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PostgreSQL Reference Manual - Volume 3 - Server Administration Guide
by The PostgreSQL Global Development Group
Paperback (6"x9"), 204 pages
ISBN 0954612043
RRP £13.95 ($24.95)

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9.1.3 Preventing transaction ID wraparound failures

PostgreSQL's MVCC transaction semantics depend on being able to compare transaction ID (XID) numbers: a row version with an insertion XID greater than the current transaction's XID is “in the future” and should not be visible to the current transaction. But since transaction IDs have limited size (32 bits at this writing) a cluster that runs for a long time (more than 4 billion transactions) would suffer transaction ID wraparound: the XID counter wraps around to zero, and all of a sudden transactions that were in the past appear to be in the future--which means their outputs become invisible. In short, catastrophic data loss. (Actually the data is still there, but that's cold comfort if you can't get at it.) To avoid this, it is necessary to vacuum every table in every database at least once every two billion transactions.

The reason that periodic vacuuming solves the problem is that PostgreSQL distinguishes a special XID FrozenXID. This XID is always considered older than every normal XID. Normal XIDs are compared using modulo-231 arithmetic. This means that for every normal XID, there are two billion XIDs that are “older” and two billion that are “newer”; another way to say it is that the normal XID space is circular with no endpoint. Therefore, once a row version has been created with a particular normal XID, the row version will appear to be “in the past” for the next two billion transactions, no matter which normal XID we are talking about. If the row version still exists after more than two billion transactions, it will suddenly appear to be in the future. To prevent data loss, old row versions must be reassigned the XID FrozenXID sometime before they reach the two-billion-transactions-old mark. Once they are assigned this special XID, they will appear to be “in the past” to all normal transactions regardless of wraparound issues, and so such row versions will be good until deleted, no matter how long that is. This reassignment of old XIDs is handled by VACUUM.

VACUUM's behavior is controlled by the configuration parameter vacuum_freeze_min_age: any XID older than vacuum_freeze_min_age transactions is replaced by FrozenXID. Larger values of vacuum_freeze_min_age preserve transactional information longer, while smaller values increase the number of transactions that can elapse before the table must be vacuumed again.

The maximum time that a table can go unvacuumed is two billion transactions minus the vacuum_freeze_min_age that was used when it was last vacuumed. If it were to go unvacuumed for longer than that, data loss could result. To ensure that this does not happen, the autovacuum facility described in section 9.1.4 The auto-vacuum daemon is invoked on any table that might contain XIDs older than the age specified by the configuration parameter autovacuum_freeze_max_age. (This will happen even if autovacuum is otherwise disabled.)

This implies that if a table is not otherwise vacuumed, autovacuum will be invoked on it approximately once every autovacuum_freeze_max_age minus vacuum_freeze_min_age transactions. For tables that are regularly vacuumed for space reclamation purposes, this is of little importance. However, for static tables (including tables that receive inserts, but no updates or deletes), there is no need for vacuuming for space reclamation, and so it can be useful to try to maximize the interval between forced autovacuums on very large static tables. Obviously one can do this either by increasing autovacuum_freeze_max_age or by decreasing vacuum_freeze_min_age.

The sole disadvantage of increasing autovacuum_freeze_max_age is that the ‘pg_clog’ subdirectory of the database cluster will take more space, because it must store the commit status for all transactions back to the autovacuum_freeze_max_age horizon. The commit status uses two bits per transaction, so if autovacuum_freeze_max_age has its maximum allowed value of a little less than two billion, ‘pg_clog’ can be expected to grow to about half a gigabyte. If this is trivial compared to your total database size, setting autovacuum_freeze_max_age to its maximum allowed value is recommended. Otherwise, set it depending on what you are willing to allow for ‘pg_clog’ storage. (The default, 200 million transactions, translates to about 50MB of ‘pg_clog’ storage.)

One disadvantage of decreasing vacuum_freeze_min_age is that it may cause VACUUM to do useless work: changing a table row's XID to FrozenXID is a waste of time if the row is modified soon thereafter (causing it to acquire a new XID). So the setting should be large enough that rows are not frozen until they are unlikely to change any more. Another disadvantage of decreasing this setting is that details about exactly which transaction inserted or modified a row will be lost sooner. This information sometimes comes in handy, particularly when trying to analyze what went wrong after a database failure. For these two reasons, decreasing this setting is not recommended except for completely static tables.

To track the age of the oldest XIDs in a database, VACUUM stores XID statistics in the system tables pg_class and pg_database. In particular, the relfrozenxid column of a table's pg_class row contains the freeze cutoff XID that was used by the last VACUUM for that table. All normal XIDs older than this cutoff XID are guaranteed to have been replaced by FrozenXID within the table. Similarly, the datfrozenxid column of a database's pg_database row is a lower bound on the normal XIDs appearing in that database--it is just the minimum of the per-table relfrozenxid values within the database. A convenient way to examine this information is to execute queries such as

SELECT relname, age(relfrozenxid) FROM pg_class WHERE relkind
 = 'r';
SELECT datname, age(datfrozenxid) FROM pg_database;

The age column measures the number of transactions from the cutoff XID to the current transaction's XID. Immediately after a VACUUM, age(relfrozenxid) should be a little more than the vacuum_freeze_min_age setting that was used (more by the number of transactions started since the VACUUM started). If age(relfrozenxid) exceeds autovacuum_freeze_max_age, an autovacuum will soon be forced for the table.

If for some reason autovacuum fails to clear old XIDs from a table, the system will begin to emit warning messages like this when the database's oldest XIDs reach ten million transactions from the wraparound point:

WARNING:  database "mydb" must be vacuumed within 177009986
 transactions
HINT:  To avoid a database shutdown, execute a full-database
 VACUUM in "mydb".

If these warnings are ignored, the system will shut down and refuse to execute any new transactions once there are fewer than 1 million transactions left until wraparound:

ERROR:  database is shut down to avoid wraparound data loss
 in database "mydb"
HINT:  Stop the postmaster and use a standalone backend to
 VACUUM in "mydb".

The 1-million-transaction safety margin exists to let the administrator recover without data loss, by manually executing the required VACUUM commands. However, since the system will not execute commands once it has gone into the safety shutdown mode, the only way to do this is to stop the server and use a single-user backend to execute VACUUM. The shutdown mode is not enforced by a single-user backend. See the postgres reference page for details about using a single-user backend.

ISBN 0954612043PostgreSQL Reference Manual - Volume 3 - Server Administration GuideSee the print edition