds data may be left in the write buffer without being

written to the disk. For example, if you specify 30, you will know that after 30 seconds any pending

writes will be written to the disk. This is true even if the disk is constantly busy servicing reads.

For example:

SYSTEM DVR:DSK/N 200K 60

One SYSTEM command is required for each different SCSI disk driver present in the system. For

example, if you have two 1.2GB SCSI-2 drives named DSK0-36 and DSK37-73 and one 540MB SCSI-2

drive named SUB0-17, you need one SYSTEM command for the DSK device (although it's really two

physical drives) and one SYSTEM command for the SUB device.

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Alpha Microsystems Eagle 250 Manuel d'instructions Page 58