Re: [PATCH 01/21] bcachefs: KEY_TYPE_accounting

[Kent Overstreet]

On Tue, Feb 27, 2024 at 10:49:19AM -0500, Brian Foster wrote:
> On Sat, Feb 24, 2024 at 09:38:03PM -0500, Kent Overstreet wrote:
> > New key type for the disk space accounting rewrite.
> > 
> >  - Holds a variable sized array of u64s (may be more than one for
> >    accounting e.g. compressed and uncompressed size, or buckets and
> >    sectors for a given data type)
> > 
> >  - Updates are deltas, not new versions of the key: this means updates
> >    to accounting can happen via the btree write buffer, which we'll be
> >    teaching to accumulate deltas.
> > 
> > Signed-off-by: Kent Overstreet <kent.overstreet@xxxxxxxxx>
> > ---
> >  fs/bcachefs/Makefile                 |   3 +-
> >  fs/bcachefs/bcachefs.h               |   1 +
> >  fs/bcachefs/bcachefs_format.h        |  80 +++------------
> >  fs/bcachefs/bkey_methods.c           |   1 +
> >  fs/bcachefs/disk_accounting.c        |  70 ++++++++++++++
> >  fs/bcachefs/disk_accounting.h        |  52 ++++++++++
> >  fs/bcachefs/disk_accounting_format.h | 139 +++++++++++++++++++++++++++
> >  fs/bcachefs/replicas_format.h        |  21 ++++
> >  fs/bcachefs/sb-downgrade.c           |  12 ++-
> >  fs/bcachefs/sb-errors_types.h        |   3 +-
> >  10 files changed, 311 insertions(+), 71 deletions(-)
> >  create mode 100644 fs/bcachefs/disk_accounting.c
> >  create mode 100644 fs/bcachefs/disk_accounting.h
> >  create mode 100644 fs/bcachefs/disk_accounting_format.h
> >  create mode 100644 fs/bcachefs/replicas_format.h
> > 
> ...
> > diff --git a/fs/bcachefs/disk_accounting_format.h b/fs/bcachefs/disk_accounting_format.h
> > new file mode 100644
> > index 000000000000..e06a42f0d578
> > --- /dev/null
> > +++ b/fs/bcachefs/disk_accounting_format.h
> > @@ -0,0 +1,139 @@
> > +/* SPDX-License-Identifier: GPL-2.0 */
> > +#ifndef _BCACHEFS_DISK_ACCOUNTING_FORMAT_H
> > +#define _BCACHEFS_DISK_ACCOUNTING_FORMAT_H
> > +
> > +#include "replicas_format.h"
> > +
> > +/*
> > + * Disk accounting - KEY_TYPE_accounting - on disk format:
> > + *
> > + * Here, the key has considerably more structure than a typical key (bpos); an
> > + * accounting key is 'struct disk_accounting_key', which is a union of bpos.
> > + *
> 
> First impression.. I'm a little confused why the key type is a union of
> bpos. I'm possibly missing something fundamental/obvious, but could you
> elaborate more on why that is here?

How's this?

 * More specifically: a key is just a muliword integer (where word endianness   
 * matches native byte order), so we're treating bpos as an opaque 20 byte                                                                                       
 * integer and mapping bch_accounting_key to that.