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SubjectRe: [GIT] Bcache version 12
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On Thu, Sep 15, 2011 at 2:15 PM, Dan Williams <dan.j.williams@gmail.com> wrote:
> On Sun, Sep 11, 2011 at 6:44 PM, Kent Overstreet
> <kent.overstreet@gmail.com> wrote:
>> On Sun, Sep 11, 2011 at 07:35:56PM -0600, Andreas Dilger wrote:
>>> On 2011-09-11, at 1:23 PM, Kent Overstreet <kent.overstreet@gmail.com> wrote:
>>> > I don't think that makes any more sense, as module paramaters AFAIK are
>>> > even more explicitly just a value you can stick in and pull out.
>>> > /sys/fs/bcache/register is really more analagous to mount().
>
> ... and you looked at module_param_call()?

Damn, nope. I still think a module parameter is even uglier than a
sysfs file, though.

As far as I can tell, the linux kernel is really lacking any sort of
coherent vision for how to make arbitrary interfaces available from
the filesystem.

We all seem to agree that it's a worthwhile thing to do - nobody likes
ioctls, /proc/sys has been around for ages; something visible and
discoverable beats an ioctl or a weird special purpose system call any
day.

But until people can agree on - hell, even come up with a decent plan
- for the right way to put interfaces in the filesystem, I'm not going
to lose much sleep over it.

>> I looked into that many months ago, spent quite a bit of time fighting
>> with the dm code trying to get it to do what I wanted and... no. Never
>> again
>
> Did you do a similar analysis of md?  I had a pet caching project that
> had it's own sysfs interface registration system, and came to the
> conclusion that it would have been better to have started with an MD
> personality.  Especially when one of the legs of the cache is a
> md-raid array it helps to keep all that assembly logic using the same
> interface.

I did spend some time looking at md, I don't really remember if I gave
it a fair chance or if I found a critical flaw.

I agree that an md personality ought to be a good fit but I don't
think the current md code is ideal for what bcache wants to do. Much
saner than dm, but I think it still suffers from the assumption that
there's some easy mapping from superblocks to block devices, with
bcache they really can't be tied together.

> And md supports assembling devices via sysfs without
> requiring mdadm which is a nice feature.

Didn't know that, I'll have to look at that. If nothing else
consistency is good...

> Also has the benefit of reusing the distro installation / boot
> enabling for md devices which turned out to be a bit of work when
> enabling external-metadata in md.

Dunno what you mean about external metadata, but it would be nice to
not have to do anything to userspace to boot from a bcache device. As
is though it's only a couple lines of bash you have to drop in your
initramfs.
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