Flash disks and FFS layout heuristics

[Poul-Henning Kamp]

In message <200803302016.m2UKGuZA015127 at apollo.backplane.com>, Matthew Dillon w
rites:

>   The right way to deal with flash is *NOT* to require that the filesystem
>   be smart about flash storage, but instead to implement an intermediate
>   storage layer which linearizes the writes to flash and removes all
>   random erases from the critical path.

Your description of a simplified version of what is commonly called
a "Flash Adaptation Layer", is a very good example of why there is
a clear difference between "camera grade" flash devices, like most
CF cards, and the new generation of "SSD" devices, like the M-Tron
disk now in my laptop.

The Camera grade Flash devices get lousy random write performance
because they implement in essense what you describe, only in a more
complete fashion where they have error correction, both the data
and on the bitmaps.

The newer generation of SSD devices do things much smarter than
that, which is why their random write performance is much better
than camera-grade devices.

See my earlier emails for references to how to do the really smart
thing.


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