Backup Process and Locks

Backup Process and Locks

During backups, the gpcrondump utility locks the pg_class system table and the tables that are backed up. Locking the pg_class table with an EXCLUSIVE lock ensures that no tables are added, deleted, or altered until gpcrondump locks tables that are to be backed up with ACCESS SHARE locks.

The steps below describe the process gpcrondump follows to dump databases, including what happens before locks are placed, while locks are held, and after locks are removed.

If more than one database is specified, this process is executed for each database in sequence.

  • gpcrondump parses the command-line arguments and verifies that options and arguments are properly specified.
  • If any of the following filter options are specified, gpcrondump prepares filters to determine the set of tables to back up. Otherwise, all tables are included in the backup.
    • -s schema_name – Includes all the tables qualified by the specified schema.
    • -S schema_name – Excludes all tables qualified by the specified schema.
    • --schema-file=filename – Includes all the tables qualified by the schema listed in filename.
    • --exclude-schema-file=filename – Excludes all tables qualified by the schema listed in filename.
    • -t schema.table_name – Dumps the specified table.
    • -T schema.table_name – Excludes the specified table.
    • --table-file=filename – Dumps the tables specified in filename.
    • --exclude-table-file=filename – Dumps all tables except tables specified in filename.
  • gpcrondump verifies the backup targets:
    • Verifies that the database exists.
    • Verifies that specified tables or schemas to be dumped exist.
    • Verifies that the current primary for each segment is up.
    • Verifies that the backup directory exists and is writable for the master and each segment.
    • Verifies that sufficient disk space is available to back up each segment. Note that if more than one segment is backed up to the same disk volume, this disk space check cannot ensure there is space available for all segments.
  • Places an EXCLUSIVE lock on the catalog table pg_class. The lock permits only concurrent read operations on a table. While the lock is on the catalog table, relations such as tables, indexes, and views cannot be created, altered, or dropped in the database.

    gpcrondump starts a thread to watch for a lock file (dump_dir/gp_lockfile_timestamp) to appear, signaling the parallel backup on the segments has completed.

  • gpcrondump locks tables that are to be backed up with an ACCESS SHARE lock.
    An ACCESS SHARE lock only conflicts with an ACCESS EXCLUSIVE lock. The following SQL statements acquire an ACCESS EXCLUSIVE lock:
    • ALTER TABLE
    • CLUSTER
    • DROP TABLE
    • REINDEX
    • TRUNCATE
    • VACUUM FULL
  • Threads are created and dispatched for the master and segments to perform the database dump.
  • When the threads have acquired ACCESS SHARED locks on all the required tables, the dump_dir/gp_lockfile_timestamp lock file is created, signaling gpcrondump to release the EXCLUSIVE lock on the pg_class catalog table, while tables are being backed up.
  • gpcrondump checks the status files for each primary segment for any errors to report. If a dump fails and the -r flag was specified, the backup is rolled back.
  • The ACCESS SHARE locks on the target tables are released.
  • If the backup succeeded and a post-dump script was specified with the -R option, gpcrondump runs the script now.
  • gpcrondump reports the backup status, sends email if configured, and saves the backup status in the public.gpcrondump_history table in the database.