Very slow writes on flash + msdosfs

[Poul-Henning Kamp]

One of the reasons that writes to flash can be extremely slow is
that the built in wear-leveling gets overwhelmed.

There is a specification somewhere, that explains to camera
manufacturers how exactly they should perform the writes to
flash media to get maximum writing speed.

Unfortunately many flash producers think that is the only thing you
can use flash devices for, and their wear-leveling support only
this write mode.

M-Systems had a patent on monitoring the free cluster map of the
fat filesystem from the wear-leveling code, but I don't know how
wide-spread that has become yet.  Sandisk bought M-Systems some
years ago, so I bet they have it.

There exists ATA/whatever commands to tell a flash device that a
given range of sectors can be reclaimed by the wear-leveling code,
but we do not issue these when we delete files.

As a result, the wearleveling ready-pool is rapidly depleted, forcing
all sector writes to perform a block evacuation, erase and rewrite.

The BIO_DELETE request was intended to give us support for this,
unfortunately, flash vendors are not at all willing to officially
support the interface and thus it never truly got implemented.

The slow write speed also indicates that you are wearing your
flash devices out up in 1% of the time they should last.

-- 
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