Re: Bcache upstreaming

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

On Thu, Jan 31, 2013 at 10:38:11PM -0500, Mike Snitzer wrote:
> I'm just seeing extra newlines (empty lines), e.g.:
> bcache: run_cache_set() invalidating existing data

Oh I see it, I screwed that up when I converted a bunch of stuff to
pr_*() macros and my brain was so used to reading the same stuff I
didn't notice it. I'll have that fixed in a minute.

> > > - bcache doesn't appear to be establishing proper holders on the devices
> > >   it uses for the backing and cache devices.
> > >   - lsblk doesn't show any associations with bcache devices.
> > 
> > How's that created - what function am I looking for?
> 
> bd_link_disk_holder and bd_unlink_disk_holder

Ok, that's done - it's in the testing branch

> 
> > >   - the fio utility isn't able to get any stats for the bcache device or
> > >     the devices bcache uses.
> > 
> > I'd been meaning to fix that, never got around to figuring out how those
> > stats are generated. Function/file you can point me to?
> 
> I think you'll get the stats for via genhd (add_disk) -- the stats are
> the 'disk_stats dkstats' member of the genhd's hd_struct.  But as of
> now you'll notice that /sys/block/bcacheX/stat only ever contains 0s.

> 
> part_stat_inc, part_stat_add are the low-level methods for driving the
> counters up.  But I'm not sure why bcache isn't getting these stats  --
> disk stats are something we get for free with DM devices so I haven't
> really had to dig into these mechanics in detail.

Ah, I remember coming across that before now. Got that fixed too, it's
in both the stable and testing branches.

And dm and md are both calling it from their make_request functions,
which is why bcache wasn't getting it.

> > > - if I 'stop' a bcache device (using sysfs) while it is mounted; once I
> > >   unmount the filesystem the device that bcache was using as a cache
> > >   still has an open count of 1 but the bcache device then no longer
> > >   exists
> > 
> > You mean the backing device isn't open, just the cache device?
> >
> > That's intended behaviour, backing and cache devices have separate
> > lifetimes (and you can attach many backing devices to a single cache).
> > 
> > You just have to stop the cache set separately, via
> > /sys/fs/bcache/<uuid>/stop or /sys/block/<cache device>/bcache/set/stop
> 
> The /dev/bcacheX device was mounted.  I issued 1 to
> /sys/block/#{bcache_name}/bcache/stop before unmounting.  I then
> unmounted /dev/bcacheX, the remaining bcache device infrastructure was
> torn down.  But after that the device that was the cache device was
> still held open -- seems like a reference leaked.

Was there still a directory in /sys/fs/bcache?