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这篇文章主要介绍“MySQL FLTWL 的堵塞和被堵塞分析”，在日常操作中，相信很多人在 MySQL FLTWL 的堵塞和被堵塞分析问题上存在疑惑，丸趣 TV 小编查阅了各式资料，整理出简单好用的操作方法，希望对大家解答”MySQL FLTWL 的堵塞和被堵塞分析”的疑惑有所帮助！接下来，请跟着丸趣 TV 小编一起来学习吧！
一、FTWRL 的作用
总的说来 flush tables with read lock 多用于备份的时候对全局表进行锁定来获取 binlog 信息。虽然持有时间很短，单容易被堵塞造成备份不能完成。那么 flush tables with read lock 到底做了什么事情呢如下：
获取 MDL GLOBAL 的 S 锁，直到 unlock tables 释放。
获取 MDL COMMIT 的 S 锁，直到 unlock tables 释放。
关闭所有的表重新打开，先释放 table cache(包含 TABLE_SHARED)，然后重新加载生成 table cache。
（下面的信息是在源码函数 acquire_lock 和 release_lock 增加输出达到的）
下面的信息可以看到 flush tables with read lock 获取 MDL LOCK 的信息：
2018-08-07T08:03:59.272547Z 7 [Note] [Call Acquire_lock] THIS MDL LOCK acquire [OK]:2018-08-07T08:03:59.272583Z 7 [Note] (MDL PRINT) |Thread id is 7|Current_state: starting|
2018-08-07T08:03:59.272599Z 7 [Note] (--- MDL PRINT) Namespace is:GLOBAL
2018-08-07T08:03:59.272613Z 7 [Note] (----- MDL PRINT) Mdl type is:MDL_SHARED(S)
2018-08-07T08:03:59.272627Z 7 [Note] (------ MDL PRINT) Mdl duration is:MDL_EXPLICIT
2018-08-07T08:03:59.272642Z 7 [Note] (------- MDL PRINT) Mdl status is:EMPTY
2018-08-07T08:03:59.292471Z 7 [Note] [Call Acquire_lock] THIS MDL LOCK acquire [OK]:
2018-08-07T08:03:59.292522Z 7 [Note] (MDL PRINT) |Thread id is 7|Current_state: starting| 2018-08-07T08:03:59.292538Z 7 [Note] (--- MDL PRINT) Namespace is:COMMIT
2018-08-07T08:03:59.292551Z 7 [Note] (----- MDL PRINT) Mdl type is:MDL_SHARED(S)
2018-08-07T08:03:59.292564Z 7 [Note] (------ MDL PRINT) Mdl duration is:MDL_EXPLICIT
2018-08-07T08:03:59.292580Z 7 [Note] (------- MDL PRINT) Mdl status is:EMPTY下面是 unlock tables 释放 MDL LOCK 的信息：
2018-08-07T08:05:43.520540Z 7 [Note] [Call release_lock] this MDL LOCK will [RELEASE]:2018-08-07T08:05:43.520571Z 7 [Note] (MDL PRINT) |Thread id is 7|Current_state: starting|
2018-08-07T08:05:43.520597Z 7 [Note] (--- MDL PRINT) Namespace is:COMMIT
2018-08-07T08:05:43.520609Z 7 [Note] (----- MDL PRINT) Mdl type is:MDL_SHARED(S)
2018-08-07T08:05:43.520620Z 7 [Note] (------ MDL PRINT) Mdl duration is:MDL_EXPLICIT
2018-08-07T08:05:43.520634Z 7 [Note] (------- MDL PRINT) Mdl status is:EMPTY
2018-08-07T08:05:43.520658Z 7 [Note] [Call release_lock] this MDL LOCK will [RELEASE]:2018-08-07T08:05:43.520671Z 7 [Note] (MDL PRINT) |Thread id is 7|Current_state: starting|
2018-08-07T08:05:43.520682Z 7 [Note] (--- MDL PRINT) Namespace is:GLOBAL
2018-08-07T08:05:43.520693Z 7 [Note] (----- MDL PRINT) Mdl type is:MDL_SHARED(S)
2018-08-07T08:05:43.520704Z 7 [Note] (------ MDL PRINT) Mdl duration is:MDL_EXPLICIT
2018-08-07T08:05:43.520727Z 7 [Note] (------- MDL PRINT) Mdl status is:EMPTY如下是关于范围 MDL LOCK 的兼容性：
| Type of active |
Request | scoped lock |
type | IS(*) IX S X |
---------+------------------+
IS | + + + + |
IX | + + - - |
S | + - + - |
X | + - - - |二、常见操作关于和 FTWRL MDL 相关锁。
对于 DML\FOR UPDATE：需要获取 GLOBAL 的 IX 锁持有到语句结束，但是 TABLE MDL 持有到事物结束一般为 (MDL_SHARED_WRITE(SW) )，DML 提交的时候会持有 COMMIT 的 IX 锁。
SELECT：不需要 GLOBAL 的 IX 锁，但是 TABLE 级别的 MDL 需要持有到事物事物结束一般为 (MDL_SHARED_READ(SR))。
DDL：需要获取 GLOBAL 的 IX 锁到语句结束，TABLE MDL 多变。
三、FTWRL 的被什么堵塞 1、长时间的 DDL\DML\FOR UPDATE 堵塞 FTWRL，因为 FTWRL 需要获取 GLOBAL 的 S 锁，而这些语句都会对 GLOBAL 持有 IX（MDL_INTENTION_EXCLUSIVE）锁，根据兼容矩阵不兼容。
等待为：Waiting for global read lock
堵塞栈帧：
#0 0x0000003f7480ba5e in pthread_cond_timedwait@@GLIBC_2.3.2 () from /lib64/libpthread.so.0#1 0x00000000018e7f33 in native_cond_timedwait (cond=0x7fff28009688, mutex=0x7fff28009640, abstime=0x7ffff0318a00)
at /root/mysql5.7.14/percona-server-5.7.14-7/include/thr_cond.h:129#2 0x00000000018e82a2 in safe_cond_timedwait (cond=0x7fff28009688, mp=0x7fff28009618, abstime=0x7ffff0318a00,
file=0x20013b8 /root/mysql5.7.14/percona-server-5.7.14-7/sql/mdl.cc , line=1899) at /root/mysql5.7.14/percona-server-5.7.14-7/mysys/thr_cond.c:88#3 0x00000000014a0149 in my_cond_timedwait (cond=0x7fff28009688, mp=0x7fff28009618, abstime=0x7ffff0318a00,
file=0x20013b8 /root/mysql5.7.14/percona-server-5.7.14-7/sql/mdl.cc , line=1899) at /root/mysql5.7.14/percona-server-5.7.14-7/include/thr_cond.h:180#4 0x00000000014a06ac in inline_mysql_cond_timedwait (that=0x7fff28009688, mutex=0x7fff28009618, abstime=0x7ffff0318a00,
src_file=0x20013b8 /root/mysql5.7.14/percona-server-5.7.14-7/sql/mdl.cc , src_line=1899)
at /root/mysql5.7.14/percona-server-5.7.14-7/include/mysql/psi/mysql_thread.h:1229#5 0x00000000014a192b in MDL_wait::timed_wait (this=0x7fff28009618, owner=0x7fff28009510, abs_timeout=0x7ffff0318a00, set_status_on_timeout=true,
wait_state_name=0x2d132c0) at /root/mysql5.7.14/percona-server-5.7.14-7/sql/mdl.cc:1899#6 0x00000000014a38a7 in MDL_context::acquire_lock (this=0x7fff28009618, mdl_request=0x7ffff0318a70, lock_wait_timeout=31536000)
at /root/mysql5.7.14/percona-server-5.7.14-7/sql/mdl.cc:3714#7 0x00000000017629ef in Global_read_lock::lock_global_read_lock (this=0x7fff2800b300, thd=0x7fff28009510)
at /root/mysql5.7.14/percona-server-5.7.14-7/sql/lock.cc:1131#8 0x00000000015ec19a in reload_acl_and_cache (thd=0x7fff28009510, options=16388, tables=0x0, write_to_binlog=0x7ffff03199fc)
at /root/mysql5.7.14/percona-server-5.7.14-7/sql/sql_reload.cc:222#9 0x00000000015a9c27 in mysql_execute_command (thd=0x7fff28009510, first_level=true) at /root/mysql5.7.14/percona-server-5.7.14-7/sql/sql_parse.cc:4379#10 0x00000000015adcd6 in mysql_parse (thd=0x7fff28009510, parser_state=0x7ffff031a600) at /root/mysql5.7.14/percona-server-5.7.14-7/sql/sql_parse.cc:5836#11 0x00000000015a1b95 in dispatch_command (thd=0x7fff28009510, com_data=0x7ffff031ad70, command=COM_QUERY)
at /root/mysql5.7.14/percona-server-5.7.14-7/sql/sql_parse.cc:1447#12 0x00000000015a09c6 in do_command (thd=0x7fff28009510) at /root/mysql5.7.14/percona-server-5.7.14-7/sql/sql_parse.cc:1010#13 0x00000000016e29d0 in handle_connection (arg=0x33d01c0) at /root/mysql5.7.14/percona-server-5.7.14-7/sql/conn_handler/connection_handler_per_thread.cc:312#14 0x0000000001d7b4b0 in pfs_spawn_thread (arg=0x38424d0) at /root/mysql5.7.14/percona-server-5.7.14-7/storage/perfschema/pfs.cc:2188#15 0x0000003f74807aa1 in start_thread () from /lib64/libpthread.so.0#16 0x0000003f740e8bcd in clone () from /lib64/libc.so.62、长时间的 select 堵塞 FTWRL，因为 FTWRL 要关闭所有的表，如果有活跃的语句正在执行 table cache 不能清空，因此需要等待。实际上即便是 flush tables 也不能在有语句执行的时候执行同样需要等待。
等待为：Waiting for table flush
堵塞栈帧：
#0 0x0000003f7480ba5e in pthread_cond_timedwait@@GLIBC_2.3.2 () from /lib64/libpthread.so.0#1 0x00000000018e7f33 in native_cond_timedwait (cond=0x7fff28009688, mutex=0x7fff28009640, abstime=0x7ffff0318be0)
at /root/mysql5.7.14/percona-server-5.7.14-7/include/thr_cond.h:129#2 0x00000000018e82a2 in safe_cond_timedwait (cond=0x7fff28009688, mp=0x7fff28009618, abstime=0x7ffff0318be0,
file=0x20013b8 /root/mysql5.7.14/percona-server-5.7.14-7/sql/mdl.cc , line=1899) at /root/mysql5.7.14/percona-server-5.7.14-7/mysys/thr_cond.c:88#3 0x00000000014a0149 in my_cond_timedwait (cond=0x7fff28009688, mp=0x7fff28009618, abstime=0x7ffff0318be0,
file=0x20013b8 /root/mysql5.7.14/percona-server-5.7.14-7/sql/mdl.cc , line=1899) at /root/mysql5.7.14/percona-server-5.7.14-7/include/thr_cond.h:180#4 0x00000000014a06ac in inline_mysql_cond_timedwait (that=0x7fff28009688, mutex=0x7fff28009618, abstime=0x7ffff0318be0,
src_file=0x20013b8 /root/mysql5.7.14/percona-server-5.7.14-7/sql/mdl.cc , src_line=1899)
at /root/mysql5.7.14/percona-server-5.7.14-7/include/mysql/psi/mysql_thread.h:1229#5 0x00000000014a192b in MDL_wait::timed_wait (this=0x7fff28009618, owner=0x7fff28009510, abs_timeout=0x7ffff0318be0, set_status_on_timeout=true,
wait_state_name=0x2d09d00) at /root/mysql5.7.14/percona-server-5.7.14-7/sql/mdl.cc:1899#6 0x00000000016a48ca in TABLE_SHARE::wait_for_old_version (this=0x7fff58984190, thd=0x7fff28009510, abstime=0x7ffff0318be0, deadlock_weight=100)
at /root/mysql5.7.14/percona-server-5.7.14-7/sql/table.cc:4585#7 0x0000000001513e3a in close_cached_tables (thd=0x7fff28009510, tables=0x0, wait_for_refresh=true, timeout=31536000)
at /root/mysql5.7.14/percona-server-5.7.14-7/sql/sql_base.cc:1289#8 0x00000000015ec1d8 in reload_acl_and_cache (thd=0x7fff28009510, options=16388, tables=0x0, write_to_binlog=0x7ffff03199fc)
at /root/mysql5.7.14/percona-server-5.7.14-7/sql/sql_reload.cc:224#9 0x00000000015a9c27 in mysql_execute_command (thd=0x7fff28009510, first_level=true) at /root/mysql5.7.14/percona-server-5.7.14-7/sql/sql_parse.cc:4379#10 0x00000000015adcd6 in mysql_parse (thd=0x7fff28009510, parser_state=0x7ffff031a600) at /root/mysql5.7.14/percona-server-5.7.14-7/sql/sql_parse.cc:5836#11 0x00000000015a1b95 in dispatch_command (thd=0x7fff28009510, com_data=0x7ffff031ad70, command=COM_QUERY)
at /root/mysql5.7.14/percona-server-5.7.14-7/sql/sql_parse.cc:1447#12 0x00000000015a09c6 in do_command (thd=0x7fff28009510) at /root/mysql5.7.14/percona-server-5.7.14-7/sql/sql_parse.cc:1010#13 0x00000000016e29d0 in handle_connection (arg=0x33d01c0) at /root/mysql5.7.14/percona-server-5.7.14-7/sql/conn_handler/connection_handler_per_thread.cc:312#14 0x0000000001d7b4b0 in pfs_spawn_thread (arg=0x38424d0) at /root/mysql5.7.14/percona-server-5.7.14-7/storage/perfschema/pfs.cc:2188#15 0x0000003f74807aa1 in start_thread () from /lib64/libpthread.so.0#16 0x0000003f740e8bcd in clone () from /lib64/libc.so.63、长时间的 commit(如大事物提交) 也会堵塞 FTWRL，因为 FTWRL 需要获取 COMMIT 的 S 锁，而 commit 语句会对 commit 持有 IX（MDL_INTENTION_EXCLUSIVE）锁，根据兼容矩阵不兼容。
等待为 Waiting for commit lock
基本我们看到所有的语句都会堵塞 FTWRL。
四、FTWRL 堵塞什么 1、FTWRL 会堵塞 DDL\DML\FOR UPDATE 操作
等待为：Waiting for global read lock
堵塞栈帧：
#0 0x0000003f7480ba5e in pthread_cond_timedwait@@GLIBC_2.3.2 () from /lib64/libpthread.so.0#1 0x00000000018e7f33 in native_cond_timedwait (cond=0x7fff58000ee8, mutex=0x7fff58000ea0, abstime=0x7ffff0359660)
at /root/mysql5.7.14/percona-server-5.7.14-7/include/thr_cond.h:129#2 0x00000000018e82a2 in safe_cond_timedwait (cond=0x7fff58000ee8, mp=0x7fff58000e78, abstime=0x7ffff0359660,
file=0x20013b8 /root/mysql5.7.14/percona-server-5.7.14-7/sql/mdl.cc , line=1899) at /root/mysql5.7.14/percona-server-5.7.14-7/mysys/thr_cond.c:88#3 0x00000000014a0149 in my_cond_timedwait (cond=0x7fff58000ee8, mp=0x7fff58000e78, abstime=0x7ffff0359660,
file=0x20013b8 /root/mysql5.7.14/percona-server-5.7.14-7/sql/mdl.cc , line=1899) at /root/mysql5.7.14/percona-server-5.7.14-7/include/thr_cond.h:180#4 0x00000000014a06ac in inline_mysql_cond_timedwait (that=0x7fff58000ee8, mutex=0x7fff58000e78, abstime=0x7ffff0359660,
src_file=0x20013b8 /root/mysql5.7.14/percona-server-5.7.14-7/sql/mdl.cc , src_line=1899)
at /root/mysql5.7.14/percona-server-5.7.14-7/include/mysql/psi/mysql_thread.h:1229#5 0x00000000014a192b in MDL_wait::timed_wait (this=0x7fff58000e78, owner=0x7fff58000d70, abs_timeout=0x7ffff0359660, set_status_on_timeout=true,
wait_state_name=0x2d132c0) at /root/mysql5.7.14/percona-server-5.7.14-7/sql/mdl.cc:1899#6 0x00000000014a38a7 in MDL_context::acquire_lock (this=0x7fff58000e78, mdl_request=0x7ffff03596e0, lock_wait_timeout=31536000)
at /root/mysql5.7.14/percona-server-5.7.14-7/sql/mdl.cc:3714#7 0x0000000001517a08 in open_table (thd=0x7fff58000d70, table_list=0x7fff58006a70, ot_ctx=0x7ffff0359b00)
at /root/mysql5.7.14/percona-server-5.7.14-7/sql/sql_base.cc:3227#8 0x000000000151b24f in open_and_process_table (thd=0x7fff58000d70, lex=0x7fff58003350, tables=0x7fff58006a70, counter=0x7fff58003410, flags=0,
prelocking_strategy=0x7ffff0359c30, has_prelocking_list=false, ot_ctx=0x7ffff0359b00) at /root/mysql5.7.14/percona-server-5.7.14-7/sql/sql_base.cc:5171#9 0x000000000151c3ab in open_tables (thd=0x7fff58000d70, start=0x7ffff0359bf0, counter=0x7fff58003410, flags=0, prelocking_strategy=0x7ffff0359c30)
at /root/mysql5.7.14/percona-server-5.7.14-7/sql/sql_base.cc:5789#10 0x000000000151d7e5 in open_tables_for_query (thd=0x7fff58000d70, tables=0x7fff58006a70, flags=0) at /root/mysql5.7.14/percona-server-5.7.14-7/sql/sql_base.cc:6564#11 0x00000000015acb58 in execute_sqlcom_select (thd=0x7fff58000d70, all_tables=0x7fff58006a70) at /root/mysql5.7.14/percona-server-5.7.14-7/sql/sql_parse.cc:5362#12 0x00000000015a5320 in mysql_execute_command (thd=0x7fff58000d70, first_level=true) at /root/mysql5.7.14/percona-server-5.7.14-7/sql/sql_parse.cc:2889#13 0x00000000015adcd6 in mysql_parse (thd=0x7fff58000d70, parser_state=0x7ffff035b600) at /root/mysql5.7.14/percona-server-5.7.14-7/sql/sql_parse.cc:5836#14 0x00000000015a1b95 in dispatch_command (thd=0x7fff58000d70, com_data=0x7ffff035bd70, command=COM_QUERY)
at /root/mysql5.7.14/percona-server-5.7.14-7/sql/sql_parse.cc:1447#15 0x00000000015a09c6 in do_command (thd=0x7fff58000d70) at /root/mysql5.7.14/percona-server-5.7.14-7/sql/sql_parse.cc:1010#16 0x00000000016e29d0 in handle_connection (arg=0x346f2b0) at /root/mysql5.7.14/percona-server-5.7.14-7/sql/conn_handler/connection_handler_per_thread.cc:312#17 0x0000000001d7b4b0 in pfs_spawn_thread (arg=0x38424d0) at /root/mysql5.7.14/percona-server-5.7.14-7/storage/perfschema/pfs.cc:2188#18 0x0000003f74807aa1 in start_thread () from /lib64/libpthread.so.0#19 0x0000003f740e8bcd in clone () from /lib64/libc.so.62、FTWRL 会堵塞 commit 操作
等待为 Waiting for commit lock
堵塞栈帧：
#0 0x0000003f7480ba5e in pthread_cond_timedwait@@GLIBC_2.3.2 () from /lib64/libpthread.so.0#1 0x00000000018e7f33 in native_cond_timedwait (cond=0x7fff58000ee8, mutex=0x7fff58000ea0, abstime=0x7ffff03599a0)
at /root/mysql5.7.14/percona-server-5.7.14-7/include/thr_cond.h:129#2 0x00000000018e82a2 in safe_cond_timedwait (cond=0x7fff58000ee8, mp=0x7fff58000e78, abstime=0x7ffff03599a0,
file=0x20013b8 /root/mysql5.7.14/percona-server-5.7.14-7/sql/mdl.cc , line=1899) at /root/mysql5.7.14/percona-server-5.7.14-7/mysys/thr_cond.c:88#3 0x00000000014a0149 in my_cond_timedwait (cond=0x7fff58000ee8, mp=0x7fff58000e78, abstime=0x7ffff03599a0,
file=0x20013b8 /root/mysql5.7.14/percona-server-5.7.14-7/sql/mdl.cc , line=1899) at /root/mysql5.7.14/percona-server-5.7.14-7/include/thr_cond.h:180#4 0x00000000014a06ac in inline_mysql_cond_timedwait (that=0x7fff58000ee8, mutex=0x7fff58000e78, abstime=0x7ffff03599a0,
src_file=0x20013b8 /root/mysql5.7.14/percona-server-5.7.14-7/sql/mdl.cc , src_line=1899)
at /root/mysql5.7.14/percona-server-5.7.14-7/include/mysql/psi/mysql_thread.h:1229#5 0x00000000014a192b in MDL_wait::timed_wait (this=0x7fff58000e78, owner=0x7fff58000d70, abs_timeout=0x7ffff03599a0, set_status_on_timeout=true,
wait_state_name=0x2d13380) at /root/mysql5.7.14/percona-server-5.7.14-7/sql/mdl.cc:1899#6 0x00000000014a38a7 in MDL_context::acquire_lock (this=0x7fff58000e78, mdl_request=0x7ffff0359a10, lock_wait_timeout=31536000)
at /root/mysql5.7.14/percona-server-5.7.14-7/sql/mdl.cc:3714#7 0x0000000000f63bd9 in ha_commit_trans (thd=0x7fff58000d70, all=true, ignore_global_read_lock=false)
at /root/mysql5.7.14/percona-server-5.7.14-7/sql/handler.cc:1783#8 0x00000000016b36bb in trans_commit (thd=0x7fff58000d70) at /root/mysql5.7.14/percona-server-5.7.14-7/sql/transaction.cc:239#9 0x00000000015aa1f1 in mysql_execute_command (thd=0x7fff58000d70, first_level=true) at /root/mysql5.7.14/percona-server-5.7.14-7/sql/sql_parse.cc:4472#10 0x00000000015adcd6 in mysql_parse (thd=0x7fff58000d70, parser_state=0x7ffff035b600) at /root/mysql5.7.14/percona-server-5.7.14-7/sql/sql_parse.cc:5836#11 0x00000000015a1b95 in dispatch_command (thd=0x7fff58000d70, com_data=0x7ffff035bd70, command=COM_QUERY)
at /root/mysql5.7.14/percona-server-5.7.14-7/sql/sql_parse.cc:1447#12 0x00000000015a09c6 in do_command (thd=0x7fff58000d70) at /root/mysql5.7.14/percona-server-5.7.14-7/sql/sql_parse.cc:1010#13 0x00000000016e29d0 in handle_connection (arg=0x346f2b0) at /root/mysql5.7.14/percona-server-5.7.14-7/sql/conn_handler/connection_handler_per_thread.cc:312#14 0x0000000001d7b4b0 in pfs_spawn_thread (arg=0x38424d0) at /root/mysql5.7.14/percona-server-5.7.14-7/storage/perfschema/pfs.cc:2188#15 0x0000003f74807aa1 in start_thread () from /lib64/libpthread.so.0#16 0x0000003f740e8bcd in clone () from /lib64/libc.so.63、FTWRL 不会堵塞 SELECT 操作五、percona 对 xtrbackup 的改进
对于 FTWRL percona 做了改进应该是 backup lock 和 binlog lock 来代替。
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