作者：趙黎明
愛可生 MySQL DBA 團隊成員，熟悉 Oracle、MySQL 等資料庫，擅長資料庫效能問題診斷、事務與鎖問題的分析等，負責處理客戶 MySQL 及我司自研 DMP 平臺日常運維中的問題，對開源資料庫相關技術非常感興趣。
本文來源：原創投稿
*愛可生開源社群出品，原創內容未經授權不得隨意使用，轉載請聯絡小編並註明來源。
背景
近日，客戶反饋某生產業務系統凌晨的物理備份都失敗了（一主二從的叢集，僅在兩個從庫上做 Xtrabackup 全備，主庫不參與備份），需排查備份失敗的原因。
案例分析
由於客戶使用的是我司愛可生的 DMP 資料庫管理平臺，當備份失敗時，在備份目錄中會寫入一個 FAIL 的標誌檔案，然後回滾掉殘留檔案，此時 Xtrabackup 自身的日誌已無法檢視，不過可以通過 urman-agent 元件（負責備份恢復）日誌來獲取備份失敗的資訊，以下是當時兩個從庫上的報錯資訊
- 從庫1日誌
- 從庫2日誌
兩個從庫雖然報錯的時間不同，但報錯的內容一致，都指向了“不記錄 redo 日誌的 DDL 操作”：
[FATAL] InnoDB: An optimized(without redo logging) DDLoperation has been performed. All modified pages may not have been flushed to the disk yet.
PXB will not be able take a consistent backup. Retry the backup operation
經確認，客戶的確是在凌晨執行了 DDL 業務變更，變更的內容為建立一張新表，並給現存的兩張表新增欄位，加欄位的表大約有幾百萬行記錄，這一資訊與日誌給出的內容吻合，看來問題大概率是出在加欄位的 DDL 操作上
那什麼是不記錄 redo 的 DDL 的操作呢？為何會存在不記錄 redo 的 DDL ？
首先，我們知道，在 MySQL 5.7 中給表加欄位屬於 ONLINE DLL ，會重建表，但允許併發 DML（PS：MySQL 8.0 加欄位不需要重建表）
由於 MySQL 採用的是索引組織表（IOT），表上的索引當然也需要重建，由於採用了 ALGORITHM=INPLACE 的方式，允許併發 DML
在 MySQL 5.7 中，對索引有一個優化，即 Sorted Index Builds ，會在建立或重建索引的時候通過 bulk load 、bottom-up 的方式來填充索引記錄
<br/>
採用 Sorted Index Build 方式建立索引時，不會記錄到 redo 日誌中，而這就是之前 Xtrabackup 報錯中所描述的“An optimized(without redo logging) DDL operation”的場景
Percona 稱這是 Xtrabackup 2.4.x 的一個 bug ，主要是為了避免得到一個錯誤的備份集，一旦檢測到有不記錄 redo 的 DDL 操作，就會將備份程式終止，而客戶生產環境中的 PXB 版本正是2.4.5
針對這一問題，Percona在Xtrabackup 2.4.8 及之後的版本中，新增了--lock-ddl，--lock-ddl-timeout，--lock-ddl-per-table這幾個引數，使其可以在備份時加上備份鎖，或給表加上 MDL 鎖來阻塞其他的 DDL 操作，使備份順利完成
原因明確了，就知道如何解決問題了，最後分別在2個從庫上執行手動備份，每個例項（500G左右）大約耗時2小時40分鐘完成備份
Percona 以增加引數的方式提供瞭解決備份失敗的方法，那如果暫時無法升級 PXB 版本，僅在 MySQL 層面，有沒有解決方法呢？其實也是可以的。
對於以上提到的幾種場景，我們都來測試一下吧
場景測試
環境準備
建立測試表
/usr/local/sysbench/share/sysbench# sysbench oltp_insert.lua --db-driver=mysql --threads=256 --time=300 --mysql-host=10.186.60.68 --mysql-port=3332 --mysql-user=zlm --mysql-password=zlm --tables=2 --table-size=2000000 --db-ps-mode=disable --report-interval=10 prepare
sysbench 1.0.17 (using bundled LuaJIT 2.1.0-beta2)
Initializing worker threads...
Creating table 'sbtest2'...
Inserting 2000000 records into 'sbtest2'
Creating table 'sbtest1'...
Inserting 2000000 records into 'sbtest1'
Creating a secondary index on 'sbtest2'...
Creating a secondary index on 'sbtest1'...準備指令碼（batch_ddl.sh）
-- 該指令碼的作用是對測試表進行持續的DDL操作（增加/刪除欄位，模擬客戶的業務變更）
dmp2 (master) ~/script# echo > batch_ddl.sh
dmp2 (master) ~/script# cat << EOF > batch_ddl.sh
> #!/bin/bash
> echo "alter table sbtest1 add sid varchar(32);"|/data/mysql/base/5.7.36/bin/mysql -h10.186.60.68 -P3332 -uzlm -pzlm sbtest
> sleep 1
> echo "alter table sbtest2 add sid varchar(32);"|/data/mysql/base/5.7.36/bin/mysql -h10.186.60.68 -P3332 -uzlm -pzlm sbtest
> sleep 10
> echo "alter table sbtest1 drop sid;"|/data/mysql/base/5.7.36/bin/mysql -h10.186.60.68 -P3332 -uzlm -pzlm sbtest
> sleep 1
> echo "alter table sbtest2 drop sid;"|/data/mysql/base/5.7.36/bin/mysql -h10.186.60.68 -P3332 -uzlm -pzlm sbtest
> EOF場景1：備份時併發執行 DDL（無額外引數）
執行指令碼
dmp2 (master) ~/script# while true; do bash batch_ddl.sh; done
mysql: [Warning] Using a password on the command line interface can be insecure.
mysql: [Warning] Using a password on the command line interface can be insecure.
... 略驗證指令碼是否生效（觀察測試表的 sid 欄位，時而有，時而無）
zlm@10.186.60.68 [sbtest]> show create table sbtest1\G
*************************** 1. row ***************************
Table: sbtest1
Create Table: CREATE TABLE `sbtest1` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`k` int(11) NOT NULL DEFAULT '0',
`c` char(120) NOT NULL DEFAULT '',
`pad` char(60) NOT NULL DEFAULT '',
PRIMARY KEY (`id`),
KEY `k_1` (`k`)
) ENGINE=InnoDB AUTO_INCREMENT=2000001 DEFAULT CHARSET=utf8
1 row in set (0.00 sec)
zlm@10.186.60.68 [sbtest]> show create table sbtest1\G
*************************** 1. row ***************************
Table: sbtest1
Create Table: CREATE TABLE `sbtest1` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`k` int(11) NOT NULL DEFAULT '0',
`c` char(120) NOT NULL DEFAULT '',
`pad` char(60) NOT NULL DEFAULT '',
`sid` varchar(32) DEFAULT NULL,
PRIMARY KEY (`id`),
KEY `k_1` (`k`)
) ENGINE=InnoDB AUTO_INCREMENT=2000001 DEFAULT CHARSET=utf8
1 row in set (0.00 sec)執行 Xtrabackup 備份
-- 檢查Xtrabackup的版本（客戶生產環境為2.4.5）
dmp2 /data/urman-agent/bin# ./xtrabackup --version
xtrabackup: recognized server arguments: --datadir=/var/lib/mysql
./xtrabackup version 2.4.20 based on MySQL server 5.7.26 Linux (x86_64) (revision id: c8b4056)
-- 執行備份
./xtrabackup --defaults-file=/data/mysql/mysql3332/my.cnf \
--user=zlm --password=zlm --target-dir=/data/backup/`date +%F_%H-%M-%S` \
--backup --host=127.0.0.1 --port=3332 --parallel 4 \
--check-privileges --no-version-check
... 略
220512 15:11:11 [01] Copying ./sbtest/db.opt to /data/backup/2022-05-12_15-10-56/sbtest/db.opt
220512 15:11:11 [01] ...done
220512 15:11:11 [01] Copying ./sbtest/sbtest2.frm to /data/backup/2022-05-12_15-10-56/sbtest/sbtest2.frm
220512 15:11:11 [01] ...done
220512 15:11:11 [01] Copying ./sbtest/sbtest1.frm to /data/backup/2022-05-12_15-10-56/sbtest/sbtest1.frm
220512 15:11:11 [01] ...done
220512 15:11:11 Finished backing up non-InnoDB tables and files
220512 15:11:11 [00] Writing /data/backup/2022-05-12_15-10-56/xtrabackup_binlog_info
220512 15:11:11 [00] ...done
220512 15:11:11 Executing FLUSH NO_WRITE_TO_BINLOG ENGINE LOGS...
xtrabackup: The latest check point (for incremental): '250912794909'
xtrabackup: Stopping log copying thread.
.InnoDB: Last flushed lsn: 250912650797 load_index lsn 250913206547
InnoDB: An optimized (without redo logging) DDL operation has been performed. All modified pages may not have been flushed to the disk yet.
PXB will not be able to make a consistent backup. Retry the backup operation
dmp2 /data/urman-agent/bin#
## 以上步驟，直接復現了客戶生產環境的故障場景終止指令碼
mysql: [Warning] Using a password on the command line interface can be insecure.
mysql: [Warning] Using a password on the command line interface can be insecure.
mysql: [Warning] Using a password on the command line interface can be insecure.
mysql: [Warning] Using a password on the command line interface can be insecure.
mysql: [Warning] Using a password on the command line interface can be insecure.
mysql: [Warning] Using a password on the command line interface can be insecure.
^C^C -- query aborted
ERROR 1317 (70100) at line 1: Query execution was interrupted小結
預設情況下，即使是 Xtrabackup 高版本，如果備份時併發執行 DDL ，並且沒有指定 DDL 鎖引數（--lock-ddl，--lock-ddl-per-table），會導致備份失敗
場景2：備份時併發執行 DDL（加--lock-ddl引數）
執行指令碼
dmp2 (master) ~/script# while true; do bash batch_ddl.sh; done
mysql: [Warning] Using a password on the command line interface can be insecure.
mysql: [Warning] Using a password on the command line interface can be insecure.
... 略執行 Xtrabackup 備份
dmp2 /data/urman-agent/bin# ./xtrabackup --defaults-file=/data/mysql/mysql3332/my.cnf --user=zlm --password=zlm --target-dir=/data/backup/`date +%F_%H-%M-%S` --backup --host=127.0.0.1 --port=3332 --parallel 4 --check-privileges --no-version-check --lock-ddl
xtrabackup: recognized server arguments: --datadir=/data/mysql/mysql3332/data --tmpdir=/data/mysql/mysql3332/tmp --open_files_limit=65535 --server-id=31782647 --log_bin=/data/mysql/mysql3332/logs/mysql-bin --innodb_buffer_pool_size=100M --innodb_data_file_path=ibdata1:100M:autoextend --innodb_flush_log_at_trx_commit=1 --innodb_log_buffer_size=8M --innodb_log_file_size=100M --innodb_log_files_in_group=3 --innodb_max_dirty_pages_pct=50 --innodb_file_per_table=1 --innodb_io_capacity=2000 --innodb_flush_method=O_DIRECT --innodb_flush_log_at_trx_commit=1 --parallel=4
xtrabackup: recognized client arguments: --port=3332 --socket=/tmp/mysql3332.sock --user=zlm --password=* --target-dir=/data/backup/2022-05-12_16-02-37 --backup=1 --host=127.0.0.1 --port=3332 --check-privileges=1 --no-version-check=1 --lock-ddl=1
220512 16:02:37 Connecting to MySQL server host: 127.0.0.1, user: zlm, password: set, port: 3332, socket: /tmp/mysql3332.sock
Using server version 5.7.36-log
220512 16:02:37 Error: LOCK TABLES FOR BACKUP is not supported.
## MySQL社群版不支援--lock-ddl引數，直接報錯小結
lock-ddl 其實就是在進行備份的時候，執行備份鎖（LOCK TABLES FOR BACKUP）來阻塞 DDL 語句，由於只有 Percona Server 支援該語法，MySQL 社群版暫時無法模擬，MariaDB 分支也同樣不支援該語法
場景3：備份時併發執行 DDL（加--lock-ddl-per-table引數）
執行指令碼
dmp2 (master) ~/script# while true; do bash batch_ddl.sh; done
mysql: [Warning] Using a password on the command line interface can be insecure.
mysql: [Warning] Using a password on the command line interface can be insecure.
... 略執行 Xtrabackup 備份
dmp2 /data/urman-agent/bin# ./xtrabackup --defaults-file=/data/mysql/mysql3332/my.cnf --user=zlm --password=zlm --target-dir=/data/backup/`date +%F_%H-%M-%S` --backup --host=127.0.0.1 --port=3332 --parallel 4 --check-privileges --no-version-check --lock-ddl-per-table
xtrabackup: recognized server arguments: --datadir=/data/mysql/mysql3332/data --tmpdir=/data/mysql/mysql3332/tmp --open_files_limit=65535 --server-id=31782647 --log_bin=/data/mysql/mysql3332/logs/mysql-bin --innodb_buffer_pool_size=100M --innodb_data_file_path=ibdata1:100M:autoextend --innodb_flush_log_at_trx_commit=1 --innodb_log_buffer_size=8M --innodb_log_file_size=100M --innodb_log_files_in_group=3 --innodb_max_dirty_pages_pct=50 --innodb_file_per_table=1 --innodb_io_capacity=2000 --innodb_flush_method=O_DIRECT --innodb_flush_log_at_trx_commit=1 --parallel=4
xtrabackup: recognized client arguments: --port=3332 --socket=/tmp/mysql3332.sock --user=zlm --password=* --target-dir=/data/backup/2022-05-12_16-07-12 --backup=1 --host=127.0.0.1 --port=3332 --check-privileges=1 --no-version-check=1 --lock-ddl-per-table=1
220512 16:07:12 Connecting to MySQL server host: 127.0.0.1, user: zlm, password: set, port: 3332, socket: /tmp/mysql3332.sock
Using server version 5.7.36-log
./xtrabackup version 2.4.20 based on MySQL server 5.7.26 Linux (x86_64) (revision id: c8b4056)
xtrabackup: uses posix_fadvise().
xtrabackup: cd to /data/mysql/mysql3332/data
xtrabackup: open files limit requested 65535, set to 65535
xtrabackup: using the following InnoDB configuration:
xtrabackup: innodb_data_home_dir = .
xtrabackup: innodb_data_file_path = ibdata1:100M:autoextend
xtrabackup: innodb_log_group_home_dir = ./
xtrabackup: innodb_log_files_in_group = 3
xtrabackup: innodb_log_file_size = 104857600
xtrabackup: using O_DIRECT
InnoDB: Number of pools: 1
220512 16:07:13 >> log scanned up to (250920385317)
xtrabackup: Generating a list of tablespaces
... 略
220512 16:07:14 Locking MDL for `sbtest`.`sbtest1` ## 備份sbtest1表資料前，先加上MDL鎖
220512 16:07:14 [04] Copying ./sbtest/sbtest1.ibd to /data/backup/2022-05-12_16-07-12/sbtest/sbtest1.ibd
220512 16:07:14 [02] Copying ./sbtest/sbtest2.ibd to /data/backup/2022-05-12_16-07-12/sbtest/sbtest2.ibd
220512 16:07:15 >> log scanned up to (250920494721)
220512 16:07:16 >> log scanned up to (250920494832)
220512 16:07:17 >> log scanned up to (250920494841)
220512 16:07:18 >> log scanned up to (250920494841)
220512 16:07:19 [02] ...done
220512 16:07:19 >> log scanned up to (250920494841)
220512 16:07:20 >> log scanned up to (250920494841)
220512 16:07:20 [01] ...done
220512 16:07:20 [04] ...done
220512 16:07:21 Starting to backup non-InnoDB tables and files
... 略
220512 16:07:21 [01] Copying ./sbtest/sbtest2.frm to /data/backup/2022-05-12_16-07-12/sbtest/sbtest2.frm
220512 16:07:21 [01] ...done
220512 16:07:21 [01] Copying ./sbtest/sbtest1.frm to /data/backup/2022-05-12_16-07-12/sbtest/sbtest1.frm
220512 16:07:21 [01] ...done
220512 16:07:21 Finished backing up non-InnoDB tables and files
220512 16:07:21 [00] Writing /data/backup/2022-05-12_16-07-12/xtrabackup_binlog_info
220512 16:07:21 [00] ...done
220512 16:07:21 Executing FLUSH NO_WRITE_TO_BINLOG ENGINE LOGS...
xtrabackup: The latest check point (for incremental): '250920494832'
xtrabackup: Stopping log copying thread.
.220512 16:07:21 >> log scanned up to (250920494841)
220512 16:07:21 Executing UNLOCK TABLES
220512 16:07:21 All tables unlocked
220512 16:07:21 [00] Copying ib_buffer_pool to /data/backup/2022-05-12_16-07-12/ib_buffer_pool
220512 16:07:21 [00] ...done
220512 16:07:21 Backup created in directory '/data/backup/2022-05-12_16-07-12/'
MySQL binlog position: filename 'mysql-bin.000012', position '472261259', GTID of the last change '0e795785-0ee5-11ec-9350-02000aba3c44:1-1619'
220512 16:07:21 [00] Writing /data/backup/2022-05-12_16-07-12/backup-my.cnf
220512 16:07:21 [00] ...done
220512 16:07:21 [00] Writing /data/backup/2022-05-12_16-07-12/xtrabackup_info
220512 16:07:21 [00] ...done
220512 16:07:21 Unlocking MDL for all tablesxtrabackup: Transaction log of lsn (250920249371) to (250920494841) was copied. ## 釋放所有表上的MDL鎖
220512 16:07:22 completed OK!終止指令碼
... 略
mysql: [Warning] Using a password on the command line interface can be insecure.
^C^C -- query aborted
ERROR 1317 (70100) at line 1: Query execution was interrupted
mysql: [Warning] Using a password on the command line interface can be insecure.
^C^C -- query aborted
ERROR 1317 (70100) at line 1: Query execution was interrupted
mysql: [Warning] Using a password on the command line interface can be insecure.
ERROR 1060 (42S21) at line 1: Duplicate column name 'sid'
mysql: [Warning] Using a password on the command line interface can be insecure.
ERROR 1060 (42S21) at line 1: Duplicate column name 'sid'
^C
dmp2 (master) ~/script#小結
備份時使用 --lock-ddl-per-table 引數，會在拷貝每個表的 ibd 檔案前先加一個 MDL 鎖，用於阻塞該表上 DDL 語句的執行，直到備份完成後，DDL 才能繼續執行
場景4：備份時併發執行DDL（設定MySQL引數：old_alter_table=1）
關閉 ONLINE DDL 特性
zlm@10.186.60.68 [(none)]> set global old_alter_table=1;
Query OK, 0 rows affected (0.00 sec)
zlm@10.186.60.68 [(none)]> show variables like 'old_alter_table';
+-----------------+-------+
| Variable_name | Value |
+-----------------+-------+
| old_alter_table | OFF |
+-----------------+-------+
1 row in set (0.00 sec)
zlm@10.186.60.68 [(none)]> exit
Bye
dmp2 (master) ~/script# m3332 -Nse "show variables like 'old_alter_table';"
mysql: [Warning] Using a password on the command line interface can be insecure.
old_alter_table ON執行指令碼
dmp2 (master) ~/script# while true; do bash batch_ddl.sh; done
mysql: [Warning] Using a password on the command line interface can be insecure.
ERROR 1060 (42S21) at line 1: Duplicate column name 'sid'
mysql: [Warning] Using a password on the command line interface can be insecure.
ERROR 1060 (42S21) at line 1: Duplicate column name 'sid'
mysql: [Warning] Using a password on the command line interface can be insecure.
mysql: [Warning] Using a password on the command line interface can be insecure.
... 略執行 Xtrabackup 備份
dmp2 /data/urman-agent/bin# ./xtrabackup --defaults-file=/data/mysql/mysql3332/my.cnf --user=zlm --password=zlm --target-dir=/data/backup/`date +%F_%H-%M-%S` --backup --host=127.0.0.1 --port=3332 --parallel 4 --check-privileges --no-version-check
xtrabackup: recognized server arguments: --datadir=/data/mysql/mysql3332/data --tmpdir=/data/mysql/mysql3332/tmp --open_files_limit=65535 --server-id=31782647 --log_bin=/data/mysql/mysql3332/logs/mysql-bin --innodb_buffer_pool_size=100M --innodb_data_file_path=ibdata1:100M:autoextend --innodb_flush_log_at_trx_commit=1 --innodb_log_buffer_size=8M --innodb_log_file_size=100M --innodb_log_files_in_group=3 --innodb_max_dirty_pages_pct=50 --innodb_file_per_table=1 --innodb_io_capacity=2000 --innodb_flush_method=O_DIRECT --innodb_flush_log_at_trx_commit=1 --parallel=4
xtrabackup: recognized client arguments: --port=3332 --socket=/tmp/mysql3332.sock --user=zlm --password=* --target-dir=/data/backup/2022-05-12_16-48-00 --backup=1 --host=127.0.0.1 --port=3332 --check-privileges=1 --no-version-check=1
220512 16:48:00 Connecting to MySQL server host: 127.0.0.1, user: zlm, password: set, port: 3332, socket: /tmp/mysql3332.sock
Using server version 5.7.36-log
./xtrabackup version 2.4.20 based on MySQL server 5.7.26 Linux (x86_64) (revision id: c8b4056)
xtrabackup: uses posix_fadvise().
xtrabackup: cd to /data/mysql/mysql3332/data
xtrabackup: open files limit requested 65535, set to 65535
xtrabackup: using the following InnoDB configuration:
xtrabackup: innodb_data_home_dir = .
xtrabackup: innodb_data_file_path = ibdata1:100M:autoextend
xtrabackup: innodb_log_group_home_dir = ./
xtrabackup: innodb_log_files_in_group = 3
xtrabackup: innodb_log_file_size = 104857600
xtrabackup: using O_DIRECT
InnoDB: Number of pools: 1
220512 16:48:02 >> log scanned up to (251659945145)
xtrabackup: Generating a list of tablespaces
... 略
220512 16:48:03 [02] Copying ./zlm/t.ibd to /data/backup/2022-05-12_16-48-00/zlm/t.ibd
220512 16:48:03 [03] ...done
220512 16:48:03 [02] ...done
220512 16:48:03 [03] Copying ./sbtest/sbtest2.ibd to /data/backup/2022-05-12_16-48-00/sbtest/sbtest2.ibd
220512 16:48:03 [04] Copying ./zlm1/t1.ibd to /data/backup/2022-05-12_16-48-00/zlm1/t1.ibd
220512 16:48:03 [02] Copying ./sbtest/sbtest1.ibd to /data/backup/2022-05-12_16-48-00/sbtest/sbtest1.ibd
220512 16:48:03 [04] ...done
220512 16:48:03 [04] Copying ./sbtest/#sql-c61_30133.ibd to /data/backup/2022-05-12_16-48-00/sbtest/#sql-c61_30133.ibd
220512 16:48:04 >> log scanned up to (251669706184)
220512 16:48:04 [04] ...done
220512 16:48:05 >> log scanned up to (251672429294)
220512 16:48:06 >> log scanned up to (251675700622)
220512 16:48:07 >> log scanned up to (251678520383)
220512 16:48:08 >> log scanned up to (251681195105)
220512 16:48:09 [02] ...done
220512 16:48:09 >> log scanned up to (251683273712)
220512 16:48:09 [01] ...done
220512 16:48:10 >> log scanned up to (251694062575)
220512 16:48:11 >> log scanned up to (251707703017)
220512 16:48:12 [03] ...done
220512 16:48:12 >> log scanned up to (251713134341)
220512 16:48:12 Executing FLUSH NO_WRITE_TO_BINLOG TABLES...
220512 16:48:13 >> log scanned up to (251721786959)
... 略
220512 16:48:38 [01] Copying ./sbtest/sbtest1.frm to /data/backup/2022-05-12_16-48-00/sbtest/sbtest1.frm
220512 16:48:38 [01] ...done
220512 16:48:38 [01] Copying ./sbtest/sbtest2.frm to /data/backup/2022-05-12_16-48-00/sbtest/sbtest2.frm
220512 16:48:38 [01] ...done
220512 16:48:38 Finished backing up non-InnoDB tables and files
220512 16:48:38 [00] Writing /data/backup/2022-05-12_16-48-00/xtrabackup_binlog_info
220512 16:48:38 [00] ...done
220512 16:48:38 Executing FLUSH NO_WRITE_TO_BINLOG ENGINE LOGS...
xtrabackup: The latest check point (for incremental): '251977260800'
xtrabackup: Stopping log copying thread.
.220512 16:48:38 >> log scanned up to (252017389382)
220512 16:48:38 Executing UNLOCK TABLES
220512 16:48:38 All tables unlocked
220512 16:48:38 [00] Copying ib_buffer_pool to /data/backup/2022-05-12_16-48-00/ib_buffer_pool
220512 16:48:38 [00] ...done
220512 16:48:38 Backup created in directory '/data/backup/2022-05-12_16-48-00/'
MySQL binlog position: filename 'mysql-bin.000012', position '472262307', GTID of the last change '0e795785-0ee5-11ec-9350-02000aba3c44:1-1625'
220512 16:48:38 [00] Writing /data/backup/2022-05-12_16-48-00/backup-my.cnf
220512 16:48:38 [00] ...done
220512 16:48:38 [00] Writing /data/backup/2022-05-12_16-48-00/xtrabackup_info
220512 16:48:38 [00] ...done
xtrabackup: Transaction log of lsn (251552488790) to (252017389382) was copied.
220512 16:48:38 completed OK!
dmp2 /data/urman-agent/bin#
## Xtrabackup備份日誌中沒有發現MDL鎖，備份也順利地執行完了，說明啟用old_alter_table也能夠解決備份失敗的問題終止指令碼
... 略
mysql: [Warning] Using a password on the command line interface can be insecure.
^C^C -- query aborted
ERROR 1317 (70100) at line 1: Query execution was interrupted
mysql: [Warning] Using a password on the command line interface can be insecure.
^C^C -- query aborted
ERROR 1317 (70100) at line 1: Query execution was interrupted
^C
04:48 PM dmp2 (master) ~/script#小結
當關閉 ONLINE DDL 特性時（old_alter_table=1），即執行 DDL 時強制其都採用 ALGORITHM=COPY 來代替 ALGORITHM=INPLACE ，此時即使備份時沒有使用 DDL 鎖的引數，也不會影響備份執行，但要注意的是，關閉 ONLINE DDL 特性會影響表上的 DML 併發，不推薦使用。
總結
- DDL 業務變更操作應儘量避開備份視窗
- Sorted Index Builds 對建立索引的優化與 PXB 2.4.x 備份存在衝突，需要替換為 PXB 高版本，並通過加備份鎖或 MDL 鎖的引數來避免備份失敗
- MySQL 自身也可以通過關閉 ONLINE DDL 特性來解決備份失敗的問題，但是會影響 DML 併發，需權衡利弊
參考連結
https://dev.mysql.com/doc/ref...