Re: [PATCH 04/21] bcachefs: Disk space accounting rewrite

[Brian Foster <bfoster@redhat.com>]

On Thu, Feb 29, 2024 at 04:16:00PM -0500, Kent Overstreet wrote:
> On Thu, Feb 29, 2024 at 01:44:27PM -0500, Brian Foster wrote:
> > On Wed, Feb 28, 2024 at 11:10:12PM -0500, Kent Overstreet wrote:
> > > I think it ended up not needing to be moved, and I just forgot to drop
> > > it - originally I disallowed accounting entries that referenced
> > > nonexistent devices, but that wasn't workable so now it's only nonzero
> > > accounting keys that aren't allowed to reference nonexistent devices.
> > > 
> > > I'll see if I can delete it.
> > > 
> > 
> > Do you mean to delete the change that moves the call, or the flush call
> > entirely?
> 
> Delte the change, I think there's further cleanup (& probably bugs to
> fix) possible with that flush call but I'm not going to get into it
> right now.
> 

Ok, just trying to determine whether I need to look back and make sure
this doesn't regress the problem this originally fixed.

> > > +/*
> > > + * Notes on disk accounting:
> > > + *
> > > + * We have two parallel sets of counters to be concerned with, and both must be
> > > + * kept in sync.
> > > + *
> > > + *  - Persistent/on disk accounting, stored in the accounting btree and updated
> > > + *    via btree write buffer updates that treat new accounting keys as deltas to
> > > + *    apply to existing values. But reading from a write buffer btree is
> > > + *    expensive, so we also have
> > > + *
> > 
> > I find the wording a little odd here, and I also think it would be
> > helpful to explain how/from where the deltas originate. For example,
> > something along the lines of:
> > 
> > "Persistent/on disk accounting, stored in the accounting btree and
> > updated via btree write buffer updates. Accounting updates are
> > represented as deltas that originate from <somewhere? trans triggers?>.
> > Accounting keys represent these deltas through commit into the write
> > buffer. The accounting/delta keys in the write buffer are then
> > accumulated into the appropriate accounting btree key at write buffer
> > flush time."
> 
> yeah, that's worth including.
> 
> There's an interesting point that you're touching on; btree write buffer
> are always dependent state changes from some other (non write buffer)
> btree; we never look at a write buffer btree and generate an update
> there - we can't, reading from a write buffer btree doesn't get you
> anything consistent or up to date.
> 
> So in normal operation it really only makes sense to do write buffer
> updates from a transactional trigger - that's the only way to use them
> and have them be consistent with the resst of the filesystem.
> 
> And since triggers work by comparing old and new, they naturally
> generate updates that are deltas.
> 

Hm that is interesting, I hadn't made that connection. Thanks.

Brian

> > > + *  - In memory accounting, where accounting is stored as an array of percpu
> > > + *    counters, indexed by an eytzinger array of disk acounting keys/bpos (which
> > > + *    are the same thing, excepting byte swabbing on big endian).
> > > + *
> > 
> > Not really sure about the keys vs. bpos thing, kind of related to my
> > comments on the earlier patch. It might be more clear to just elide the
> > implementation details here, i.e.:
> > 
> > "In memory accounting, where accounting is stored as an array of percpu
> > counters that are cheap to read, but not persistent. Updates to in
> > memory accounting are propagated from the transaction commit path."
> > 
> > ... but NBD, and feel free to reword, drop and/or correct any of that
> > text.
> 
> It's there because bch2_accounting_mem_read() takes a bpos when it
> should be a disk_accounting_key. I'll fix that if I can...
> 
> > > + *    Cheap to read, but non persistent.
> > > + *
> > > + * To do a disk accounting update:
> > > + * - initialize a disk_accounting_key, to specify which counter is being update
> > > + * - initialize counter deltas, as an array of 1-3 s64s
> > > + * - call bch2_disk_accounting_mod()
> > > + *
> > > + * This queues up the accounting update to be done at transaction commit time.
> > > + * Underneath, it's a normal btree write buffer update.
> > > + *
> > > + * The transaction commit path is responsible for propagating updates to the in
> > > + * memory counters, with bch2_accounting_mem_mod().
> > > + *
> > > + * The commit path also assigns every disk accounting update a unique version
> > > + * number, based on the journal sequence number and offset within that journal
> > > + * buffer; this is used by journal replay to determine which updates have been
> > > + * done.
> > > + *
> > > + * The transaction commit path also ensures that replicas entry accounting
> > > + * updates are properly marked in the superblock (so that we know whether we can
> > > + * mount without data being unavailable); it will update the superblock if
> > > + * bch2_accounting_mem_mod() tells it to.
> > 
> > I'm not really sure what this last paragraph is telling me, but granted
> > I've not got that far into the code yet either.
> 
> yeah that's for a whole different subsystem that happens to be slaved to
> the accounting - replicas.c, which also used to help out quite a bit
> with the accounting but now it's pretty much just for managing the
> superblock replicas section.
> 
> The superblock replicas section is just a list of entries, where each
> entry is a list of devices - "there is replicated data present on this
> set of devices". We also have full counters of how much data is present
> replicated across each set of devices, so the superblock section is just
> a truncated version of the accounting - "data exists on these devices",
> instead of saying how much.
>