Re: make-bcache's bucket size and block size values are ignored

[Kent Overstreet <koverstreet-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org>]

On Tue, Apr 09, 2013 at 10:07:36AM -0400, Mike Snitzer wrote:
> Hey Kent,
> 
> No matter what I specify via make-bcache's -b and -w args they are
> always 512k and 512b respectively, e.g.:
> 
> # make-bcache -B /dev/mapper/test-dev-174467 -C /dev/mapper/test-dev-24832 --cache_replacement_policy=fifo -b 2048k --writeback --discard
>     UUID:                       3705b1d8-4b44-406c-abf2-594c0cb146be
>     Set UUID:           a16dc554-c054-4a24-90fa-40f624e3a652
>     nbuckets:           8192
>     block_size:         1
>     bucket_size:                1024
>     nr_in_set:          1
>     nr_this_dev:                0
>     first_bucket:               1
>     UUID:                       46a37bba-2582-4533-a95b-1eb29c71e53f
>     Set UUID:           a16dc554-c054-4a24-90fa-40f624e3a652
>     nbuckets:           512
>     block_size:         1
>     bucket_size:                1024
>     nr_in_set:          1
>     nr_this_dev:                0
>     first_bucket:               1
> 
> # cat /sys/block/bcache3/bcache/cache/bucket_size 
> 512k
> 
> I can take a look at the make-bcache source in a bit but figured I'd let
> you know sooner rather than later.

Oh, it's because you specified the bucket size after the device. If you
specify the devices last it'll work.

So it's definitely unintuitive and semi broken, the reason it works that
way is for formatting backing devices and cache devices at the same
time; it walks through the options, and when it sees a device it formats
it with the options seen so far.

Right way to do it would be to make two passes... I suppose I should
just do that so it doesn't trip other people up.