Serial ATA Flash Drive
APS25H12xxxG-XTM1X
8
© 2015 Apacer Technology Inc. Rev. 1.0
3. Flash Management
3.1 Error Correction/Detection
Flash memory cells will deteriorate with use, which might generate random bit errors in the stored data. Thus,
this SSD applies the BCH ECC Algorithm, which can detect and correct errors occur during Read process,
ensure data been read correctly, as well as protect data from corruption. This ECC supports up to 72bit/1K
data.
3.2 Bad Block Management
Bad blocks are blocks that include one or more invalid bits, and their reliability is not guaranteed. Blocks that
are identified and marked as bad by the manufacturer are referred to as “Initial Bad Blocks”. Bad blocks that
are developed during the lifespan of the flash are named “Later Bad Blocks”. Apacer implements an efficient
bad block management algorithm to detect the factory-produced bad blocks and manages any bad blocks that
appear with use. This practice further prevents data being stored into bad blocks and improves the data
reliability.
3.3 Wear Leveling
NAND Flash devices can only undergo a limited number of program/erase cycles, and in most cases, the flash
media are not used evenly. If some areas get updated more frequently than others, the lifetime of the device
would be reduced significantly. Thus, Wear Leveling technique is applied to extend the lifespan of NAND
Flash by evenly distributing write and erase cycles across the media.
Apacer provides advanced Wear Leveling algorithm, which can efficiently spread out the flash usage through
the whole flash media area. Moreover, by implementing both dynamic and static Wear Leveling algorithms,
the life expectancy of the NAND Flash is greatly improved.
3.4 Power Failure Management
Power Failure Management plays a crucial role when experiencing unstable power supply. Power disruption
may occur when users are storing data into the SSD. In this urgent situation, the controller would run multiple
flush cycles to store the metadata for later block rebuilding. This urgent operation requires about several
milliseconds to get it done. At the next power up, the firmware will perform a status tracking to retrieve the
mapping table and resume previously programmed NAND blocks to check if there is any incompleteness of
transmission.
3.5 TRIM
TRIM is a feature which helps improve the read/write performance and speed of solid-state drives (SSD).
Unlike hard disk drives (HDD), SSDs are not able to overwrite existing data, so the available space gradually
becomes smaller with each use. With the TRIM command, the operating system can inform the SSD which
blocks of data are no longer in use and can be removed permanently. Thus, the SSD will perform the erase
action, which prevents unused data from occupying blocks all the time.
