Re: [PATCH 01/21] bcachefs: KEY_TYPE_accounting

[Kent Overstreet]

On Fri, Mar 01, 2024 at 10:03:06AM -0500, Brian Foster wrote:
> On Thu, Feb 29, 2024 at 04:24:37PM -0500, Kent Overstreet wrote:
> > On Thu, Feb 29, 2024 at 01:43:15PM -0500, Brian Foster wrote:
> > > Hmm.. I think the connection I missed on first look is basically
> > > disk_accounting_key_to_bpos(). I think what is confusing is that calling
> > > this a key makes me think of bkey, which I understand to contain a bpos,
> > > so then overlaying it with a bpos didn't really make a lot of sense to
> > > me conceptually.
> > >
> > > So when I look at disk_accounting_key_to_bpos(), I see we are actually
> > > using the bpos _pad field, and this structure basically _is_ the bpos
> > > for a disk accounting btree bkey. So that kind of makes me wonder why
> > > this isn't called something like disk_accounting_pos instead of _key,
> > > but maybe that is wrong for other reasons.
> > 
> > hmm, I didn't consider calling it disk_accounting_pos. I'll let that
> > roll around in my brain.
> > 
> > 'key' is more standard terminology to me outside bcachefs, but 'pos'
> > does make more sense within bcachefs.
> > 
> 
> Ok, so I'm not totally crazy at least. :)
> 
> Note again that wasn't an explicit suggestion, just that it seems more
> logical to me based on my current understanding. I'm just trying to put
> down my initial thoughts/confusions in hopes that at least some of this
> triggers ideas for improvements...

I liked it because it makes the relationship between disk_accounting_pos
and bpos more explicit - they're both the same kind of thing.

> > > Either way, what I'm trying to get at is that I think this documentation
> > > would be better if it explained conceptually how disk_accounting_key
> > > relates to bkey/bpos, and why it exists separately from bkey vs. other
> > > key types, rather than (or at least before) getting into the lower level
> > > side effects of a union with bpos.
> > 
> > Well, that gets into some fun territory - ideally bpos would not be a
> > fixed thing that every btree was forced to use, we'd be able to define
> > different types per btree.
> > 
> 
> Ok, but this starts to sound orthogonal to the accounting bits. Since I
> don't really grok why this is called a key, here's how I would add to
> the existing documentation:
> 
> "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. We do this because disk_account_key actually is bpos for
> the related bkey that ends up in the accounting btree.
> 
> This btree uses nontraditional bpos semantics because accounting btree
> keys are indexed differently <reasons based on the counter
> structures..?>. Yadda yadda..
> 
> Unlike with other key types, <continued existing comment> ...

I'm just going to go with my latest revision for now, I think it's a
reasonable balance between terse and explanatory:

 * 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_pos to that.