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GitHub release

PTRACK

Overview

PTRACK allows speed up incremental backups for the huge PostgreSQL databases. PTRACK store changes of physical blocks in the memory. You can effectively use PTRACK engine for taking incremental backups by pg_probackup.

Current patch are available for PostgreSQL 11, 12, 13, 14, 15

Enterprise edition

Enterprise PTRACK are part of Postgres Pro Enterprise and offers the capability to track more than 100 000 tables and indexes concurrently without any degradation in speed CFS (compressed file system). According to benchmarks, it operated up to 5 times faster and useful for ERP and DWH with huge amount of tables and relations between them.

Installation

  1. Get latest PTRACK sources:
git clone https://github.com/postgrespro/ptrack.git
  1. Get latest PostgreSQL sources:
git clone https://github.com/postgres/postgres.git -b REL_14_STABLE && cd postgres
  1. Apply PostgreSQL core patch:
git apply -3 ../ptrack/patches/REL_14_STABLE-ptrack-core.diff

  1. Compile and install PostgreSQL

  2. Set ptrack.map_size (in MB)

echo "shared_preload_libraries = 'ptrack'" >> postgres_data/postgresql.conf
echo "ptrack.map_size = 64" >> postgres_data/postgresql.conf
  1. Compile and install PTRACK extension
USE_PGXS=1 make -C /path/to/ptrack/ install
  1. Run PostgreSQL and create PTRACK extension
postgres=# CREATE EXTENSION ptrack;

Configuration

The only one configurable option is ptrack.map_size (in MB). Default is 0, which means PTRACK is turned off. In order to reduce number of false positives it is recommended to set ptrack.map_size to 1 / 1000 of expected PGDATA size (i.e. 1000 for a 1 TB database).

To disable PTRACK and clean up all remaining service files set ptrack.map_size to 0.

Public SQL API

  • ptrack_version() — returns ptrack version string.
  • ptrack_init_lsn() — returns LSN of the last ptrack map initialization.
  • ptrack_get_pagemapset(start_lsn pg_lsn) — returns a set of changed data files with a number of changed blocks and their bitmaps since specified start_lsn.
  • ptrack_get_change_stat(start_lsn pg_lsn) — returns statistic of changes (number of files, pages and size in MB) since specified start_lsn.

Usage example:

postgres=# SELECT ptrack_version();
 ptrack_version 
----------------
 2.4
(1 row)

postgres=# SELECT ptrack_init_lsn();
 ptrack_init_lsn 
-----------------
 0/1814408
(1 row)

postgres=# SELECT * FROM ptrack_get_pagemapset('0/185C8C0');
        path         | pagecount |                pagemap                 
---------------------+-----------+----------------------------------------
 base/16384/1255     |         3 | \x001000000005000000000000
 base/16384/2674     |         3 | \x0000000900010000000000000000
 base/16384/2691     |         1 | \x00004000000000000000000000
 base/16384/2608     |         1 | \x000000000000000400000000000000000000
 base/16384/2690     |         1 | \x000400000000000000000000
(5 rows)

postgres=# SELECT * FROM ptrack_get_change_stat('0/285C8C8');
 files | pages |        size, MB        
-------+-------+------------------------
    20 |    25 | 0.19531250000000000000
(1 row)

Upgrading

Usually, you have to only install new version of PTRACK and do ALTER EXTENSION 'ptrack' UPDATE;. However, some specific actions may be required as well:

Upgrading from 2.0.0 to 2.1.*:

  • Put shared_preload_libraries = 'ptrack' into postgresql.conf.
  • Rename ptrack_map_size to ptrack.map_size.
  • Do ALTER EXTENSION 'ptrack' UPDATE;.
  • Restart your server.

Upgrading from 2.1.* to 2.2.*:

Since version 2.2 we use a different algorithm for tracking changed pages. Thus, data recorded in the ptrack.map using pre 2.2 versions of PTRACK is incompatible with newer versions. After extension upgrade and server restart old ptrack.map will be discarded with WARNING and initialized from the scratch.

Upgrading from 2.2.* to 2.3.*:

  • Stop your server
  • Update ptrack binaries
  • Remove global/ptrack.map.mmap if it exist in server data directory
  • Start server
  • Do ALTER EXTENSION 'ptrack' UPDATE;.

Upgrading from 2.3.* to 2.4.*:

  • Stop your server
  • Update PTRACK binaries
  • Start server
  • Do ALTER EXTENSION 'ptrack' UPDATE;.

Limitations

  1. You can only use PTRACK safely with wal_level >= 'replica'. Otherwise, you can lose tracking of some changes if crash-recovery occurs, since certain commands are designed not to write WAL at all if wal_level is minimal, but we only durably flush PTRACK map at checkpoint time.

  2. The only one production-ready backup utility, that fully supports PTRACK is pg_probackup.

  3. You cannot resize PTRACK map in runtime, only on postmaster start. Also, you will lose all tracked changes, so it is recommended to do so in the maintainance window and accompany this operation with full backup.

  4. You will need up to ptrack.map_size * 2 of additional disk space, since PTRACK uses additional temporary file for durability purpose. See Architecture section for details.

Benchmarks

Briefly, an overhead of using PTRACK on TPS usually does not exceed a couple of percent (~1-3%) for a database of dozens to hundreds of gigabytes in size, while the backup time scales down linearly with backup size with a coefficient ~1. It means that an incremental PTRACK backup of a database with only 20% of changed pages will be 5 times faster than a full backup. More details here.

Architecture

It is designed to permit false positives (i.e., block/page is marked as altered in the PTRACK map when it hasn't actually been changed), but it never tolerates false negatives (i.e., it never loses any PGDATA modifications, barring hint-bits).

At present, the PTRACK codebase is divided between a small PostgreSQL core patch and an extension. The public SQL API methods and the main engine are housed in the PTRACK extension, whereas the core patch only includes specific hooks and modifies binary utilities to disregard ptrack.map.* files.

In PTRACK, we use a single shared hash table. Due to the fixed size of the map, there can be false positives (when a block is marked as changed without actual modification), but false negatives are not allowed. Nevertheless, these false positives can be completely removed by setting a sufficiently high ptrack.map_size.

All reads/writes are performed using atomic operations on uint64 entries, making the map completely lockless during standard PostgreSQL operation. Since we do not utilize locks for read/write access, PTRACK maintains a map (ptrack.map) from the last checkpoint unaltered and uses a maximum of one additional temporary file.

  • temporary file ptrack.map.tmp to durably replace ptrack.map during checkpoint.

Map is written on disk at the end of checkpoint atomically block by block involving the CRC32 checksum calculation that is checked on the next whole map re-read after crash-recovery or restart.

To gather the whole changeset of modified blocks in ptrack_get_pagemapset() we walk the entire PGDATA (base/**/*, global/*, pg_tblspc/**/*) and verify using map whether each block of each relation was modified since the specified LSN or not.

Contribution

Feel free to send pull requests, fill up issues. See also the list of authors who participated in this project.

Tests

Everything is tested automatically with travis-ci.com and codecov.io, but you can also run tests locally via Docker:

export PG_BRANCH=REL_14_STABLE
export TEST_CASE=all
export MODE=paranoia

./make_dockerfile.sh

docker-compose build
docker-compose run tests

Available test modes (MODE) are basic (default) and paranoia (per-block checksum comparison of PGDATA content before and after backup-restore process). Available test cases (TEST_CASE) are tap (minimalistic PostgreSQL tap test), all or any specific pg_probackup test, e.g. test_ptrack_simple.

Acknowledgments

PTRACK development is supported by Postgres Professional You can ask any question about contributing or usage in Russian chat or Internation chat of pg_probackup.