cbd/TODO

Cache object sizing:
  - lbdcache size: multiple of num_online_cpus().
  - lbatviewcache:
    Min: one.
    Max: one per lbd.
    Avg: 1/2 lbdcache size.
    => Alloc 1/2 lbdcache size.
  - lbpatpage cache:
    Min: lbatviewcache size.
    Max: 2 * lbatviewcache size.
    Avg: 1.5 * lbatviewcache size.
    => alloc 1.5 * lbatviewcache size.
  - pbatcache size:
    Min: 1
    Max: lbatviewcache size.
    Avg: 1/2 lbatviewcache size.
    => alloc ???
       1/2 lbatviewcache size is way too large.
       Ratio of lbatview to pbat is 1:lbat_per_pbat.


TODO:
  - Allocate pbat pages using allocv and vmap them
    (but only when pbat_len() > 1).
    Can we vmap compression workspace?
  - In *cache_get, call *_reset outside cache lock.
  - Implement vectorized lbatview_elem_pblk().
  - Dirty flag in compress header.
  - Implement stats.
  - Keep working lbd, flush on timer.
    Need per-cpu lbd.
  - Move back to module based build system.
  - Create utilities for:
    - Resizing a compressed device.
    - Checking and repairing a compressed device.
  - Check parameters in compress_ctr:
    - Compressed device must be large enough.
    - Backing device must be large enough.
   - Remove workqueue.