lastes sources don't include "drop_cache" option

[Linda A. Walsh]

Was there some reason that patch got dropped?

Otherwise rsync eats up all the buffer memory.

Note -- I tried directio -- didn't work due to alignment
issues -- buffers have to be aligned to sectors.

The kernel, if I remember correctly, has been on again/off again
on requiring alignment on directio -- because most of the drivers
and devices do for directio to work, at.

"dd" throughput is about 20-30% faster with direct_IO (on the target side,
not on the read side... no benefit on read -- but that's where drop
cache helps..