Error Rates; Unrecoverable Errors; Interface Errors; Endurance Management - Seagate ST400FM0323 Product Manual

5.1

Error rates

The error rates stated in this manual assume the following:
• The drive is operated in accordance with this manual using DC power as defined in paragraph 6.2.1, "Direct Current Consumption
by Voltage Rail."
• Errors caused by host system failures are excluded from error rate computations.
• Assume random data.
• Default OEM error recovery settings are applied. This includes AWRE, ARRE, full read retries, full write retries and full retry time.
5.1.1

Unrecoverable Errors

An unrecoverable data error is defined as a failure of the drive to recover data from the media. These errors occur due to read or write
problems. Unrecoverable data errors are only detected during read operations, but not caused by the read. If an unrecoverable data
error is detected, a MEDIUM ERROR (03h) in the Sense Key will be reported. Multiple unrecoverable data errors resulting from the
same cause are treated as 1 error.
5.1.2

Interface errors

An interface error is defined as a failure of the receiver on a port to recover the data as transmitted by the device port connected to
the receiver. The error may be detected as a running disparity error, illegal code, loss of word sync, or CRC error.
5.2

Endurance Management

Customer satisfaction with Solid State Drives can be directly related to the internal algorithms which an SSD uses to manage the
limited number of Program-Erase (PE) cycles that NAND Flash can withstand. These algorithms consist of Wearleveling, Garbage
Collection, Write Amplification, Unmap, Data Retention, Lifetime Endurance Management.
5.2.1

Wear Leveling

Wear Leveling is a technique used by the drive to ensure that all Flash cells are written to or exercised as evenly as possible to avoid
any hot spots where some cells are used up faster than other locations. Wear Leveling is automatically managed by the drive and
requires no user interaction. The Seagate algorithm is tuned to operate only when needed to ensure reliable product operation.
5.2.2

Garbage Collection

Garbage Collection is a technique used by the drive to consolidate valid user data into a common cell range freeing up unused or
obsolete locations to be erased and used for future storage needs. Garbage Collection is automatically managed by the drive and
requires no user interaction. The Seagate algorithm is tuned to operate only when needed to ensure reliable product operation.
5.2.3

Write Amplification

While Write Amplification is not an algorithm, it is a major characteristic of SSD's that must be accounted for by all the algorithms
that the SSD implements. The Write Amplification Factor of an SSD is defined as the ratio of Host/User data requested to be written to
the actual amount of data written by the SSD internal to account for the user data and the housekeeping activities such as Wear
Leveling and Garbage Collection. The Write Amplification Factor of an SSD can also be directly affected by the characteristics of the
host data being sent to the SSD to write. The best Write Amplification Factor is achieved for data that is written in sequential LBA's
that are aligned on 4KB boundaries. The worst case Write Amplification Factor typically occurs for randomly written LBA's of transfer
sizes that are less than 4KB and that originate on LBA's that are not on 4KB boundaries.
5.2.4

UNMAP

A new SCSI command has been added to the SSD as part of the Thin Provisioning feature set. Use of the UNMAP command reduces
the Write Amplification Factor of the drive during housekeeping tasks such as Wear Leveling and Garbage Collection. This is
accomplished because the drive does not need to retain data which has been classified by the host as obsolete.
5.2.5

Data Retention

Data Retention is another major characteristic of SSD's that must be accounted for by all the algorithms that the SSD implements.
While powered up, the Data Retention of SSD cells are monitored and rewritten if the cell levels decay to an unexpected level. Data
Retention when the drive is powered off is affected by Program and Erase (PE) cycles and the temperature of the drive when stored.
Seagate 1200.2 SAS SSD Product Manual, Rev. A 22