On Sat, 13 Sep 2014 18:23:01 +0200 martin f krafft [off-list ref]
wrote:

also sprach Roberto Spadim [off-list ref] [2014-09-13 13:56 +0200]:

quoted

Try a bcache or something like it

The problem with a cache is that the data have to be read once from
the slow disk anyway, whereas I am proposing to keep a shadow copy
on rotational disk, really, just in case the MMC may need to be
removed at some point.

It is a persistent cache though.  So it only needs to be read once.
If the cache SD is the same sizes as the main device then it should never
ever need to read again.

But I suspect you will find that SD cards aren't really fast enough for your
needs.  It wouldn't hurt to experiment on just an SD card and see how it
performs.

NeilBrown