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When the RC bit is one, reallocations are disabled even if the ARRE or AWRE bits are one. The drive will still
perform data recovery actions within the limits defined by the Read Retry Count, Write Retry Count, and
Recovery Time Limit parameters. However, the drive does not report any unrecovered errors.
Table 14:
Read and write retry count maximum recovery times
Maximum recovery time per
Read retry count
LBA (cumulative, msec)
0
67.43
1
77.81
2
264.54
3
300.85
4
344.60
5
378.72
6
420.21
7
529.22
8
617.62
9
659.11
10
705.80
11 (default)
1693.67
Setting these retry counts to a value below the default setting could result in degradation of the unrecovered
error rate. For example, suppose the read/write recovery page has the RC bit = 0, the read retry count set to 4,
and the recovery time limit set to 450. A 4-block read command can take up to 371 msec recovery time for
each block and a maximum of 450 msec recovery for all four blocks. If either of these limits is reached and a
block has not yet been recovered, the command will end with Check Condition status and an unrecoverable
read error will be reported.
7.3
SAS system errors
Information on the reporting of operational errors or faults across the interface is given in the SAS Interface
Manual. The SSP Response returns information to the host about numerous kinds of errors or faults. The
Receive Diagnostic Results reports the results of diagnostic operations performed by the drive.
Status returned by the drive to the initiator is described in the SAS Interface Manual. Status reporting plays a
role in systems error management and its use in that respect is described in sections where the various com-
mands are discussed.
7.4
Background Media Scan
Background Media Scan (BMS) is a self-initiated media scan. BMS is defined in the T10 document SPC-4
available from the T10 committee. BMS performs sequential reads across the entire pack of the media while
the drive is idle. In RAID arrays, BMS allows hot spare drives to be scanned for defects prior to being put into
service by the host system. On regular duty drives, if the host system makes use of the BMS Log Page, it can
avoid placing data in suspect locations on the media. Unreadable and recovered error sites will be logged or
reallocated per ARRE/AWRE settings.
With BMS, the host system can consume less power and system overhead by only checking BMS status and
results rather than tying up the bus and consuming power in the process of host-initiated media scanning activ-
ity.
Savvio 10K.2 SAS Product Manual, Rev. B
Maximum recovery time per
Write retry count
LBA (cumulative, msec)
0
31.12
1
46.68
2
72.62
3
88.18
4
155.73
5 (default)
192.04
39
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