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leveling erases.
Again, considering the necessity
to maintain optimal read/write
performance in the flash, a flash
manager should never incur wear-
leveling erases if they are not
necessary. It will effectively move high
use areas of the flash to low use areas
over time and will keep the average
erase cycle count within a predefined
range. It is important to note that this
system utilizes the entire flash disk
by forcing static areas of the flash to
move, which would otherwise never
be rewritten fully.
In order to ensure that the wear-
leveling process will not degrade
performance or compromise the
integrity of fixed file system meta
data, the flash manager must begin
erases well before the cumulative
writes equals the size of the flash disk.
Advanced wearleveling schemes (like
Datalight FlashFX family) keep
endurance cycle differences in this
example between 700 and 1400, at a
maximum.
Simplistic Calculation of
Flash Cycles
In order to determine whether a
certain application will challenge the
endurance cycles of the flash that are
specified by the flash manufacturer,
a number of variables must be
considerd.
Assume the flash is used efficiently
by the application and that all flash
described by Stotal is available to be
written prior to an erase cycle being
incurred. Given this assumption, a
single erase cycle will be incurred once
the entire flash (Stotal) is written. The
period of time elapsed
over a given cycle count, and therefore
the period of time before a flash chip
wears out, is then calculated with the
following formula:
P = C [Stotal ÷ ( F * Dwrite) ]
Stotal describes the total flash memory available (KB) to be used as a disk.
Dwrite describes the amount of data (KB) being regularly written into the disk.
F is the frequency of Dwrite to the disk per day.
C is a given number of cycles to calculate the period.
Example – the difference with Static Wear-leveling
In this example, we examine a field data recorder that collects and stores
coordinates and other statistics at a rate of 1280 bytes per minute, and must
maintain the last hour of data (75KB), for a total of 1800KB of writes per day.
This device has 1 MB of flash media total, and the application and configuration
files will take up more than half of that space - meaning only 256 KB can be used
to store data.
With only dynamic wear-leveling, the media will regularly swap between two 128
KB erase blocks – remember, we cannot erase just 75 KB. The chart below shows
three different choices for flash media, rated for the total number of erase cycles.
The other values are fixed:
S total = 256 KB, D write = 1800 KB, F = 1 day
If this field recorder’s expected lifetime of 12 years, flash rated for 100,000 cycle
flash is required.
With both static and dynamic wear-leveling, as implemented by the FlashFX
family, the entire media is eventually available for data. This results in one change
to the equation values above.
S total = 1024 KB, D write = 1800 KB, F = 1 day
For the same expected lifetime of 12 years, flash rated for 10,000 erase cycles is
now perfectly appropriate, though a 20-year lifetime would still require flash with
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