diff options
| author | Andres Freund | 2017-05-06 23:47:40 +0000 |
|---|---|---|
| committer | Andres Freund | 2017-05-06 23:55:07 +0000 |
| commit | e6c44eef55cda493c759e926cecceb92186159b8 (patch) | |
| tree | 96f6da115893bd9bbd4d7c4e1def1337c4cecdd3 | |
| parent | 334b82cd56a65e09154d9f930d35a761a9c5cfab (diff) | |
Fix off-by-one possibly leading to skipped XLOG_RUNNING_XACTS records.
Since 6ef2eba3f57f1 ("Skip checkpoints, archiving on idle systems."),
GetLastImportantRecPtr() is used to avoid performing superfluous
checkpoints, xlog switches, running-xact records when the system is
idle. Unfortunately the check concerning running-xact records had a
off-by-one error, leading to such records being potentially skipped
when only a single record has been inserted since the last
running-xact record.
An alternative approach would have been to change
GetLastImportantRecPtr()'s definition to point to the end of records,
but that would make the checkpoint code more complicated.
Author: Andres Freund
Discussion: https://postgr.es/m/[email protected]
Backpatch: no, code only present in master
| -rw-r--r-- | src/backend/postmaster/bgwriter.c | 7 | ||||
| -rw-r--r-- | src/backend/postmaster/checkpointer.c | 3 |
2 files changed, 7 insertions, 3 deletions
diff --git a/src/backend/postmaster/bgwriter.c b/src/backend/postmaster/bgwriter.c index dcb4cf249c..48efe15e82 100644 --- a/src/backend/postmaster/bgwriter.c +++ b/src/backend/postmaster/bgwriter.c @@ -325,10 +325,13 @@ BackgroundWriterMain(void) /* * Only log if enough time has passed and interesting records have - * been inserted since the last snapshot. + * been inserted since the last snapshot. Have to compare with <= + * instead of < because GetLastImportantRecPtr() points at the + * start of a record, whereas last_snapshot_lsn points just past + * the end of the record. */ if (now >= timeout && - last_snapshot_lsn < GetLastImportantRecPtr()) + last_snapshot_lsn <= GetLastImportantRecPtr()) { last_snapshot_lsn = LogStandbySnapshot(); last_snapshot_ts = now; diff --git a/src/backend/postmaster/checkpointer.c b/src/backend/postmaster/checkpointer.c index fe9041f748..d12db0d5a7 100644 --- a/src/backend/postmaster/checkpointer.c +++ b/src/backend/postmaster/checkpointer.c @@ -611,7 +611,8 @@ CheckArchiveTimeout(void) { /* * Switch segment only when "important" WAL has been logged since the - * last segment switch. + * last segment switch (last_switch_lsn points to end of segment + * switch occurred in). */ if (GetLastImportantRecPtr() > last_switch_lsn) { |