Seagate ST336754SS Cheetah 15K.4 SAS Product Manual - Page 23

Cheetah 15K.4 SAS Product Manual, Rev. B

13

If the drive receives a START STOP UNIT command with the START bit and IMMED bit equal to 1 and does

not receive a NOTIFY (ENABLE SPINUP) primitive within 5 seconds, the drive fails the START STOP UNIT

command.

The START STOP UNIT command may be used to command the drive to stop the spindle. Stop time is 30 sec-

onds (maximum) from removal of DC power. There is no power control switch on the drive.

4.4

Prefetch/multi-segmented cache control

The drive provides a prefetch (read look-ahead) and multi-segmented cache control algorithms that in many

cases can enhance system performance. Cache refers to the drive buffer storage space when it is used in

cache operations. To select this feature, the host sends the Mode Select command with the proper values in

the applicable bytes in page 08h. Prefetch and cache operations are independent features from the standpoint

that each is enabled and disabled independently using the Mode Select command; however, in actual opera-

tion, the prefetch feature overlaps cache operation somewhat as described in sections 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 9.3.2.1.

4.5

Cache operation

Note.

Refer to the

SAS Interface Manual

for more detail concerning the cache bits.

Of the 8,192 kbytes physical buffer space in the drive, approximately 7,077 kbytes can be used as a cache.

The buffer can be divided into logical segments (using Mode Select Page 08h, byte 13) 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 buffer. If the

cache is enabled (see RCD bit in the


), data requested by the host with a read command

is retrieved from the buffer, if possible, before any disc access is initiated. If cache operation is not enabled, the

buffer (still segmented with the 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 the explanation provided with the information about Mode Page 02h (disconnect/reconnect

control) in the


.

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.

2.

When a requested logical block is reached that is not in any cache segment, the drive fetches it and any

remaining requested logical block addresses from the disc and puts them in a segment of the cache. The

drive transfers the remaining requested logical blocks from the cache to the host in accordance with the

Mode Select Disconnect/Reconnect parameters, page 02h.

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, and the first logical block is not in any segment of the cache:

1.

The drive fetches the requested logical blocks from the disc and transfers them into a segment, and then

from there to the host in accordance with the Mode Select Disconnect/Reconnect parameters, page 02h.

2.

If the prefetch feature is enabled, refer to section 4.5.2 for operation from this point.

Section	Page
1.0 Scope	11
2.0 Standards, compliance and reference documents	13
2.1 Standards	13
2.1.1 Electromagnetic compatibility	13
2.1.1.1 Electromagnetic susceptibility	13
2.2 Compliance	14
2.2.1 Electromagnetic compliance	14
2.3 Reference documents	15
3.0 General description	17
3.1 Standard features	18
3.2 Media description	18
3.3 Performance	18
3.4 Reliability	19
3.5 Formatted capacities	19
3.6 Programmable drive capacity	19
3.7 Factory-installed accessories	19
3.8 Factory-installed options	20
3.9 User-installed accessories	20
4.0 Performance characteristics	21
4.1 Internal drive characteristics	21
4.2 Seek performance characteristics	21
4.2.1 Access time	21
4.2.2 Format command execution time (minutes)	22
4.2.3 General performance characteristics	22
4.3 Start/stop time	22
4.4 Prefetch/multi-segmented cache control	23
4.5 Cache operation	23
4.5.1 Caching write data	24
4.5.2 Prefetch operation	24
4.5.3 Optimizing cache performance for desktop and server applications	25
5.0 Reliability specifications	27
5.1 Error rates	27
5.1.1 Recoverable Errors	27
5.1.2 Unrecoverable Errors	27
5.1.3 Seek errors	28
5.1.4 Interface errors	28
5.2 Reliability and service	28
5.2.1 Annualized Failure Rate (AFR)	28
5.2.2 Preventive maintenance	28
5.2.3 Hot plugging the drive	28
5.2.4 S.M.A.R.T.	29
5.2.5 Thermal monitor	30
5.2.6 Drive Self Test (DST)	31
5.2.6.1 DST failure definition	31
5.2.6.2 Implementation	31
5.2.6.2.1 State of the drive prior to testing	32
5.2.6.2.2 Invoking DST	32
5.2.6.2.3 Short and extended tests	32
5.2.6.2.4 Log page entries	32
5.2.6.2.5 Abort	33
5.2.7 Product warranty	33
6.0 Physical/electrical specifications	35
6.1 AC power requirements	35
6.2 DC power requirements	35
6.2.1 Conducted noise immunity	37
6.2.2 Power sequencing	37
6.2.3 Current profiles	37
6.3 Power dissipation	41
6.4 Environmental limits	44
6.4.1 Temperature	44
6.4.2 Relative humidity	45
6.4.3 Effective altitude (sea level)	45
6.4.4 Shock and vibration	45
6.4.4.1 Shock	45
6.4.4.2 Vibration	47
6.4.5 Air cleanliness	47
6.4.6 Corrosive environment	47
6.4.7 Acoustics	48
6.4.8 Electromagnetic susceptibility	48
6.5 Mechanical specifications	48
7.0 Defect and error management	49
7.1 Drive internal defects/errors	49
7.2 Drive error recovery procedures	49
7.3 SAS system errors	50
8.0 Installation	51
8.1 Drive orientation	51
8.2 Cooling	52
8.3 Drive mounting	53
8.4 Grounding	53
9.0 Interface requirements	55
9.1 SAS features	55
9.1.1 task management functions	55
9.1.2 task management responses	55
9.2 Dual port support	56
9.3 SCSI commands supported	57
9.3.1 Inquiry data	61
9.3.2 Mode Sense data	61
9.3.2.1 ST3146854SS Mode Sense data	63
9.3.2.2 ST373454SS Mode Sense data	64
9.3.2.3 ST336754SS Mode Sense data	65
9.4 Miscellaneous operating features and conditions	66
9.4.1 SAS physical interface	67
9.4.2 Physical characteristics	69
9.4.3 Connector requirements	69
9.4.4 Electrical description	70
9.4.5 Pin descriptions	70
9.4.6 SAS transmitters and receivers	71
9.4.7 Power	71
9.5 Signal characteristics	71
9.5.1 Ready LED Out	71
9.5.2 Differential signals	72
9.5.2.1 Eye masks	72
9.5.2.1.1 Eye masks overview	72
9.5.2.1.2 Receive eye mask	73
9.5.2.1.3 Jitter tolerance masks	73
9.5.2.2 Transmitter signal characteristics	75
9.5.2.3 Receiver signal characteristics	76
9.5.2.3.1 Jitter	76
9.5.2.3.2 Receiver jitter tolerance	77
9.5.2.3.3 Compliant jitter test pattern (CJTPAT)	77
9.5.2.3.4 Impedance specifications	77
9.5.2.4 Electrical TxRx connections	78
9.5.2.4.1 Transmitter characteristics	78
9.5.2.5 Receiver characteristics	81
10.0 Seagate Technology support services	83
Numerics	89
A	89
B	89
C	89
D	89
E	89
F	90
G	90
H	90
I	90
J	90
L	90
M	90
N	91
O	91
P	91
Q	91
R	91
S	91
T	92
U	92
V	92
W	92

Seagate ST336754SS Cheetah 15K.4 SAS Product Manual - Page 23

Prefetch/multi-segmented cache control, Cache operation

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