Wear Leveling; Garbage Collection; Write Amplification; Deallocation - Seagate Nytro 5350S NVMe SSD Product Manual

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8.4.1

Wear leveling

The drive uses Wear Leveling to make sure that all Flash cells are written to or exercised as evenly as possible to avoid
hot spots where some cells are used up faster than other locations. The drive automatically manages Wear Leveling
without user interaction. The Seagate algorithm operates only when needed to ensure reliable product operation.
8.4.2

Garbage collection

The drive uses Garbage Collection 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. The drive automatically manages Garbage
Collection without user interaction. The Seagate algorithm operates only when needed to ensure reliable product
operation.
8.4.3

Write amplification

While Write Amplification is not an algorithm, it is a major characteristic of SSDs. Write Amplification must be
accounted for by all the algorithms that the SSD implements. The Write Amplification Factor of an SSD is 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 LBAs that are aligned on 4KB
boundaries. The worst case Write Amplification Factor 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.
8.4.4

Deallocation

Deallocation is when the firmware uses the Dataset Management command, the Write Zeroes command, or the
Sanitize command to deallocate logical blocks. This deallocation reduces the Write Amplification Factor of the drive
during housekeeping tasks such as Wear Leveling and Garbage Collection. This happens because the drive does not
need to retain data which has been classified by the host as obsolete.
8.4.5

Data retention

Data Retention is another major characteristic of SSDs 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.
8.4.6

SSD percentage used endurance indicator

An application can interrogate the drive through the host to determine an estimate of the percentage of device life
that has been used. To accomplish this, issue a Get Log command to Log Identifier 2h. This allows applications to read
the contents of the Percentage Used field in Byte 5. The Percentage Used field is defined in the NVM Express
specifications available from the NVM Express consortium.
Seagate Nytro 5350S NVMe SSD Product Manual, Rev A 18