34
Cheetah NS SAS Product Manual, Rev. B
The drive firmware error recovery algorithms consists of 11 levels for read recoveries and five levels for write.
Each level may consist of multiple steps, where a step is defined as a recovery function involving a single re-
read or re-write attempt. The maximum level used by the drive in LBA recovery is determined by the read and
write retry counts.
Table 3 equates the read and write retry count with the maximum possible recovery time for read and write
recovery of individual LBAs. The times given do not include time taken to perform reallocations. Reallocations
are performed when the ARRE bit (for reads) or AWRE bit (for writes) is one, the RC bit is zero, and the recov-
ery time limit for the command has not yet been met. Time needed to perform reallocation is not counted
against the recovery time limit.
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.
[1]
These values are subject to change.
Setting these retry counts to a value below the default setting could result in degradation of the unrecov-
ered error rate which may exceed the value given in this product manual. A setting of zero (0) will result in
the drive not performing error recovery.
For example, suppose the read/write recovery page has the RC bit set to 0, read retry count set to 4, and
the recovery time limit field (Mode Sense page 01, bytes 10 and 11) set to FF FF hex (maximum). A four
LBA Read command is allowed to take up to 253.11 msec recovery time for each of the four LBAs in the
command. If the recovery time limit is set to 00 C8 hex (200 msec decimal) a four LBA read command is
allowed to take up to 200 msec for all error recovery within that command. The use of the Recovery Time
Limit field allows finer granularity on control of the time spent in error recovery. The recovery time limit
only starts counting when the drive is executing error recovery and it restarts on each command. There-
fore, each command’s total recovery time is subject to the recovery time limit. Note: A recovery time limit
of 0 will use the drive’s default value of FF FF. Minimum recovery time limit is achieved by setting the
Recovery Time Limit field to 00 01.
Table 3:
Read and write retry count maximum recovery times
Read retry count
1
Maximum recovery time per
LBA (cumulative, msec)
Write retry count
1
Maximum recovery time per
LBA (cumulative, msec)
0
77.87
0
23.94
1
89.85
1
35.91
2
305.49
2
55.86
3
329.45
3
67.83
4
347.42
4
119.79
5
397.98
5 (default)
147.72
6
437.86
7
485.78
8
611.66
9
713.68
10
761.6
11 (default)
815.51
12
2159.13
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