Summary
BasebackupSync is a PostgreSQL IO wait event. The official server documentation describes it as: “Waiting for data written by a base backup to reach durable storage.” (verbatim from PostgreSQL’s wait_event_names.txt catalog).
Classification
- wait_event:
BasebackupSync - wait_event_type:
IO - Internal enum:
WAIT_EVENT_BASEBACKUP_SYNC - Reported in:
pg_stat_activity
What the IO class indicates
(Paraphrased explanation.) The process is waiting for an I/O operation (read, write, or fsync) to complete. Sustained IO waits often indicate storage pressure or cold cache.
How to observe it
(Illustrative query — not from the catalog.) You can see which sessions are currently reporting this wait event in the cumulative statistics view:
SELECT pid, state, wait_event_type, wait_event, query
FROM pg_stat_activity
WHERE wait_event_type = 'IO'
AND wait_event = 'BasebackupSync';Version applicability
Confirmed present in PostgreSQL major version(s): 15, 16, 17, 18, 19 (verified against each release’s server source).
This wait event has existed since at least PostgreSQL 15.
Naming note: in PostgreSQL 16 and earlier this event was reported as BaseBackupSync. PostgreSQL 17 normalized wait-event spellings (via the generated wait_event_names.txt), so it now appears as BasebackupSync.
The machine-readable wait_event_names.txt catalog exists from PostgreSQL 17 onward; presence in PostgreSQL 12–16 was verified directly from the wait-event, lock, and lightweight-lock definitions in those releases’ source code.
References
- PostgreSQL documentation — Wait Events
- PostgreSQL source —
src/backend/utils/activity/wait_event_names.txt