Barracuda 4LP Product Manual, Rev. D
11
Notes for Section 4.2.
[1]
Execution time measured from receipt of the last Byte of the Command Descriptor Block (CDB) to the
request for a Status Byte Transfer to the Initiator (excluding connect/disconnect).
[2]
Maximum times are specified over the worst case conditions of temperature, voltage margins and drive
orientation. When comparing specified access times, care should be taken to distinguish between typical
access times and maximum access times. The best comparison is obtained by system benchmark tests
conducted under identical conditions. Maximum times do not include error recovery.
[3]
Typical Access times are measured under nominal conditions of temperature, voltage, and horizontal ori-
entation as measured on a representative sample of drives.
[4]
Assumes no errors and no sector has been relocated.
[5]
Rate measured from the start of the first sector transfer to or from the Host.
[6]
Assumes system ability to support the rates listed and no cable loss.
[7]
Simulated.
[8]
Access time = controller ov average seek time
Access to data = controller ov average seek time + latency time
4.3
Start/stop time
After DC power at nominal voltage has been applied, the drive becomes ready within 20 seconds if the Motor
Start Option is disabled (i.e. the motor starts as soon as the power has been applied). If a recoverable error
condition is detected during the start sequence, the drive executes a recovery procedure which may cause the
time to become ready to exceed 20 seconds. During spin up to ready time the drive responds to some com-
mands over the SCSI interface in less than 3 seconds after application of power. Stop time is less than 20 sec-
onds from removal of DC power.
If the Motor Start Option is enabled, the internal controller accepts the commands listed in the SCSI Interface
Product Manual less than 3 seconds after DC power has been applied. After the Motor Start Command has
been received the drive becomes ready for normal operations within 13 seconds typically (excluding an error
recovery procedure). The Motor Start Command can also be used to command the drive to stop the spindle
(see SCSI Interface Product Manual, 77738479).
There is no power control switch on the drive.
4.4
Prefetch/multi-segmented cache control
The drive provides prefetch (read look-ahead) and multi-segmented cache control algorithms that in many
cases can enhance system performance. “Cache” as used herein refers to the drive buffer storage space when
it is used in “cache” operations. To select prefetch and cache features the host sends the Mode Select com-
mand with the proper values in the applicable bytes in Mode Page 08h (see SCSI Interface Product Manual,
77738479). Prefetch and cache operation are independent features from the standpoint that each is enabled
and disabled independently via the Mode Select command. However, in actual operation the prefetch feature
overlaps cache operation somewhat as is noted in Section 4.5.1 and 4.5.2.
All default cache and prefetch Mode parameter values (Mode Page 08h) for standard OEM versions of this
drive family are given in Tables 8a and 8b.
4.5
Cache operation
In general, 480 Kbytes of the 512 Kbytes of physical buffer space in the drive can be used as storage space for
cache operations. The buffer can be divided into logical segments (Mode Select Page 08h, byte 13) from which
data is read and to which data is written. The drive maintains a table of logical block disk medium addresses of
the data stored in each segment of the buffer. If cache operation is enabled (RCD bit = 0 in Mode Page 08h,
byte 2, bit 0. See SCSI Interface Product Manual P/N 77738479), data requested by the host with a Read com-
mand is retrieved from the buffer (if it is there), before any disc access is initiated. If cache operation is not
enabled, the buffer (still segmented with required number of segments) is still used, but only as circular buffer
segments during disc medium read operations (disregarding Prefetch operation for the moment). That is, the
drive does not check in the buffer segments for the requested read data, but goes directly to the medium to
retrieve it. The retrieved data merely passes through some buffer segment on the way to the host. On a cache
“miss”, all data transfers to the host are in accordance with “buffer-full” ratio rules. On a cache “hit” the drive
ignores the “buffer-full” ratio rules. See explanations associated with Mode page 02h (disconnect/reconnect
control) in the SCSI Interface Product Manual, 77738479.
