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

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

[Mike Snitzer]

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.

Mike

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

[Kent Overstreet]

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.

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

[Mike Snitzer]

On Tue, Apr 09 2013 at  3:35pm -0400,
Kent Overstreet <koverstreet-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org> wrote:

> 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.

OK, thanks for the insight.

> Right way to do it would be to make two passes... 

Or just defer the formatting until all args are processed.

> I suppose I should just do that so it doesn't trip other people up.

Yeah, probably for the best.

Thanks.