From 347ceb27c7fcdc6c356aee3dffced55d3ee60e9d Mon Sep 17 00:00:00 2001 From: Alina Derkach Date: Wed, 30 Oct 2024 15:25:12 +0200 Subject: [PATCH] PXB-3254 Document reduction in locks modified: docs/lock-options.md new file: docs/reduction-in-locks.md modified: docs/xtrabackup-option-reference.md modified: mkdocs-base.yml --- docs/lock-options.md | 5 +- docs/reduction-in-locks.md | 98 +++++++++++++++++++++++++++++ docs/xtrabackup-option-reference.md | 15 +++-- mkdocs-base.yml | 2 +- 4 files changed, 113 insertions(+), 7 deletions(-) create mode 100644 docs/reduction-in-locks.md diff --git a/docs/lock-options.md b/docs/lock-options.md index 0e40e8d5e..7817857e1 100644 --- a/docs/lock-options.md +++ b/docs/lock-options.md @@ -1,5 +1,9 @@ # lock-ddl-per-table option improvements +!!! important + + The lock-ddl-per-table option is deprecated in Percona Server for MySQL 8.0. Use --lock-ddl instead of this variable. + To block DDL statements on an instance, Percona Server implemented LOCK TABLES FOR BACKUP. Percona XtraBackup uses this lock for the duration of the backup. This lock does not affect DML statements. @@ -60,4 +64,3 @@ The following improvements have been added: * A `SELECT` query that acquires an MDL does not retrieve any data. - diff --git a/docs/reduction-in-locks.md b/docs/reduction-in-locks.md new file mode 100644 index 000000000..83030b7f1 --- /dev/null +++ b/docs/reduction-in-locks.md @@ -0,0 +1,98 @@ +# Reduced backup lock time + +!!! important + + The `--lock-ddl=REDUCED` option is a [tech preview](./glossary.md#tech-preview). Before using this option in production, we recommend that you test restoring production from physical backups in your environment, and also use the alternative backup method for redundancy. + +Percona Server for MySQL implemented `LOCK TABLES FOR BACKUP` to block Data Definition Language (DDL) statements on a server. Percona XtraBackup utilizes this lock during the backup process to ensure that any `DDL` event does not corrupt the backup. The lock does not affect the Data Manipulation Language (DML) statements. + +## How the backup lock works + +With the [`--lock-ddl=ON`](./xtrabackup-option-reference.md/#lock-ddl) option the backup lock is enabled by default from the start of the backup process. + +* Set a backup lock on the server to prevent any `DDL` operations, ensuring no new tables are created, renamed, or dropped during the backup. + +* Copy all redo logs from the checkpoint up to the current Log Sequence Number (LSN) and start tracking new entries. + +* Identify and copy all InnoDB tablespace, `.ibd`, files. This step takes most of the time and is performed under the lock. + +* Copy non-InnoDB files. + +* Gather a sync points from all engines (InnoDB LSN, binlog, Global Transaction Identifier (GTID), etc.) by querying the log status. + +* Stop the redo flow thread once it copies at least up to sync point. + +* Release the backup lock on the server. + +If the backup lock is disabled with the [`--lock-ddl=OFF`](./xtrabackup-option-reference.md/#lock-ddl) option, a backup continues while concurrent DDL events are executed. These backups may be invalid and may fail at either the `backup` or the `--prepare` step. + +## The `--lock-ddl=REDUCED` option overview + +[Percona XtraBackup 8.4.0-2](./release-notes/8.4.0-2.md) adds the [`--lock-ddl=REDUCED`](./xtrabackup-option-reference.md/#lock-ddl) option to reduce the time the server remains locked by `xtrabackup`. Now, you can execute more `DDL` operations concurrently while the backup is in progress. The backup lock is now taken after copying the `.ibd` files and before copying the `non-InnoDB` files. + +The new backup design includes the following phases: + +### Phase 1: `Operations performed without the lock` + +1. Copy all redo logs from the checkpoint up to the current `LSN` and start tracking new entries. +2. Start the redo log thread. +3. Track file operations from the redo log by parsing the `MLOG_FILE_*` records. +4. Copy of all the `.ibd` files. + +### Phase 2: `Operations performed under the lock` + +1. Acquire the backup lock (Lock Tables For Backup (LTFB)/Lock Instance For Backup (LIFB)) to prevent new DDL operations, such as creating or altering tables. +2. Check the file operations that were tracked and recopy the tablespaces. +3. Create additional `meta` files to perform the required actions (deletions or renames) on the already copied files. This approach ensures that the backup remains consistent and accurate without disrupting the streaming process. This step is required for taking streaming backups. + + The meta files are the following: + + | File | Description | + |--------------|------------------------------------------------------------------------------------------------------------------| + | `.new` | For files that need to be recopied due to encryption or `ADD INDEX` operations. | + | `.del` | For deleted files. If a file has been copied, create a `space_id.del` file. | + | `.ren` | For renamed files after copying. Create a `space_id.ren` file. | + | `.crpt` | For files that cannot be fully copied due to encryption changes. These will be recopied, and a `.crpt` file will be created for the existing file. | + | `.new.meta` | Similar to `.new`, but for incremental backups. | + | `.new.delta` | Similar to `.new`, but for incremental backups. Create `t1.ibd.meta` and `t1.ibd.delta` instead of `t1.ibd`. | + +4. Gather a sync point from all engines (InnoDB LSN, binlog, GTID, etc.) by querying the `log_status`. +5. Stop the redo flow thread once it copies at least up to sync point at step 4. +6. Release the backup lock (LTFB/LIFB). + +### Phase 3. Processing the new metadata files during the `--prepare` phase before crash recovery starts. + +1. The `.crpt` files are removed matching the name after stripping the extension. This step is crucial before the IBD scan because these files are incomplete (they could even be zero size). +2. Perform a scan to create a mapping of `space_id` to file names. +3. The `space_id.del` deletes the file matching the `space_id`. In case of incremental backups, also deletes the corresponding `.new.meta` and `.new.delta` files. +4. The `space_id.ren` renames the file matching the `space_id` to the name specified in the file. +5. The `.new` extension replaces the file that matches the name without the `.new` extension. +6. The `.new.meta/.delta` replace the meta and delta files that match the name without the `.new` in the name. + +After Phase 3 is completed, the regular crash recovery starts. + +### Limitations + +1. The `ALTER INSTANCE ROTATE MASTER KEY` command should not be executed during [Phase 1](#phase-1-operations-performed-without-the-lock) if there are encrypted tables, as it will cause the backup to fail. + +2. The number of open file handles in your Operation System (OS) should be configured to match the number of files in the server data directory. + +3. When using the `--lock-ddl=REDUCED` option, backups may be larger because new `DDL` operations are executed concurrently on the server, and the files they generate are included in the backup. Additionally, certain DDL operations, such as `ADD INDEX` or encryption changes to existing data files, will cause these data files to be recopied, further increasing the backup size. + +### Benefits + +The `--lock-ddl=REDUCED` option key features are as follows: + +* Reduced locking: Rather than holding the backup lock throughout the entire backup process, it is only acquired for a very short period. + +* DDL statements: The server remains accessible for `DDL` operations during the backup process. + +* Consistency: Consistency is maintained by using the redo log to track file operations. During the backup process, new metadata files are created to ensure consistency. In the `--prepare` phase, these metadata files are processed first to bring the database to a consistent state. + + + + + + + + diff --git a/docs/xtrabackup-option-reference.md b/docs/xtrabackup-option-reference.md index afd45e55c..a7f1cc48e 100644 --- a/docs/xtrabackup-option-reference.md +++ b/docs/xtrabackup-option-reference.md @@ -27,7 +27,6 @@ $ xtrabackup --prepare --target-dir=/data/backup/mysql/ For all modes, the default options are read from the xtrabackup and mysqld configuration groups from the following files in the given order: - 1. `/etc/my.cnf` @@ -598,7 +597,14 @@ Usage: `--lock-ddl` Enabled by default to ensure that any DDL event does not corrupt the backup. Any DML events continue to occur. A DDL lock protects table and view definitions. -If the option is disabled, a backup continues while concurrent DDL events are executed. These backups are invalid and fail in the Prepare step. +The available values are the following: + +|Option| Description| +|`--lock-ddl=ON`| The backup lock is enabled and is taken at the beginning of the backup.| +|`--lock-ddl=OFF`|The backup lock is not taken.| +|`--lock-ddl=REDUCED`|This option value is a [tech preview](./glossary.md#tech-preview). The option value has been added in [Percona XtraBackup 8.4.0-2](./release-notes/8.4.0-2.md) to reduce the time the instance remains under backup lock. The backup lock is taken after copying the `.ibd` files and before copying the `non-InnoDB` files.| + +If the backup lock is disabled with the `--lock-ddl=OFF` option, a backup continues while concurrent DDL events are executed. These backups may be invalid and may fail at either the `backup` or the `--prepare` step. Use a [safe-slave-backup](#safe-slave-backup) option to stop a SQL replica thread before copying the InnoDB files. @@ -606,10 +612,9 @@ Use a [safe-slave-backup](#safe-slave-backup) option to stop a SQL replica threa Usage: `--lock-ddl-per-table` -Deprecated. Use the [`–lock-ddl`](#lock-ddl) option instead +Deprecated in Percona XtraBackup 8.0. Use the [`–lock-ddl`](#lock-ddl) option instead -Lock DDL for each table before xtrabackup starts to copy -it and until the backup is completed. +Lock DDL for each table before xtrabackup starts to copy it and until the backup is completed. ### lock-ddl-timeout diff --git a/mkdocs-base.yml b/mkdocs-base.yml index c7af762cf..f730e11e0 100644 --- a/mkdocs-base.yml +++ b/mkdocs-base.yml @@ -159,6 +159,7 @@ nav: - lru-dump-backup.md - throttling-backups.md - store-backup-history.md + - reduction-in-locks.md - Prepare features: - dictionary-cache.md - Restore features: @@ -166,7 +167,6 @@ nav: - restore-individual-tables.md - flush-tables-with-read-lock.md - log-enhancements.md - - lock-options.md - smart-memory-estimation.md - working-with-binary-logs.md - Binaries: