Re: BUG #17257: (auto)vacuum hangs within lazy_scan_prune() - Mailing list pgsql-bugs
From | Noah Misch |
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Subject | Re: BUG #17257: (auto)vacuum hangs within lazy_scan_prune() |
Date | |
Msg-id | 20240108182125.f8.nmisch@google.com Whole thread Raw |
In response to | Re: BUG #17257: (auto)vacuum hangs within lazy_scan_prune() (Peter Geoghegan <pg@bowt.ie>) |
Responses |
Re: BUG #17257: (auto)vacuum hangs within lazy_scan_prune()
(Peter Geoghegan <pg@bowt.ie>)
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List | pgsql-bugs |
On Mon, Jan 08, 2024 at 12:02:01PM -0500, Peter Geoghegan wrote: > On Sat, Jan 6, 2024 at 5:44 PM Noah Misch <noah@leadboat.com> wrote: > > Tied to that decision is the choice of semantics when the xmin horizon moves > > backward during one VACUUM, e.g. when a new walsender xmin does so. Options: > > > > 1. Continue to remove tuples based on the OldestXmin from VACUUM's start. We > > could have already removed some of those tuples, so the walsender xmin > > won't achieve a guarantee anyway. (VACUUM would want ratchet-like behavior > > in GlobalVisState, possibly by sharing OldestXmin with pruneheap like you > > say.) > > > > 2. Move OldestXmin backward, to reflect the latest xmin horizon. (Perhaps > > VACUUM would just pass GlobalVisState to a function that returns the > > compatible OldestXmin.) > > > > Which way is better? > > I suppose that a hybrid of these two approaches makes the most sense. > A design that's a lot closer to your #1 than to your #2. > > Under this scheme, pruneheap.c would be explicitly aware of > OldestXmin, and would promise to respect the exact invariant that we > need to avoid getting stuck in lazy_scan_prune's loop (or avoid > confused NewRelfrozenXid tracking on HEAD, which no longer has this > loop). But it'd be limited to that exact invariant; we'd still avoid > unduly imposing any requirements on pruning-away deleted tuples whose > xmax was >= OldestXmin. lazy_scan_prune/vacuumlazy.c shouldn't care if > we prune away any "extra" heap tuples, just because we can (or just > because it's convenient to the implementation). Andres has in the past > placed a lot of emphasis on being able to update the > GlobalVisState-wise bounds on the fly. Not sure that it's really that > important that VACUUM does that, but there is no reason to not allow > it. So we can keep that property (as well as the aforementioned > high-level OldestXmin immutability property). > > More importantly (at least to me), this scheme allows vacuumlazy.c to > continue to treat OldestXmin as an immutable cutoff for both pruning > and freezing -- the high level design doesn't need any revisions. We > already "freeze away" multixact member XIDs >= OldestXmin in certain > rare cases (i.e. we remove lockers that are determined to no longer be > running in FreezeMultiXactId's "second pass" slow path), so there is > nothing fundamentally novel about the idea of removing some extra XIDs > >= OldestXmin in passing, just because it happens to be convenient to > some low-level piece of code that's external to vacuumlazy.c. > > What do you think of that general approach? That all sounds good to me. > I see no reason why it > matters if OldestXmin goes backwards across two VACUUM operations, so > I haven't tried to avoid that. That may be fully okay, or we may want to clamp OldestXmin to be no older than relfrozenxid. I don't feel great about the system moving relfrozenxid backward unless it observed an older XID, and observing an older XID would be a corruption signal. I don't have a specific way non-monotonic relfrozenxid breaks things, though.
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