Re: Issues dropping vm fs caches with kernel 3.5.2 + bcache master

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

On Tue, Aug 21, 2012 at 02:31:18AM +0200, Javier Marcet wrote:
> >> >> I've recently set up a software RAID5 + bcache.
> >> >>
> >> >> I'm using kernel 3.5.2 merged with bcache master branch. I'm quite
> >> >> happy with the performance but
> >> >> there is an important issue. I can't drop caches. Writing any number
> >> >> to /proc/sys/vm/drop_caches
> >> >> makes the system hung.
> >> >>
> >> >> Furthermore, so far I've been unable to curb bcache's memory
> >> >> consumption using the available
> >> >> sysctl knobs.
> >> >>
> >> >> Is this problem known?
> >> >
> >> > No I didn't know about that - thanks, I'll look into it.
> >
> > Just tried it - I can reproduce it, but it's gone in 3.6. Looks like a
> > 3.5 bug, not a bcache bug.
> 
> Oh, it was a coincidence then. I hadn't needed to use that for a long
> time.

Yeah. Weird bug.

> > I'm gonna reset the master branch to my current dev branch soon as I've
> > tested it a bit more, but I'd suggest the bcache-3.2 branch - that's the
> > best tested code.
> 
> I cannot use a 3.2 kernel. This an HTPC too, besides a NAS and host for
> vm guests. The tuners I have are supported from 3.4 onwards.
> 
> Do you mean there might be more problems with the master branch?

Perhaps, but mainly it tracks mainline pretty closely. 

> By the way. I have read in the manual how to add new cache devices, but
> can you remove completely the caches in a 'clean way'? Imagine to change
> it to a different disk which for some reason you cannot use at the same time.

Yep. echo 1 > /sys/fs/bcache/<cache set>/unregister