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The Buffer Full Ratio applies when the drive is reading from
the disk.
The drive fills buffer memory while the SCSI bus
is emptying it.
If the drive must seek for more data, the
SCSI bus may empty the buffer, at which time the drive dis-
connects.
Now assume the drive is much faster than the SCSI
bus: then the drive should reconnect as soon as there is
anything in the buffer, and the Buffer Full Ratio should be
set to 00.
Conversely, if the SCSI bus is much faster than
the drive, the drive shouldn't reconnect until the buffer is
full (Buffer Full Ratio
=
FF).
In many situations, the host will request a transfer of a
specific size; near optimum bus utilization is obtained by
setting the Buffer Full Ratio so the drive has filled the
buffer with that much data before reconnecting: if the host
asks for 4 KBytes, wait for the drive to transfer 4 KBytes
to the buffer before reconnecting.
This strategy avoids
excessive disconnect/reconnect SCSI bus overhead.
Buffer Empty Ratio applies when the drive is writing, and is
the mirror image of the situation when the drive is reading.
The SCSI bus fills buffer memory while the drive is emptying
it.
If the drive must seek, the SCSI bus may fill the buf-
fer, at which time the drive disconnects.
Now assume that
the SCSI bus is much faster than the drive: then the drive
should reconnect even if the buffer is nearly empty (Buffer
Empty Ratio
=
00).
Conversely, if the drive is much faster
than the SCSI bus, the drive should reconnect when the buf-
fer is full (Buffer Empty Ratio
=
FF).
If the host transfers data in known size blocks, near opti-
mum bus utilization is obtained by setting the Buffer Empty
Ratio so the buffer is filled with that much data before
reconnecting.
3.
When transferring data, use the maximum number of blocks
within a single read or write command.
Since each indivi-
dual command has an overhead associated with it, the more
individual commands used, the lower the data throughput.
If
a file consists of several blocks located in sequentially
increasing LBAs (logical block addresses), use one command
to read all of the blocks, instead of many commands of one
block reads or writes.
Also, adjust the block size with the MODE SELECT command
to allow up to 4 sectors to be read with a single block
command.
Since the Q250/Q280 drives are hard sectored and
have a fixed interleave of 1:1, optimizing the block size
and number of blocks per read can have a dramatic impact on
overall performance.
4-4
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