現象：一次磁盤告警，提示剩餘空間超閾值，df -h查看根目錄的Avail已剩不到2G。
原因：MySQL的數據目錄掛載在根目錄的data目錄下，撐爆了
解決方法：遷移數據目錄。
實際處理過程：set global innodb_fast_shutdown = 0; 後關閉數據庫，mv走數據目錄，修改配置文件的datadir，重啓數據庫。
然後。。。
報錯了：
Starting MySQL….. ERROR! The server quit without updating PID file
以下爲故障重現過程：
數據庫原datadir=/data/mysql3306、log-bin = /data/mysql3306/mybinlog，準備遷移到/home路徑下：
mkdir /home/mysqldata
mv /data/mysql3306 /home/mysqldata/
那麼，新的datadir=/home/mysqldata/mysql3306，新的log-bin = /home/mysqldata/mysql3306/mybinlog
在my.cnf中按需修改相關路徑後，另一一個session執行 # tail -f error.log 監控錯誤日誌的輸出，然後重啓數據庫。
（1）使用support-files下的mysql.server 啓動一般報錯如下：
Starting MySQL...... ERROR! The server quit without updating PID file (/home/mysqldata/mysql3306/mysqldb.pid).
（2）使用mysqld啓動，一般直接退出，如下：
[1] 14789
#
[1]+ Exit 1 /usr/local/mysql3306/bin/mysqld --defaults-file=/usr/local/mysql3306/my.cnf
（3）新數據目錄的權限未調整，或者採用rpm包方式安裝的MySQL受到SELinux的影響，同樣會導致 Starting MySQL...... ERROR! 的問題，但錯誤日誌與（1）（2）不同，本文不討論。
回到頭兩種情況，由於mysql.server是調用的mysqld_safe，再調用mysqld進行啓動，故最終錯誤日誌是一樣的，截取部分輸出如下：
2019-12-10T10:10:03.319664+08:00 0 [Note] --secure-file-priv is set to NULL. Operations related to importing and exporting data are disabled
2019-12-10T10:10:03.319837+08:00 0 [Note] /usr/local/mysql3306/bin/mysqld (mysqld 5.7.28-log) starting as process 11147 ...
2019-12-10T10:10:03.330161+08:00 0 [Note] InnoDB: PUNCH HOLE support available
2019-12-10T10:10:03.330222+08:00 0 [Note] InnoDB: Mutexes and rw_locks use GCC atomic builtins
2019-12-10T10:10:03.330242+08:00 0 [Note] InnoDB: Uses event mutexes
2019-12-10T10:10:03.330258+08:00 0 [Note] InnoDB: GCC builtin __sync_synchronize() is used for memory barrier
2019-12-10T10:10:03.330274+08:00 0 [Note] InnoDB: Compressed tables use zlib 1.2.11
2019-12-10T10:10:03.330289+08:00 0 [Note] InnoDB: Using Linux native AIO
2019-12-10T10:10:03.330858+08:00 0 [Note] InnoDB: Number of pools: 1
2019-12-10T10:10:03.331056+08:00 0 [Note] InnoDB: Using CPU crc32 instructions
2019-12-10T10:10:03.334548+08:00 0 [Note] InnoDB: Initializing buffer pool, total size = 6G, instances = 4, chunk size = 128M
2019-12-10T10:10:04.421360+08:00 0 [Note] InnoDB: Completed initialization of buffer pool
2019-12-10T10:10:04.510523+08:00 0 [Note] InnoDB: If the mysqld execution user is authorized, page cleaner thread priority can be changed. See the man page of setpriority().
2019-12-10T10:10:04.535726+08:00 0 [Note] InnoDB: Opened 95 undo tablespaces
2019-12-10T10:10:04.535771+08:00 0 [Note] InnoDB: 95 undo tablespaces made active
2019-12-10T10:10:04.536161+08:00 0 [Note] InnoDB: Highest supported file format is Barracuda.
2019-12-10T10:10:04.764728+08:00 0 [Note] InnoDB: Creating shared tablespace for temporary tables
2019-12-10T10:10:04.764878+08:00 0 [Note] InnoDB: Setting file './ibtmp1' size to 12 MB. Physically writing the file full; Please wait ...
2019-12-10T10:10:04.788994+08:00 0 [Note] InnoDB: File './ibtmp1' size is now 12 MB.
2019-12-10T10:10:04.790995+08:00 0 [Note] InnoDB: 96 redo rollback segment(s) found. 96 redo rollback segment(s) are active.
2019-12-10T10:10:04.791030+08:00 0 [Note] InnoDB: 32 non-redo rollback segment(s) are active.
2019-12-10T10:10:04.791550+08:00 0 [Note] InnoDB: Waiting for purge to start
2019-12-10T10:10:04.841937+08:00 0 [Note] InnoDB: 5.7.28 started; log sequence number 2711531
2019-12-10T10:10:04.842111+08:00 0 [Note] InnoDB: Loading buffer pool(s) from /home/mysqldata/mysql3306/ib_buffer_pool
2019-12-10T10:10:04.842291+08:00 0 [Note] Plugin 'FEDERATED' is disabled.
mysqld: File '/data/mysql3306/mybinlog.000002' not found (Errcode: 2 - No such file or directory)2019-12-10T10:10:04.847771+08:00 0 [Note] InnoDB: Buffer pool(s) load completed at 191210 10:10:04
2019-12-10T10:10:04.847822+08:00 0 [ERROR] Failed to open log (file '/data/mysql3306/mybinlog.000002', errno 2)
2019-12-10T10:10:04.847848+08:00 0 [ERROR] Could not open log file
2019-12-10T10:10:04.847867+08:00 0 [ERROR] Can't init tc log
2019-12-10T10:10:04.847890+08:00 0 [ERROR] Aborting
2019-12-10T10:10:04.847915+08:00 0 [Note] Binlog end
顯然，是由於未找到binlog導致啓動失敗，mysqld依然從老的數據目錄下去尋找。但之前已在配置文件中指定log-bin的路徑，那麼是什麼原因導致的呢？查閱官方文檔：
The default location for binary log files is the data directory. You can use the --log-bin option to specify an alternative location, by adding a leading absolute path name to the base name to specify a different directory. When the server reads an entry from the binary log index file, which tracks the binary log files that have been used, it checks whether the entry contains a relative path. If it does, the relative part of the path is replaced with the absolute path set using the --log-bin option. An absolute path recorded in the binary log index file remains unchanged; in such a case, the index file must be edited manually to enable a new path or paths to be used. (In older versions of MySQL, manual intervention was required whenever relocating the binary log or relay log files.) (Bug #11745230, Bug #12133)
可知是通過index文件來追蹤binlog文件，並且如果 --log-bin 選項中含有絕對路徑時，將會在index文件中以絕對路徑表示binlog文件，而不再是在默認的datadir下。
將如下index文件中的路徑做修改
實測已能正常修改
To keep track of which binary log files have been used, mysqld also creates a binary log index file that contains the names of the binary log files. By default, this has the same base name as the binary log file, with the extension '.index'. You can change the name of the binary log index file with the --log-bin-index[=file_name] option. You should not manually edit this file while mysqld is running; doing so would confuse mysqld.
若配置文件中不指定--log-basename 或者 log-bin = /data/mysql/mybinlog，顯然會避免上述問題，但會帶來另外一個問題：一旦主庫hostname有修改並重啓後，會導致主從複製斷連。
The name for the binary log index file, which contains the names of the binary log files. By default, it has the same location and base name as the value specified for the binary log files using the --log-bin option, plus the extension .index. If you do not specify --log-bin, the default binary log index file name is binlog.index. If you omit the file name and do not specify one with --log-bin, the default binary log index file name is host_name-bin.index, using the name of the host machine.
Log file names are based on the server host name if you do not specify a file name with the startup option. To retain the same log file names if you change your host name to something else, you must explicitly use options such as --log-bin=old_host_name-bin. See Section 5.1.6, “Server Command Options”. Alternatively, rename the old files to reflect your host name change. If these are binary logs, you must edit the binary log index file and fix the binary log file names there as well. (The same is true for the relay logs on a slave server.)
參考文檔：
16.1.6.4 Binary Logging Options and Variables：log-bin
