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CONSTELLATION ES.1 SAS PRODUCT MANUAL, REV. G 39
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 483.14ms recovery time for each block and a maximum of 450ms 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.
9.3 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.
9.4 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 activity.
Since the background scan functions are only done during idle periods, BMS causes a negligible impact to system performance. The first
BMS scan for a newly manufactured drive is performed as quickly as possible to verify the media and protect data by setting the “Start time
after idle” to 5ms, all subsequent scans begin after 500ms of idle time. Other features that normally use idle time to function will function
normally because BMS functions for bursts of 800ms and then suspends activity for 100ms to allow other background functions to operate.
BMS interrupts immediately to service host commands from the interface bus while performing reads. BMS will complete any BMS-initiated
error recovery prior to returning to service host-initiated commands. Overhead associated with a return to host-servicing activity from BMS
only impacts the first command that interrupted BMS, this results in a typical delay of about 1 ms.
9.5 MEDIA PRE-SCAN
Media Pre-Scan is a feature that allows the drive to repair media errors that would otherwise have been found by the host system during
critical data accesses early in the drive’s life. The default setting for Media Pre-Scan is enabled on standard products. Media Pre-Scan
checks each write command to determine if the destination LBAs have been scanned by BMS. If the LBAs have been verified, the drive
proceeds with the normal write command. If the LBAs have not been verified by BMS, Pre-Scan will convert the write to a write verify to
certify that the data was properly written to the disk.
Note. During Pre-Scan write verify commands, write performance may decrease by 50% until Pre-Scan completes. Write perfor-
mance testing should be performed after Pre-Scan is complete. This may be checked by reading the BMS status.
To expedite the scan of the full pack and subsequently exit from the Pre-Scan period, BMS will begin scanning immediately when the drive
goes to idle during the Pre-Scan period. In the event that the drive is in a high transaction traffic environment and is unable to complete a
BMS scan within 24 power on hours BMS will disable Pre-Scan to restore full performance to the system.
Table 5 Read and write retry count maximum recovery times
Read retry count Maximum recovery time per LBA
(cumulative, ms)
Write retry count Maximum recovery time per LBA
(cumulative, ms)
0 108.29 0 23.94
1 124.95 1 35.91
2 424.83 2 55.86
3 458.15 3 67.83
4 483.14 4 119.79
5 590.16 5 (default) 147.72
6 644.92
7 711.56
8 898.54
9 1049.49
10 1116.13
11 1174.44
12 (default) 2311.47