Serial ATA Flash Drive
APS25MBAxxxx-XTX
8
©2014 Apacer Technology Inc. Rev. 1.1
3. Flash Management
3.1 Error Correction/Detection
SFD25M5 implements a hardware ECC scheme, based on the BCH algorithm. It can detect and correct up to
40 bits error in 1K bytes.
3.2 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.
3.3 Wear Leveling
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.
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
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.
Note: The controller unit of this product model is designed with a DRAM as a write cache for improved performance and data efficiency.
Though unlikely to happen in most cases, the data cached in the volatile DRAM might be potentially affected if a sudden power loss takes
place before the cached data is flushed into non-volatile NAND flash memory.
3.5 ATA Secure Erase
ATA Secure Erase is an ATA disk purging command currently embedded in most of the storage drives. Defined
in ATA specifications, (ATA) Secure Erase is part of Security Feature Set that allows storage drives to erase all
user data areas. The erase process usually runs on the firmware level as most of the ATA-based storage
media currently in the market are built-in with this command. ATA Secure Erase can securely wipe out the user
data in the drive and protects it from malicious attack.
