o_sync in vfat driver

[col-pepper]

part 3 (we'll get past these filters in the end...)

These devices do present special problems since they are a rw media that
can be abruptly removed at any time without even the chance for the OS to
interrupt on-going IO.

This is compounded by the fact that flash memory has to be zeroed and then
rewritten with the new data. If the device is physically removed before a
block is written the update will be lost. If it is removed _during_ write
the new and the old data will likely be lost.

If the block being written is the FAT , the principal record of the
structure of the whole disk will very likely be erased.

Since there is a heavy performance penalty involved (typically around an
_order of magnitude_ slower), it seems that the sole aim here is security
of data at any cost in the case of premature withdrawal.
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