Re: How to create bcachefs?

[Christopher James Halse Rogers <chris@cooperteam.net>]

On Thu, Aug 25, 2016 at 7:21 PM, Marcin Mirosław <marcin@mejor.pl> 
wrote:
> W dniu 25.08.2016 o 02:03, Christopher James Halse Rogers pisze:
> 
> Hi!
> 
>>  On Thu, Aug 25, 2016 at 7:21 AM, marcin@mejor.pl wrote:
> [...]
>>>  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.