The strategy is:
- Periodical flushing of inmemory blocks to files, so they aren't lost on unclean shutdown.
- Periodical syncing of metadata for persisted queues, so the metadata remains in sync with the persisted data.
- Automatic adjusting of too big chunk size when opening the queue. The chunk size may be bigger than the writer offset after unclean shutdown.
- Skipping of broken chunk file if it cannot be read.
- Fsyncing finalized chunk files.
Updates https://github.com/VictoriaMetrics/VictoriaMetrics/issues/687
Metric `vm_persistentqueue_bytes_pending` is a gauge that shows current amount
of bytes in persistentqueue flushed on disk as a difference between write and read
offsets. This metric is very similar to `vmagent_remotewrite_pending_data_bytes`
except of accounting for bytes in-memory.
