On Thu, Aug 25, 2016 at 7:21 PM, Marcin Mirosław [off-list ref] 
wrote:

W dniu 25.08.2016 o 02:03, Christopher James Halse Rogers pisze:

Hi!

quoted

 On Thu, Aug 25, 2016 at 7:21 AM, marcin@mejor.pl wrote:

[...]

quoted

quoted

 Does it means that cache is unavailable and only tiering will be in
 bcachefs?
 And... How to mount tiered FS? When I pass one device in mount I'm
 getting:
  bcache: bch_open_as_blockdevs() register_cache_set err 
insufficient
 devices

 Tiering gets you all the advantages of caching, plus you can (with 
some
 effort) have the combined filesystem size be the sum of the SSD + 
HDD
 capacities, rather than the capacity be determined solely by the
 capacity of the slow tier (this is not currently the case for 
bcachefs).

I think that cacheing has at least such advantages over tiering:
- allow fast read and write to files compressed with slow alghoritm 
(gzip)

I think this is getting into ā€œwhat should we call this thingā€ 
arguments. A naive cache is just going to promote the gzipped data to 
the fast storage. On the other end, there's nothing much preventing a 
sophisticated tiering system from compressing/decompressing as a part 
of tier demotion/promotion.

- can be optimized for using SSD drives

It's not clear to me how? Tiering and caching are doing the same sort 
of things.