[writeboost] Add test_invalidate_prev_cache

Writeboost usually writes back cache blocks in background but
there is a slow path (marked as unlikely) in foregruond which
performs single writeback.

To see how badly this path performs, this test submits 512B writes
in stride access patten and thus always pass through the inefficient path.

An experiment shows that throughtput decreses from 387KB/sec (HDD only) to
293KB/sec (With Writeboost) which is 25% loss.

Signed-off-by: Akira Hayakawa <ruby.wktk@gmail.com>
[writeboost] Add test_sync_writes

Add a benchmark test to see the worst-case effect of FLUSH request.

Signed-off-by: Akira Hayakawa <ruby.wktk@gmail.com>
[writeboost] Update for new tunable read_cache_threshold

Signed-off-by: Akira Hayakawa <ruby.wktk@gmail.com>
[writeboost] Enable read-caching in some existing tests

- ruby test
- dbench test

Signed-off-by: Akira Hayakawa <ruby.wktk@gmail.com>
[writeboost] Add split_overhead

This test submits big sequential read to wb'd device
to examine the overhead of splitting

Signed-off-by: Akira Hayakawa <ruby.wktk@gmail.com>
[writeboost] Add tests for read-caching

add following three tests
- no_read_cache: disabling works?
- read_cache: really staged and read hit?
- read_cache_threshold: threshold really works?

Signed-off-by: Akira Hayakawa <ruby.wktk@gmail.com>
Merge pull request #37 from akiradeveloper/for-joe

update for writeboost