Table 6 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, if reallocations are performed. Reallocations are performed when the ARRE bit (for reads) or AWRE bit (for writes) is one, the RC bit is zero, and the Recovery Time Limit for the command has not yet been met. Time needed to perform reallocation is not counted against the Recovery Time Limit.
The Read Continuous (RC) bit, when set to one, requests the disc drive to transfer the requested data length without adding delays (for retries or ECC correction) that may be required to insure data integrity. The disc drive may send erroneous data in order to maintain the continuous flow of data. The RC bit should only be used when data integrity is not a concern and speed is of utmost importance. If the Recovery Time Limit or retry count is reached during error recovery, the state of the RC bit is examined. If the RC bit is set, the drive will transfer the unrecovered data with no error indication and continue to execute the remaining command. If the RC bit is not set, the drive will stop data transfer with the last good LBA, and report a “Check Condition, Unrecovered Read Error.”
Table 6: Read and write retry count maximum recovery times [1]
Read retry | Maximum recovery time per LBA |
count [1] | (cumulative, msec) |
|
|
0 | 51.87 |
|
|
1 | 59.85 |
|
|
2 | 203.49 |
|
|
3 | 219.45 |
|
|
4 | 253.11 |
|
|
5 | 279.35 |
|
|
6 | 311.27 |
|
|
7 | 395.12 |
|
|
8 | 463.12 |
|
|
9 | 495.04 |
|
|
10 | 530.95 |
|
|
11 (default) | 1,282.97 |
|
|
Write retry | Maximum recovery time per LBA |
count | (cumulative, msec) |
|
|
0 | 23.94 |
|
|
1 | 35.91 |
|
|
2 | 55.86 |
|
|
3 | 67.83 |
|
|
4 | 119.79 |
|
|
5 (default) | 147.72 |
|
|
[1]These values are subject to change.
Setting these retry counts to a value below the default setting could result in an increased unrecovered 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. Therefore, 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.
36 | Cheetah 15K.5 SCSI Product Manual, Rev. J |
