Seagate ST33000651SS Constellation ES.2 SAS Product Manual - Page 47

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 recovery 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. Table 3: Read and write retry count maximum recovery times Maximum recovery time per Read retry count* LBA (cumulative, msec) 1 5 10 15 20 (default) 124.32 621.62 1243.23 1864.85 2486.47 Maximum recovery time per Write retry count LBA (cumulative, msec) 0 23.94 1 35.91 2 55.86 3 67.83 4 119.79 5 (default) 147.72 * For read retry count, every tick ~ 5% of total error recovery. Valid range setting is 1-20. e.g. 1 ~ 5% 5 ~ 25% 20 ~ 100% 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 and if the read retry count is set to 5, this means ~ 25% of error recovery will be executed which consumes 621.62 ms (please refer to the table above). If the limit is reached and a LBA has not yet been recovered (i.e. requires retries beyond 621.62 ms), the command will end with Check Condition status report and unrecoverable read error will be reported. 10.3 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 commands are discussed. 10.4 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. Constellation ES.2 SAS Product Manual, Rev. C 39