M-Systems Flash Disk Pioneers Flash Memory - Manual (Page 7)

Implementing MLC NAND Flash for Cost-Effective, High-Capacity Memory
91-SR-014-02-8L 7
Sustained Read
When comparing sustained read performance values in real-world scenarios for Binary Flash with
MLC, the gap lessens considerably: MLC performance is 98 percent of Binary flash performance.
Operations that both Binary flash and MLC require to support a sustained read operation – such as
running the driver code and the file system code, and accumulating bus cycles to support address,
command, error correction code and control information – account for closing the gap.
Sustained Write
A comparison of sustained write performance for both technologies in real-world scenarios must take
into account an additional factor: making room for new data when no free space is available. This
means adding to the calculation the time it takes to erase a flash unit and, depending on the time it
takes to manage the flash (using M-Systems’ TrueFFS®, for instance, adds 5 percent of the time
required to write a unit), this time as well. For Binary flash, these calculations result in a sustained
write performance rate of 250KBytes per second on a low MIPS platform, or 4
µsec per byte for a
typical mix of files, as compared with 172KBytes per second for MLC. (Note that the number of
sectors per unit for MLC is twice the corresponding number for Binary flash.) When these figures
are translated into percentages, MLC sustained write performance is approximately 69 percent of
Binary flash write performance.
Write performance greatly varies according to the user’s access patterns, mainly the average file size.
For large files the rate is much higher (up to approximately 600 KBytes per second); for very small
files it is much lower. Here, unlike in read operations, the time that is required for file system
handling is more significant than device driver time, especially when dealing with small files. Bus
cycle time for writing is practically the same as for reading. All the remaining time is spent on
software overhead.
Flash Management
Because of MLC’s unique architecture, pages can only be written sequentially, whereas in Binary
flash they can be written randomly within the erase block. MLC also makes partial page
programming impossible, as opposed to Binary flash technology that enables it. This means that the
existing translation layers used by TrueFFS to support Binary flash devices, NFTL and INFTL, are
unusable, since they rely on random page access. Sequential write only and the lack of partial page
programming impose limitations on MLC that affect reliability as well as performance.