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SubjectRe: [patch 2/6] mm: bdi: export BDI attributes in sysfs
From
Date
> On Tue, 29 Jan 2008 16:49:02 +0100
> Miklos Szeredi <miklos@szeredi.hu> wrote:
>
> > From: Peter Zijlstra <a.p.zijlstra@chello.nl>
> >
> > Provide a place in sysfs (/sys/class/bdi) for the backing_dev_info
> > object. This allows us to see and set the various BDI specific
> > variables.
> >
> > In particular this properly exposes the read-ahead window for all
> > relevant users and /sys/block/<block>/queue/read_ahead_kb should be
> > deprecated.
>
> This description is not complete. It implies that the readahead window is
> not "properly" exposed for some "relevant" users. The reader is left
> wondering what on earth this is referring to. I certainly don't know.
> Perhaps when this information is revealed, we can work out what was
> wrong with per-queue readahead tuning.

I think Peter meant, that the readahead window was only exposed for
block devices, and not things like NFS or FUSE.

> > --- /dev/null 1970-01-01 00:00:00.000000000 +0000
> > +++ linux/Documentation/ABI/testing/sysfs-class-bdi 2008-01-29 13:02:46.000000000 +0100
> > @@ -0,0 +1,50 @@
> > +What: /sys/class/bdi/<bdi>/
> > +Date: January 2008
> > +Contact: Peter Zijlstra <a.p.zijlstra@chello.nl>
> > +Description:
> > +
> > +Provide a place in sysfs for the backing_dev_info object.
> > +This allows us to see and set the various BDI specific variables.
> > +
> > +The <bdi> identifyer can take the following forms:
>
> "identifier"

Arrgh. Must run spellchecker on doc files :)

> > +blk-NAME
> > +
> > + Block devices, NAME is 'sda', 'loop0', etc...
>
> But if I've done `mknod /dev/pizza-party 8 0', I'm looking for
> blk-pizza-party, not blk-sda.
>
> But I might still have /dev/sda, too.

An alternative would be to uniformly use MAJOR:MINOR in there. It
would work for block devices and anonymous devices (NFS/FUSE) as well.

Would that be any better?

>
> > +FSTYPE-MAJOR:MINOR
> > +
> > + Non-block device backed filesystems which provide their own
> > + BDI, such as NFS and FUSE. MAJOR:MINOR is the value of st_dev
> > + for files on this filesystem.
> > +
> > +default
> > +
> > + The default backing dev, used for non-block device backed
> > + filesystems which do not provide their own BDI.
> > +
> > +Files under /sys/class/bdi/<bdi>/
> > +---------------------------------
> > +
> > +read_ahead_kb (read-write)
> > +
> > + Size of the read-ahead window in kilobytes
> > +
> > +reclaimable_kb (read-only)
> > +
> > + Reclaimable (dirty or unstable) memory destined for writeback
> > + to this device
> > +
> > +writeback_kb (read-only)
> > +
> > + Memory currently under writeback to this device
> > +
> > +dirty_kb (read-only)
> > +
> > + Global threshold for reclaimable + writeback memory
> > +
> > +bdi_dirty_kb (read-only)
> > +
> > + Current threshold on this BDI for reclaimable + writeback
> > + memory
> > +
>
> I dunno. A number of the things which you're exposing are closely tied to
> present-day kernel implementation and may be irrelevant or even
> unimplementable in a few years' time.

Which ones? They could possibly be moved to debugfs, or something.

I agree, that sysfs should be relatively stable.

Thanks,
Miklos


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