On Mon, Sep 5, 2022 at 1:28 PM Thomas Munro <thomas.mu...@gmail.com> wrote: > I had this more or less figured out on Friday when I wrote last, but I > got stuck on a weird problem with 026_overwrite_contrecord.pl. I > think that failure case should report an error, no? I find it strange > that we end recovery in silence. That was a problem for the new > coding in this patch, because it is confused by XLREAD_FAIL without > queuing an error, and then retries, which clobbers the aborted recptr > state. I'm still looking into that.
On reflection, it'd be better not to clobber any pre-existing error there, but report one only if there isn't one already queued. I've done that in this version, which I'm planning to do a bit more testing on and commit soonish if there are no comments/objections, especially for that part. I'll have to check whether a doc change is necessary somewhere to advertise that maintenance_io_concurrency=0 turns off prefetching, but IIRC that's kinda already implied. I've tested quite a lot of scenarios including make check-world with maintenance_io_concurrency = 0, 1, 10, 1000, and ALTER SYSTEM for all relevant GUCs on a standby running large pgbench to check expected effect on pg_stat_recovery_prefetch view and generate system calls.
From 956d0613ecad68ba694b8a492895aa02002dbed5 Mon Sep 17 00:00:00 2001 From: Thomas Munro <thomas.mu...@gmail.com> Date: Mon, 5 Sep 2022 12:07:24 +1200 Subject: [PATCH v2] Fix recovery_prefetch with low maintenance_io_concurrency. The LSN-based logic for knowing when IOs complete ran operations in the wrong order. We should process completed IOs first before trying to start more, so that it is always possible to decode one more record when the decoded record queue is empty, even if maintenance_io_concurrency is set so low that a single earlier WAL record might have saturated the IO queue. That thinko was hidden because the effect of maintenance_io_concurrency was arbitrarily clamped to be at least 2. Fix the ordering, and also remove that clamp. We need a special case for 0, which is now treated the same as recovery_prefetch=off, but otherwise the number is used directly. This allows for testing with 1, which would have made the problem obvious in simple test scenarios. Back-patch to 15. Reported-by: Justin Pryzby <pry...@telsasoft.com> Discussion: https://postgr.es/m/20220831140128.GS31833%40telsasoft.com --- src/backend/access/transam/xlogprefetcher.c | 58 ++++++++++++++------- src/backend/access/transam/xlogreader.c | 12 +++++ 2 files changed, 52 insertions(+), 18 deletions(-) diff --git a/src/backend/access/transam/xlogprefetcher.c b/src/backend/access/transam/xlogprefetcher.c index 9aa56411d5..33491d7325 100644 --- a/src/backend/access/transam/xlogprefetcher.c +++ b/src/backend/access/transam/xlogprefetcher.c @@ -72,7 +72,9 @@ int recovery_prefetch = RECOVERY_PREFETCH_TRY; #ifdef USE_PREFETCH -#define RecoveryPrefetchEnabled() (recovery_prefetch != RECOVERY_PREFETCH_OFF) +#define RecoveryPrefetchEnabled() \ + (recovery_prefetch != RECOVERY_PREFETCH_OFF && \ + maintenance_io_concurrency > 0) #else #define RecoveryPrefetchEnabled() false #endif @@ -1000,7 +1002,8 @@ XLogPrefetcherReadRecord(XLogPrefetcher *prefetcher, char **errmsg) if (RecoveryPrefetchEnabled()) { - max_inflight = Max(maintenance_io_concurrency, 2); + Assert(maintenance_io_concurrency > 0); + max_inflight = maintenance_io_concurrency; max_distance = max_inflight * XLOGPREFETCHER_DISTANCE_MULTIPLIER; } else @@ -1018,14 +1021,39 @@ XLogPrefetcherReadRecord(XLogPrefetcher *prefetcher, char **errmsg) } /* - * Release last returned record, if there is one. We need to do this so - * that we can check for empty decode queue accurately. + * Release last returned record, if there is one, and perform tasks that + * are we can do now that the caller has replayed it. */ - XLogReleasePreviousRecord(prefetcher->reader); + if (likely(record = prefetcher->reader->record)) + { + XLogRecPtr replayed_up_to = record->next_lsn; + + XLogReleasePreviousRecord(prefetcher->reader); + + /* + * Can we drop any filters yet? If we were waiting for a relation to + * be created or extended, it is now OK to access blocks in the + * covered range. + */ + XLogPrefetcherCompleteFilters(prefetcher, replayed_up_to); + + /* + * All IO initiated by earlier WAL is now completed. This might + * trigger further prefetching. + */ + lrq_complete_lsn(prefetcher->streaming_read, replayed_up_to); + } - /* If there's nothing queued yet, then start prefetching. */ + /* + * If there's nothing queued yet, then start prefetching to cause at least + * one record to be queued. + */ if (!XLogReaderHasQueuedRecordOrError(prefetcher->reader)) + { + Assert(lrq_inflight(prefetcher->streaming_read) == 0); + Assert(lrq_completed(prefetcher->streaming_read) == 0); lrq_prefetch(prefetcher->streaming_read); + } /* Read the next record. */ record = XLogNextRecord(prefetcher->reader, errmsg); @@ -1039,12 +1067,13 @@ XLogPrefetcherReadRecord(XLogPrefetcher *prefetcher, char **errmsg) Assert(record == prefetcher->reader->record); /* - * Can we drop any prefetch filters yet, given the record we're about to - * return? This assumes that any records with earlier LSNs have been - * replayed, so if we were waiting for a relation to be created or - * extended, it is now OK to access blocks in the covered range. + * If maintenance_io_concurrency is set very low, we might have started + * prefetching some but not all of the blocks referenced in the record + * we're about to return. Forget about the rest of the blocks in this + * record by dropping the prefetcher's reference to it. */ - XLogPrefetcherCompleteFilters(prefetcher, record->lsn); + if (record == prefetcher->record) + prefetcher->record = NULL; /* * See if it's time to compute some statistics, because enough WAL has @@ -1053,13 +1082,6 @@ XLogPrefetcherReadRecord(XLogPrefetcher *prefetcher, char **errmsg) if (unlikely(record->lsn >= prefetcher->next_stats_shm_lsn)) XLogPrefetcherComputeStats(prefetcher); - /* - * The caller is about to replay this record, so we can now report that - * all IO initiated because of early WAL must be finished. This may - * trigger more readahead. - */ - lrq_complete_lsn(prefetcher->streaming_read, record->lsn); - Assert(record == prefetcher->reader->record); return &record->header; diff --git a/src/backend/access/transam/xlogreader.c b/src/backend/access/transam/xlogreader.c index cdcacc7803..1fefaf2826 100644 --- a/src/backend/access/transam/xlogreader.c +++ b/src/backend/access/transam/xlogreader.c @@ -907,6 +907,18 @@ err: */ state->abortedRecPtr = RecPtr; state->missingContrecPtr = targetPagePtr; + + /* + * If we got here without reporting an error, report one now so that + * XLogPrefetcherReadRecord() doesn't bring us back a second time and + * clobber the above state. Otherwise, the existing error takes + * precedence. + */ + if (!state->errormsg_deferred) + report_invalid_record(state, + "aborted record pointer at %X/%X: missing contrecord at %X/%X", + LSN_FORMAT_ARGS(state->abortedRecPtr), + LSN_FORMAT_ARGS(state->missingContrecPtr)); } if (decoded && decoded->oversized) -- 2.35.1
