Seagate Enterprise Performance 10K HDD v8 Product Manual, Rev. A
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* 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 440 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 440 ms), the command will end with Check Condition status report and unrecoverable read error will be
reported.
8.3
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.
8.4
Deferred Auto-Reallocation
Deferred Auto-Reallocation (DAR) simplifies reallocation algorithms at the system level by allowing the drive to reallocate
unreadable locations on a subsequent write command. Sites are marked for DAR during read operations performed by the drive.
When a write command is received for an LBA marked for DAR, the auto-reallocation process is invoked and attempts to rewrite the
data to the original location. If a verification of this rewrite fails, the sector is re-mapped to a spare location.
This is in contrast to the system having to use the Reassign Command to reassign a location that was unreadable and then generate
a write command to rewrite the data. DAR is most effective when AWRE and ARRE are enabled—this is the default setting from the
Seagate factory. With AWRE and ARRE disabled DAR is unable to reallocate the failing location and will report an error sense code
indicating that a write command is being attempted to a previously failing location.
8.5
Protection Information (PI)
Protection Information is intended as a standardized approach to system level LRC traditionally provided by systems using 520 byte
formatted LBAs. Drives formatted with PI information provide the same, common LBA count (i.e. same capacity point) as non-PI
formatted drives. Sequential performance of a PI drive will be reduced by approximately 1.56% due to the extra overhead of PI being
transferred from the media that is not calculated as part of the data transferred to the host. To determine the full transfer rate of a PI
drive, transfers should be calculated by adding the 8 extra bytes of PI to the transferred LBA length, i.e. 512 + 8 = 520. PI formatted
drives are physically formatted to 520 byte sectors that store 512 bytes of customer data with 8 bytes of Protection Information
appended to it. The advantage of PI is that the Protection Information bits can be managed at the HBA and HBA driver level. Allowing
a system that typically does not support 520 LBA formats to integrate this level of protection.
Protection Information is valid with any supported LBA size. 512 LBA size is used here as common example.
Table 4
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
25
1
120
1
50
5
440
2
78
10
810
3
95
15
1260
4
186
20 (default)
1500
5 (default)
295
