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... > > 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> ... " Hm? Brian > And we're actually going to need to be able to do that in order to do > configurationless autotiering - i.e. tracking how hot/cold data is on an > inode:offset basis, because LRU btree backreferences need to go in the > key (bpos), not the value, in order to avoid collisions, and bpos isn't > big enough for that. > > disk_accounting_(key|pos) is an even trickier situation, because of > endianness issues. The trick we do with bpos of defining the field order > differently based on endianness so that byte order matches word order - > that really wouldn't work here, so there is at present no practical way > that I know of to avoid the byte swabbing when going back and forth > between bpos and disk_accounting_pos on big endian. > > But gcc does have an attribute now that lets you specify that an integer > struct member is big or little endian... I if we could get them to go > one step further and give us an attribute to control whether members are > laid out in ascending or descending order... >
