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    SubjectRe: Bcache version 9
    On Tue, Nov 23, 2010 at 12:07:36AM -0800, Kent Overstreet wrote:
    > 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.

    True, but I thought configfs could handle "bare" config items, you might
    want to look a bit closer as to how people are using it. But I could be
    totally wrong however.

    > 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.

    /sys/fs is for different filesystem specific things.

    > 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...

    No, sys/fs/cgroup/ is where the cgroup filesystem is mounted.

    > 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.

    It's not ad hoc, it's module specific things.

    > 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 :)

    What is "bcache"? Is it related to filesystems? If so, use
    /sys/fs/bcache and I have no issues with it. But don't put it in
    /sys/kernel/ without at least asking.


    greg k-h

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