ATA Disk Module
AP-FMxxxxD30XXS-XTX
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© 2015 Apacer Technology Inc. Rev. 1.1
5. Flash Management
5.1 Advanced wear-leveling algorithms
Flash memory devices differ from Hard Disk Drives (HDDs) in terms of how blocks are utilized. For HDDs,
when a change is made to stored data, like erase or update, the controller mechanism on HDDs will
perform overwrites on blocks. Unlike HDDs, flash blocks cannot be overwritten and each P/E cycle wears
down the lifespan of blocks gradually. Repeatedly program/erase cycles performed on the same memory
cells will eventually cause some blocks to age faster than others. This would bring flash storages to their
end of service term sooner. Wear leveling is an important mechanism that level out the wearing of blocks
so that the wearing-down of blocks can be almost evenly distributed. This will increase the lifespan of
SSDs. Commonly used wear leveling types are Static and Dynamic.
5.2 Built-in Hardware ECC
The properties of NAND flash memory make it ideal for applications that require high integrity while
operating in challenging environments. The integrity of data to NAND flash memory is generally
maintained through ECC algorithms. This ATA-Flash Drive is programmed with a hardware ECC engine
which correct up to 72 bits per 1KB.
5.3 Flash Block Management
Current production technology is unable to guarantee total reliability of NAND flash memory array. When a
flash memory device leaves factory, it comes with a minimal number of initial bad blocks during production
or out-of-factory as there is no currently known technology that produce flash chips free of bad blocks. In
addition, bad blocks may develop during program/erase cycles. When host performs program/erase
command on a block, bad block may appear in Status Register. Since bad blocks are inevitable, the
solution is to keep them in control. Apacer flash devices are programmed with ECC, block mapping
technique and S.M.A.R.T to reduce invalidity or error. Once bad blocks are detected, data in those blocks
will be transferred to free blocks and error will be corrected by designated algorithms.
5.4 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 write-to-flash 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.
5.5 Mean Time Between Failures (MTBF)
Mean Time Between Failures (MTBF) is predicted based on reliability data for the individual components in
the drive. The prediction result for the drive is more than 1,000,000 hours.
Notes about the MTBF:
The MTBF is predicated and calculated based on “Telcordia Technologies Special Report, SR-332, Issue
2” method.
