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SubjectRe: Bcache version 9
On 11/21/2010 05:09 PM, Greg KH wrote:
> On Sun, Nov 21, 2010 at 06:09:34AM -0800, Kent Overstreet wrote:
>> +++ b/Documentation/bcache.txt
>
> For new sysfs files, please create Documentation/ABI files.
>
>> +All configuration is done via sysfs. To use sde to cache md1, assuming the
>> +SSD's erase block size is 128k:
>> +
>> + make-bcache -b128k /dev/sde
>> + echo "/dev/sde"> /sys/kernel/bcache/register_cache
>> + echo "<UUID> /dev/md1"> /sys/kernel/bcache/register_dev
>
> /sys/kernel/bcache/? Really?

That was a completely arbitrary choice dating from when I first started
hacking on it. No point in moving it when it might be moved again :p

> Come on, shouldn't this be somewhere else? You only have 1 file here,
> right?

Two files (really three, but the third is for gimpy latency tracing and
will die eventually). register_dev is there so on bootup you don't have
to wait for the cache to be discovered - when you add a cache device if
there's a backing device waiting for a cache, and the cache has seen
that UUID before it'll do what you want.

> Shouldn't it be a configfs file instead as that is what you are doing?

That was one of the possibilities I had in mind. My main issue with that
though is I don't see any way to just have a bare config_item - per the
documentation, the user must do a mkdir() first, which just doesn't make
any sense for bcache. There's no point in having a persistent object
besides the one associated with the block device. Maybe there would be
in the future, with multiple cache devices, but I still think it's a
lousy interface for that problem - what bcache wants is something more
like a syscall; you wouldn't use configfs to replace mount(), for example.

There do exist global interfaces in sysfs, not attached to any device -
besides /sys/kernel, there's /sys/fs which doesn't have any rhyme or
reason to it I can discern. ecryptfs has /sys/ext4/ecryptfs/version,
ext4 has per device stuff that you can't find from the device's dir (you
woludn't know /sys/fs/ext4/md0 exists from looking at /sys/block/md0).
There's also /sys/fs/cgroup, which is another unique thing as far as I
can tell...

Then there's /sys/module which has a bunch of ad hoc stuff, but as far
as I can tell that's all still module parameters and register_cache and
register_dev certainly aren't module parameters.

So anyways, I absolutely agree that there are better solutions than
/sys/kernel/bcache but I want to replace it with something correct, not
something that sucks less. Ideas/flames are of course more than welcome :)


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