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Drive Operations Chapter 3
Technical Reference Manual Page 23
Buffer Management and Command Execution
The drive contains a 512Kbyte segmentable buffer which is dynamically
configured to adapt to the particular drive configuration or operating
environment.
Read Look Ahead Code
The Read Retention RAM code executes commands sequentially as they are
received from the initiator(s). Commands from multiple initiators may be
queued and overlapped so that the subsequent command can be parsed while the
current command is being executed.
The drive's 512K byte buffer is configured as four segments. These segments
allow the drive to cache sequential data from four separate areas on the disk.
This can significantly improve performance in any environment in which
multiple disk files are kept open simultaneously and operated upon in some
interleaved fashion.
The Look-Ahead RAM code segments the 512 KB buffer into four 130,548 byte
(FD
H
blocks) segments. The remainder of the RAM is used by the
microprocessor as a scratch pad area and for non read or write data information
transfers. The buffer block size is equal to the data block size (typically 512 B)
plus the 4 bytes of buffer CRC appended to each block. (refer to the Electrical
Design Feature section in Chapter 1 for a description of the buffer CRC).
Buffer operations default on Power-up to Read Look Ahead enabled and Write
Caching disabled. MODE SELECT page 8, byte 2, bit 0 (RCD), when set to one
disables the read look-ahead cache function and bit 2 (WCE), when set to one
enables write cache. In addition, MODE SELECT page 8, byte 3 contains two
fields which control the retention priority for reads and writes. Refer to the
Eleventh Generation Disk Drive SCSI Interface Manual for additional details.
When a read command is received by the disk drive, the cache tables are
searched to determine if the requested data is contained in any of the four cache
segments (a cache hit). If there is no cache hit, the Least Recently Used (LRU)
segment is selected and a read from disk is initiated into that segment which is
now considered the Active Segment. The retention of data already transferred to
the host and read look ahead in the Active Segment buffer is controlled by the
state of the Read Retention Priority.