The loader bcache is implementing simple read-ahead to boost the cache.
The bcache is built based on 512B block sizes, and the read ahead is attempting
to read number of cache blocks, based on amount of the free bcache space.
However, there are devices using larger sector sizes than 512B, most obviously
the CD media is based on 2k sectors. This means the read-ahead can not be just
random number of blocks, but we should use value suitable also for use with
larger sectors, as for example, with CD devices, we should read multiple of 2KB.
Since the sector size from disk interface is not too reliable, i guess we can
just use "good enough" value, so the implementation is rounding down the read
ahead block count to be multiple of 16.
This means we have covered sector sizes to 8k.
In addition, the update does implement the end of cache marker, to help to
detect the possible memory corruption - I have not seen it happening so far,
but it does not hurt to have the detection mechanism in place.
Reviewed by: allanjude
Approved by: allanjude (mentor)
Differential Revision: https://reviews.freebsd.org/D9179
7a60826b65