MDEV-14992 BACKUP SERVER

[dr-m]

The following SQL statements will be introduced:

BACKUP SERVER TO '/path/to/directory';
BACKUP SERVER TO '/path/to/directory' 1 CONCURRENT;
BACKUP SERVER WITH 'command';
BACKUP SERVER WITH 1 CONCURRENT 'command';

In place of the 1, any positive number of threads may be specified. For the first variant, '/path/to' must exist and '/path/to/directory' must not exist; that is where the backup will be written to.

For the second variant, 'command' must be the name of a script or command that will be executed in a child process. The standard input of that command will be in a format that is compatible with GNU tar --format=oldgnu (and also BSD tar variants that are also part of Microsoft Windows and Apple macOS). The command is expected to optionally compress and encrypt the stream and redirect it to a file on a local or a remote server. The BACKUP SERVER WITH will append an additional argument, a positive base-ten number in ASCII, starting with 1, to identify the current thread. In this way, each concurrent stream can write a separate file.

The backup or the first stream will contain a file backup.cnf, which includes parameters needed for restoring the backup. Currently, these are innodb_log_recovery_start and innodb_log_recovery_target. If innodb_log_recovery_target>0, InnoDB will be in read-only mode, not allowing any writes to persistent files other than via the log application.

To restore a streaming backup made with BACKUP SERVER WITH, an empty directory needs to be created and all streams be extracted there using the standard tar utility of the operating system, optionally after undoing any encryption or compression that had been added by the backup command. Then, the backup is prepared or MariaDB server started up on the extracted directory, similar to as if the BACKUP SERVER TO statement had been used.

Note: The parameter innodb_log_recovery_start in backup.cnf is STRICTLY NECESSARY TO AVOID CORRUPTION! By default, InnoDB crash recovery starts from the latest available log checkpoint. However, for restoring a backup, recovery must start from the checkpoint that was the latest when the backup was started. Starting recovery from a possible later checkpoint will result in a corrupted database!

The following will be implemented separately:

MDEV-39061 mariadb-backup compatible wrapper script for BACKUP SERVER
MDEV-40163 Partial backup and restore
MDEV-39091 Back up ENGINE=RocksDB
MDEV-39092 Less blocking backup of ENGINE=Aria

The implementation introduces a basic driver Sql_cmd_backup, storage engine interfaces, and basic copying of the storage engines InnoDB, Aria, MyISAM, MERGE (MyISAM), Archive, CSV.

backup_target: A structured data type to represent a target directory. On Microsoft Windows, we must use directory paths because there is no variant of CopyFileEx() that would work on file handles.

backup_sink: Wraps a per-thread output stream as well as storage engine specific context.

handlerton::backup_start(), handlerton::backup_end(): Invoked at the start or end of a backup phase, in the thread that executes a BACKUP SERVER statement.

handlerton::backup_step(): A backup step that can be invoked from multiple threads concurrently, between the execution of the corresponding handlerton::backup_start() and handlerton::backup_end() of the same phase.

copy_entire_file(): A file copying service for POSIX systems.

copy_file(): A partial or sparse file-copying service for all systems.

backup_stream_append(): Equivalent to copy_file(), but appending to a stream. On Linux, this uses sendfile(2), which assumes that the source data will not be changed before the data has been consumed from the pipe.

backup_stream_append_async(): A variant of backup_stream_append() where the source file region is guaranteed to be immutable after the call returns. We must not use Linux sendfile(2) for copying data files that may be modified in place, because it could introduce a race condition between a page write that runs concurrently with a child process that is reading the data from the pipe.

InnoDB_backup::context: Backup context, attached to backup_sink so that context can continue to exist between the time a BACKUP SERVER releases all locks and another BACKUP SERVER starts executing, with innodb_backup pointing to the new backup, while the old backup is still being finished.

fil_space_t::write_or_backup: Keep track of in-flight page writes and pending backup operation. We must not allow them concurrently, because that could lead into torn pages in the backup.

fil_space_t::backup_end: The first page number that is not being backed up (by default 0, to indicate that no backup is in progress).

fil_space_t::BACKUP_BATCH_SIZE: The number of preceding pages that will be covered by fil_space_t::backup_end. This is the unit of "page range locking" during InnoDB backup.

log_sys.backup: Whether BACKUP SERVER is in progress. The purpose of this is to make BACKUP SERVER prevent the concurrent execution of SET GLOBAL innodb_log_archive=OFF or SET GLOBAL innodb_log_file_size when innodb_log_archive=OFF.

log_sys.archived_checkpoint: Keep track of the earliest available checkpoint, corresponding to log_sys.archived_lsn. This reflects SET GLOBAL innodb_log_recovery_start (which is settable now), for incremental backup.

buf_flush_list_space(): Check for concurrent backup before writing each page. This is inefficient, but this function may be invoked from multiple threads concurrently, and it cannot be changed easily, especially for fil_crypt_thread().

fil_system.have_all_spaces: Whether all tablespace metadata is guaranteed to be known. To speed up startup, InnoDB does not normally open all tablespace files.

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❌ dr-m
❌ Thirunarayanan
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[Thirunarayanan]

BACKUP SERVER WITH ''` runs an arbitrary shell command as the mysqld OS user, gated only by global RELOAD + SELECT_ACL. Is a dedicated privilege planned? Should there be a server switch to disable the WITH variant, and is the command captured by the audit log?

[Thirunarayanan]

1024 lines is sufficient for shell script commands?

[dr-m]

I think yes. The script should likely be in $PATH anyway. On Linux, extfs and its successors such as ext4 would limit the length of a path component (a file name) to 255 bytes.

[Thirunarayanan]

should the unlink() deferred till copy() succeeds?

[dr-m]

The log file that we are duplicating here is one in the BACKUP SERVER target directory. If the duplication failed, we did not remove the hard link to the active (last) log of the server, and the user started to use the output directory despite the error, they could end up corrupting the log of the server that is being backed up.