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Pbckp 690 update readme #38

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[![Build Status](https://travis-ci.com/postgrespro/ptrack.svg?branch=master)](https://travis-ci.com/postgrespro/ptrack)
[![codecov](https://codecov.io/gh/postgrespro/ptrack/branch/master/graph/badge.svg)](https://codecov.io/gh/postgrespro/ptrack)
[![GitHub release](https://img.shields.io/github/v/release/postgrespro/ptrack?include_prereleases)](https://github.com/postgrespro/ptrack/releases/latest)

# ptrack
## PTRACK allows speed up incremental backups for the huge PostgreSQL databases.

## Overview
## Overview

Ptrack is a block-level incremental backup engine for PostgreSQL. You can [effectively use](https://postgrespro.github.io/pg_probackup/#pbk-setting-up-ptrack-backups) `ptrack` engine for taking incremental backups with [pg_probackup](https://github.com/postgrespro/pg_probackup) backup and recovery manager for PostgreSQL.
PTRACK saves changes of physical blocks in the memory. You can [effectively use](https://postgrespro.github.io/pg_probackup/#pbk-setting-up-ptrack-backups) `PTRACK` engine for taking incremental backups by [pg_probackup](https://github.com/postgrespro/pg_probackup).

It is designed to allow false positives (i.e. block/page is marked in the `ptrack` map, but actually has not been changed), but to never allow false negatives (i.e. loosing any `PGDATA` changes, excepting hint-bits).
Current patch are available for [11](https://github.com/postgrespro/ptrack/blob/master/patches/REL_11_STABLE-ptrack-core.diff), [12](https://github.com/postgrespro/ptrack/blob/master/patches/REL_12_STABLE-ptrack-core.diff), [13](https://github.com/postgrespro/ptrack/blob/master/patches/REL_13_STABLE-ptrack-core.diff), [14](https://github.com/postgrespro/ptrack/blob/master/patches/REL_14_STABLE-ptrack-core.diff), [15](https://github.com/postgrespro/ptrack/blob/master/patches/REL_15_STABLE-ptrack-core.diff)
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@demonolock demonolock Aug 3, 2023 •
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for PostgreSQL


Currently, `ptrack` codebase is split between small PostgreSQL core patch and extension. All public SQL API methods and main engine are placed in the `ptrack` extension, while the core patch contains only certain hooks and modifies binary utilities to ignore `ptrack.map.*` files.
## Enterprise edition

This extension is compatible with PostgreSQL [11](https://github.com/postgrespro/ptrack/blob/master/patches/REL_11_STABLE-ptrack-core.diff), [12](https://github.com/postgrespro/ptrack/blob/master/patches/REL_12_STABLE-ptrack-core.diff), [13](https://github.com/postgrespro/ptrack/blob/master/patches/REL_13_STABLE-ptrack-core.diff), [14](https://github.com/postgrespro/ptrack/blob/master/patches/REL_14_STABLE-ptrack-core.diff).
Enterprise PTRACK are part of [Postgres Pro Backup Enterprise](https://postgrespro.ru/products/postgrespro/enterprise) and share posibility to track more than 100 000 tables and indexes per time without speed degradation with [CFS (compressed file system)](https://postgrespro.ru/docs/enterprise/15/cfs).
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and shares

Benchmarks are x5 time faster and useful for ERP and DWH with huge amounth of tables and relations between them.

## Installation

1) Get latest `ptrack` sources:
1) Get latest `PTRACK` sources:

```shell
git clone https://github.com/postgrespro/ptrack.git
Expand All @@ -43,23 +42,23 @@ echo "shared_preload_libraries = 'ptrack'" >> postgres_data/postgresql.conf
echo "ptrack.map_size = 64" >> postgres_data/postgresql.conf
```

6) Compile and install `ptrack` extension
6) Compile and install `PTRACK` extension

```shell
USE_PGXS=1 make -C /path/to/ptrack/ install
```

7) Run PostgreSQL and create `ptrack` extension
7) Run PostgreSQL and create `PTRACK` extension

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

## Public SQL API

Expand Down Expand Up @@ -102,7 +101,7 @@ postgres=# SELECT * FROM ptrack_get_change_stat('0/285C8C8');

## 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:
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.*:

Expand All @@ -113,7 +112,7 @@ Usually, you have to only install new version of `ptrack` and do `ALTER EXTENSIO

#### 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.
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.*:

Expand All @@ -126,29 +125,33 @@ Since version 2.2 we use a different algorithm for tracking changed pages. Thus,
#### Upgrading from 2.3.* to 2.4.*:

* Stop your server
* Update ptrack binaries
* 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](https://www.postgresql.org/docs/12/populate.html#POPULATE-PITR), but we only durably flush `ptrack` map at checkpoint time.
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](https://www.postgresql.org/docs/12/populate.html#POPULATE-PITR), 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](https://github.com/postgrespro/pg_probackup).
2. The only one production-ready backup utility, that fully supports `PTRACK` is [pg_probackup](https://github.com/postgrespro/pg_probackup).

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


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](#Architecture) for details.
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](#Architecture) 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](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](benchmarks).

## Architecture

We use a single shared hash table in `ptrack`. Due to the fixed size of the map there may be false positives (when some block is marked as changed without being actually modified), but not false negative results. However, these false postives may be completely eliminated by setting a high enough `ptrack.map_size`.
It is designed to allow false positives (i.e. block/page is marked in the `PTRACK` map, but actually has not been changed), but to never allow false negatives (i.e. loosing any `PGDATA` changes, excepting hint-bits).

Currently, `PTRACK` codebase is split between small PostgreSQL core patch and extension. All public SQL API methods and main engine are placed in the `PTRACK` extension, while the core patch contains only certain hooks and modifies binary utilities to ignore `ptrack.map.*` files.

We use a single shared hash table in `PTRACK`. Due to the fixed size of the map there may be false positives (when some block is marked as changed without being actually modified), but not false negative results. However, these false postives may be completely eliminated by setting a high enough `ptrack.map_size`.

All reads/writes are made using atomic operations on `uint64` entries, so the map is completely lockless during the normal PostgreSQL operation. Because we do not use locks for read/write access, `ptrack` keeps a map (`ptrack.map`) since the last checkpoint intact and uses up to 1 additional temporary file:
All reads/writes are made using atomic operations on `uint64` entries, so the map is completely lockless during the normal PostgreSQL operation. Because we do not use locks for read/write access, `PTRACK` keeps a map (`ptrack.map`) since the last checkpoint intact and uses up to 1 additional temporary file:

* temporary file `ptrack.map.tmp` to durably replace `ptrack.map` during checkpoint.

Expand All @@ -158,7 +161,7 @@ To gather the whole changeset of modified blocks in `ptrack_get_pagemapset()` we

## Contribution

Feel free to [send pull requests](https://github.com/postgrespro/ptrack/compare), [fill up issues](https://github.com/postgrespro/ptrack/issues/new), or just reach one of us directly (e.g. <[Alexey Kondratov](mailto:[email protected]?subject=[GitHub]%20Ptrack), [@ololobus](https://github.com/ololobus)>) if you are interested in `ptrack`.
Feel free to [send pull requests](https://github.com/postgrespro/ptrack/compare), [fill up issues](https://github.com/postgrespro/ptrack/issues/new), or just reach one of us directly (e.g. <[Alexey Kondratov](mailto:[email protected]?subject=[GitHub]%20Ptrack), [@ololobus](https://github.com/ololobus)>) if you are interested in `PTRACK`.

### Tests

Expand All @@ -176,9 +179,3 @@ 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](https://github.com/postgrespro/ptrack/blob/master/t/001_basic.pl)), `all` or any specific [pg_probackup test](https://github.com/postgrespro/pg_probackup/blob/master/tests/ptrack.py), e.g. `test_ptrack_simple`.

### TODO

* Should we introduce `ptrack.map_path` to allow `ptrack` service files storage outside of `PGDATA`? Doing that we will avoid patching PostgreSQL binary utilities to ignore `ptrack.map.*` files.
* Can we resize `ptrack` map on restart but keep the previously tracked changes?
* Can we write a formal proof, that we never loose any modified page with `ptrack`? With TLA+?