Seagate ST15150N Product Manual - Page 25

Start/stop time, Prefetch/multi-segmented cache control

Get Seagate ST15150N - Barracuda 4.3 GB Hard Drive PDF manuals and user guides View all Seagate ST15150N manuals
Add to My Manuals
Save this manual to your list of manuals

Page 25 highlights

ST15150N/ND/W/WD/WC/DC Product Manual, Rev. D 15 5.4 Start/stop time Disabling the Motor Start option causes the drive to become ready within 30 seconds after DC power is applied. If a recoverable error condition is detected during the start sequence, the drive executes a recovery procedure−this may cause the time it takes for the drive to become ready to exceed 30 seconds. During the start sequence, the drive responds to some commands over the SCSI interface. Stop time is less than 30 seconds after removing DC power. This means the motor starts as soon as power is applied. During this time the drive responds to some commands over the SCSI interface. Stop time is less than 30 seconds from removal of DC power. Enabling the Motor Start option causes the internal controller to accept the commands listed in the SCSI-2 Interface Product Manual (77738479) less than 3 seconds after applying DC power. After receiving the Motor Start command, the drive becomes ready for normal operations within 30 seconds (excluding an error recovery procedure, if needed). The Motor Start command can also be used to command the drive to stop the spindle (see the Start/Stop command information in the SCSI-2 Interface Product Manual). There is no power control switch on the drive. 5.5 Prefetch/multi-segmented cache control The drive provides a prefetch/multi-segmented cache algorithm that in many cases enhances system performance. To select this feature the host sends the Mode Select command with the proper values in the applicable bytes in page 08h (see theSCSI-2 Interface Product Manual). Default is prefetch and cache operation enabled. Of the 1,024 Kbytes physical buffer space, approximately 954 Kbytes can be used as a cache. The cache can be divided into logical segments from which data is read and to which data is written. The drive keeps track of the logical block addresses of the data stored in each segment of the cache. If the cache is enabled (see RCD bit, Table 5.2.1-27 in the SCSI-2 Interface Product Manual), data requested by the host with a read command is retrieved from the cache, if possible, before any disc access is initiated. Data in contiguous logical blocks immediately beyond that requested by the Read command can be retrieved and stored in the cache for immediate transfer to the initiator on subsequent read commands. This is referred to as the prefetch operation. Since data that is prefetched may replace data already in the cache segment, an initiator can limit the amount of prefetch data to optimize system performance. The drive never prefetches more sectors than the number specified in bytes 8 and 9 of Mode page 08h (see the SCSI-2 Interface Product Manual). If the cache is not enabled, 954 Kbytes of the buffer are used as a circular buffer for read/writes, with no prefetch operation and no segmented cache operation. The following is a simplified description of the prefetch/cache operation: Case A. Read command is received and the first logical block is already in the cache. 1. Drive transfers to the initiator the first logical block requested plus all subsequent contiguous logical blocks that are already in the cache. This data may be in multiple segments.

  • 1
  • 2
  • 3
  • 4
  • 5
  • 6
  • 7
  • 8
  • 9
  • 10
  • 11
  • 12
  • 13
  • 14
  • 15
  • 16
  • 17
  • 18
  • 19
  • 20
  • 21
  • 22
  • 23
  • 24
  • 25
  • 26
  • 27
  • 28
  • 29
  • 30
  • 31
  • 32
  • 33
  • 34
  • 35
  • 36
  • 37
  • 38
  • 39
  • 40
  • 41
  • 42
  • 43
  • 44
  • 45
  • 46
  • 47
  • 48
  • 49
  • 50
  • 51
  • 52
  • 53
  • 54
  • 55
  • 56
  • 57
  • 58
  • 59
  • 60
  • 61
  • 62
  • 63
  • 64
  • 65
  • 66
  • 67
  • 68
  • 69
  • 70
  • 71
  • 72
  • 73
  • 74
  • 75
  • 76
  • 77
  • 78
  • 79
  • 80
  • 81
  • 82
  • 83
  • 84
  • 85
  • 86
  • 87
  • 88
  • 89
  • 90
  • 91
  • 92
  • 93
  • 94
  • 95
  • 96
  • 97
  • 98
  • 99
  • 100
  • 101
  • 102
ST15150N/ND/W/WD/WC/DC Product Manual, Rev. D
15
5.4
Start/stop time
Disabling the Motor Start option causes the drive to become ready within 30
seconds after DC power is applied. If a recoverable error condition is
detected during the start sequence, the drive executes a recovery proce-
dure
-
this may cause the time it takes for the drive to become ready to exceed
30 seconds. During the start sequence, the drive responds to some com-
mands over the SCSI interface. Stop time is less than 30 seconds after
removing DC power. This means the motor starts as soon as power is
applied. During this time the drive responds to some commands over the
SCSI interface. Stop time is less than 30 seconds from removal of DC power.
Enabling the Motor Start option causes the internal controller to accept the
commands listed in the
SCSI-2 Interface Product Manual
(77738479) less
than 3 seconds after applying DC power. After receiving the Motor Start
command, the drive becomes ready for normal operations within 30 seconds
(excluding an error recovery procedure, if needed). The Motor Start com-
mand can also be used to command the drive to stop the spindle (see the
Start/Stop command information in the
SCSI-2 Interface Product Manual).
There is no power control switch on the drive.
5.5
Prefetch/multi-segmented cache control
The drive provides a prefetch/multi-segmented cache algorithm that in many
cases enhances system performance. To select this feature the host sends
the Mode Select command with the proper values in the applicable bytes in
page 08h (see the
SCSI-2 Interface Product Manual).
Default is prefetch and
cache operation enabled.
Of the 1,024 Kbytes physical buffer space, approximately 954 Kbytes can be
used as a cache. The cache can be divided into logical segments from which
data is read and to which data is written.
The drive keeps track of the logical block addresses of the data stored in each
segment of the cache. If the cache is enabled (see RCD bit, Table 5.2.1-27
in the
SCSI-2 Interface Product Manual),
data requested by the host with a
read command is retrieved from the cache, if possible, before any disc
access is initiated. Data in contiguous logical blocks immediately beyond that
requested by the Read command can be retrieved and stored in the cache
for immediate transfer to the initiator on subsequent read commands. This
is referred to as the prefetch operation. Since data that is prefetched may
replace data already in the cache segment, an initiator can limit the amount
of prefetch data to optimize system performance. The drive never prefetches
more sectors than the number specified in bytes 8 and 9 of Mode page 08h
(see the
SCSI-2 Interface Product Manual).
If the cache is not enabled, 954
Kbytes of the buffer are used as a circular buffer for read/writes, with no
prefetch operation and no segmented cache operation.
The following is a simplified description of the prefetch/cache operation:
Case A.
Read command is received and the first logical block is already in
the cache.
1.
Drive transfers to the initiator the first logical block requested plus all
subsequent contiguous logical blocks that are already in the cache. This
data may be in multiple segments.