Barracuda 18XL Product Manual, Rev. A 11
4.3 Start/stop time
After DC power at nominal voltage has been applied, the drive typically becomes ready for media access com-
mands within 15 seconds if the Motor Start Option is disabled (i.e. the motor starts as soon as the power has
been applied). Maximum is 25 seconds. 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 15, but to
become ready within 25 seconds. During spin up to ready time, the drive responds to some commands over
the SCSI interface in less than 1.5 seconds after application of power. Stop time is typically <10 seconds from
removal of DC power. Maximum is <20 seconds.
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, part number 75789509).
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,
part number 75789509). 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 Table 9a.
4.5 Cache operation
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, part number 75789509), data requested by the host with a Read command 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 dur-
ing 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, part number 75789509.
The following is a simplified description of a read operation with cache operation enabled:
Case A -
A Read command is received and the first logical block (LB) is already in cache:
1. Drive transfers to the initiator the first LB requested plus all subsequent contiguous LB’s that are already in
the cache. This data may be in multiple segments.
2. When a requested LB is reached that is not in any cache segment, the drive fetches it and any remaining
requested LBs from the disc and puts them in a segment of the cache. The drive transfers the remaining
requested LBs from the cache to the host in accordance with the disconnect/reconnect specification men-
tioned above.
3. If the prefetch feature is enabled, refer to Section 4.5.2 for operation from this point.
Case B -
A Read command requests data, the first LB of which is not in any segment of the cache:
1. The drive fetches the requested LB’s from the disc and transfers them into a segment, and from there to
the host in accordance with the disconnect/reconnect specification referred to in case A.
2. If the prefetch feature is enabled, refer to Section 4.5.2 for operation from this point.
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