linux 磁盘io利用率高，分析的正确姿势 mysql数据库sql

一、背景简介作为一个DBA难免不了会遇到性能问题，那么我们遇到性能问题该如何进行排查呢？例如我们在高并发的业务下，出现业务响应慢，处理时间长我们又该如何入手进行排查，本片文章将分析io高的情况下如何分析及定位。
二、环境复现

环境配置：本次测试使用128C_512G_4TSSD服务器配置，MySQL版本为8.0.27
场景模拟：使用sysbench创建5个表，每个表2亿条数据，执行产生笛卡尔积查询的sql语句，产生io,可以模拟业务压力。
首先使用sysbench进行数据压测

三、系统层面底层故障排查

Shell> sysbench --test=/usr/local/share/sysbench/oltp_insert.lua --mysql-host=XXX --mysql-port=3306 --mysql-user=pcms --mysql-password=abc123 --mysql-db=sysbench --percentile=99 --table-size=2000000000 --tables=5 --threads=1000 prepare

使用sysbench进行模拟高并发

shell> sysbench --test=/usr/local/share/sysbench/oltp_write_only.lua --mysql-host=xxx --mysql-port=3306 --mysql-user=pcms --mysql-password=abc123 --mysql-db=sysbench --percentile=99 --table-size=2000000000 --tables=5 --threads=1000 --max-time=60000 --report-interval=1 --threads=1000 --max-requests=0 --mysql-ignore-errors=all run

执行笛卡尔积sql语句

mysql> select SQL_NO_CACHE b.id,a.k from sbtest_a a left join sbtest_b b on a.id=b.idgroup by a.k order by b.c desc;

3.1 检查当前服务器状态

shell> top top - 19:49:05 up 10 days,8:16,2 users,load average: 72.56, 40.21, 17.08 Tasks: 1288 total,1 running, 586 sleeping,0 stopped,0 zombie %Cpu(s): 19.7 us,4.2 sy,0.0 ni, 75.9 id,1.0 wa,0.0 hi,0.2 si,0.0 st KiB Mem : 53542118+total, 23667507+free, 22735366+used, 71392448 buff/cache KiB Swap:0 total,0 free,0 used. 23128006+avail Mem

由上可知：目前一分钟负载为72.56，且呈上升趋势，并且存在io压力
3.2查看当前各个磁盘设备的io情况

shell> iostat -m -x 1 Linux 4.14.0-115.el7a.0.1.aarch64 (mysql-4)01/08/2022_aarch64_(128 CPU) Device:rrqm/swrqm/sr/sw/srMB/swMB/s avgrq-sz avgqu-szawait r_await w_awaitsvctm%util sda0.000.00 14213.00 27430.00222.08465.1533.805.390.130.140.120.0286.00 sdb0.000.000.000.000.000.000.000.000.000.000.000.000.00 dm-00.000.000.000.000.000.000.000.000.000.000.000.000.00 dm-10.000.000.000.000.000.000.000.000.000.000.000.000.00 dm-20.000.000.000.000.000.000.000.000.000.000.000.000.00

【linux 磁盘io利用率高，分析的正确姿势】由上可知：目前有多块物理磁盘，sda磁盘的io压力较大
3.3 检查sda磁盘当前的io读写情况

shell> iostat -d /dev/sda -m -x 1 Linux 4.14.0-115.el7a.0.1.aarch64 (mysql-4)01/08/2022_aarch64_(128 CPU) Device:rrqm/swrqm/sr/sw/srMB/swMB/s avgrq-sz avgqu-szawait r_await w_awaitsvctm%util sda0.000.177.8681.230.293.9697.880.232.530.222.760.040.33 Device:rrqm/swrqm/sr/sw/srMB/swMB/s avgrq-sz avgqu-szawait r_await w_awaitsvctm%util sda0.000.00 21109.00 42839.00329.81710.9033.3319.470.300.160.370.0296.00 Device:rrqm/swrqm/sr/sw/srMB/swMB/s avgrq-sz avgqu-szawait r_await w_awaitsvctm%util sda0.000.00 19287.00 41404.00301.36692.2933.5315.730.260.180.300.0293.00 Device:rrqm/swrqm/sr/sw/srMB/swMB/s avgrq-sz avgqu-szawait r_await w_awaitsvctm%util sda0.000.00 22135.00 43044.00345.861165.1847.48100.871.550.202.240.0197.00

由上可知：目前sda磁盘的压力比较大，每秒写入比每秒读差距较大，证明目前有大量的io写入
3.4 检查sda磁盘中哪个应用程序占用的io比较高

shell> pidstat -d1 Linux 4.14.0-115.el7a.0.1.aarch64 (mysql-4)01/08/2022_aarch64_(128 CPU) 08:01:43 PMUIDPIDkB_rd/skB_wr/s kB_ccwr/sCommand 08:01:44 PM10007373962018.35 171346.790.00mysqld 08:01:44 PMUIDPIDkB_rd/skB_wr/s kB_ccwr/sCommand 08:01:45 PM100073739 145328.00 435216.000.00mysqld 08:01:45 PMUIDPIDkB_rd/skB_wr/s kB_ccwr/sCommand 08:01:46 PM100073739 141488.00 433232.000.00mysqld

由上可知：占用io高的应用程序是mysql，且pid为73739
3.5 分析应用程序中哪一个线程占用的io比较高

shell> pidstat -dt -p 73739 1执行两三秒即可 Average:1000-738230.00 233133.980.00|__mysqld Average:1000-746740.00 174291.260.00|__mysqld 11:56:18 PM1000-74770 124928.0074688.000.00|__mysqld 11:56:17 PM1000-74770 124603.7773358.490.00|__mysqld Average:1000-74770 124761.1774003.880.00|__mysqld

由上可知：74770这个线程占用的io比较高
3.6 分析这个线程在干什么？

Shell> perf trace -t 74770 -o /tmp/tmp_aa.pstrace Shell> cat /tmp/tmp_aa.pstrace 2850.656 ( 1.915 ms): futex(uaddr: 0x653ae9c4, op: WAIT|PRIVATE_FLAG, val: 1)= 0 2852.572 ( 0.001 ms): futex(uaddr: 0x653ae990, op: WAKE|PRIVATE_FLAG, val: 1)= 0 2852.601 ( 0.001 ms): clock_gettime(which_clock: MONOTONIC, tp: 0xfff7bd470f68)= 0 2852.690 ( 0.040 ms): write(fd: 159, buf: 0xd7a30020, count: 65536)= 65536 2852.796 ( 0.001 ms): clock_gettime(which_clock: MONOTONIC, tp: 0xfff7bd470f68)= 0 2852.798 ( 0.001 ms): clock_gettime(which_clock: MONOTONIC, tp: 0xfff7bd470f58)= 0 2852.939 ( 0.001 ms): clock_gettime(which_clock: MONOTONIC, tp: 0xfff7bd470f38)= 0 2852.950 ( 0.001 ms): clock_gettime(which_clock: MONOTONIC, tp: 0xfff7bd470f68)= 0 2852.977 ( 0.001 ms): clock_gettime(which_clock: MONOTONIC, tp: 0xfff7bd470f68)= 0 2853.029 ( 0.035 ms): write(fd: 64, buf: 0xcd51e020, count: 65536)= 65536 2853.164 ( 0.001 ms): clock_gettime(which_clock: MONOTONIC, tp: 0xfff7bd470f68)= 0 2853.167 ( 0.001 ms): clock_gettime(which_clock: MONOTONIC, tp: 0xfff7bd470f58)= 0 2853.302 ( 0.001 ms): clock_gettime(which_clock: MONOTONIC, tp: 0xfff7bd470f38)= 0

由上可知：目前这个线程在写入多个文件，fd为文件句柄，文件句柄号有64、159
3.7 查看这个文件句柄是什么

shell> lsof -p 73739|grep 159u mysqld 73739 mysql159uREG8,02121432467046482357 /mysql/mysqldata/16320fff-5fd5-4c47-889a-a9e1a8591d0d/tmp/#7046482357 (deleted) [root@mysql-4 ~]# lsof -p 73739|grep 64u mysqld 73739 mysql64uREG8,02118727246979323031 /mysql/mysqldata/16320fff-5fd5-4c47-889a-a9e1a8591d0d/tmp/#6979323031 (deleted)

由上可知：这个线程在大量的写入临时文件
四、分析MySQL应用程序 4.1 查看当前的会话列表

mysql> select * from information_schema.processlist where command !='sleep'; |9 | pcms| 172.16.76.12:57596 | sysbench | Query|67 | executing| select SQL_NO_CACHE b.id,a.k from sbtest_a a left join sbtest_b b on a.id=b.idgroup by a.k order by b.c desc |66477 |0 |0 |

由上可知：目前这个sql已经执行了67s，且此sql使用了group by和order by，必然会产生io
4.2 通过线程号查询会话

mysql> select * from threads where thread_os_id=74770\G; *************************** 1. row *************************** THREAD_ID: 95 NAME: thread/sql/one_connection TYPE: FOREGROUND PROCESSLIST_ID: 9 PROCESSLIST_USER: pcms PROCESSLIST_HOST: 172.16.76.12 PROCESSLIST_DB: sysbench PROCESSLIST_COMMAND: Query PROCESSLIST_TIME: 91 PROCESSLIST_STATE: NULL PROCESSLIST_INFO: select SQL_NO_CACHE b.id,a.k from sbtest_a a left join sbtest_b b on a.id=b.idgroup by a.k order by b.c desc PARENT_THREAD_ID: 1 ROLE: NULL INSTRUMENTED: YES HISTORY: YES CONNECTION_TYPE: TCP/IP THREAD_OS_ID: 74770 RESOURCE_GROUP: USR_default 1 row in set (0.00 sec)

由上可知：通过查询threads表可以进行验证，该线程在频繁创建临时表的原因就来源于此sql
4.3 查看该sql语句的执行计划，进行进一步认证

mysql> explain select SQL_NO_CACHE b.id,a.k from sbtest_a a left join sbtest_b b on a.id=b.idgroup by a.k order by b.c desc\G; *************************** 1. row *************************** id: 1 select_type: SIMPLE table: a partitions: NULL type: ALL possible_keys: NULL key: NULL key_len: NULL ref: NULL rows: 1 filtered: 100.00 Extra: Using temporary; Using filesort *************************** 2. row *************************** id: 1 select_type: SIMPLE table: b partitions: NULL type: eq_ref possible_keys: PRIMARY key: PRIMARY key_len: 4 ref: sysbench.a.id rows: 1 filtered: 100.00 Extra: NULL 2 rows in set, 2 warnings (0.00 sec)

由上可知：该sql的执行计划用到了临时表及临时文件，符合
4.4 查看全局状态进一步进行确认

mysql> show global status like '%tmp%'; +-------------------------+-------+ | Variable_name| Value | +-------------------------+-------+ | Created_tmp_disk_tables | 3| | Created_tmp_files| 22| | Created_tmp_tables| 8| +-------------------------+-------+

多执行几次，可以看出tmp_files和tmp_disk_tables的值在增长，证明在大量的创建临时文件及磁盘临时表，符合该线程的行为
五、故障处理通过上述的一系列排查，我们已经分析出来，目前sda磁盘的io使用率最高，且mysqld程序占用的最多，通过排查有一个线程在频繁的创建临时表或临时文件且通过登录mysql排查会话及线程视图可以找到是由某一个慢sql导致的，查看此慢sql的执行计划也会创建临时表和临时文件符合我们之前排查的预期。
此时我们就需要针对此慢sql进行优化，优化步骤由DBA进行处理，此处进行忽略。慢sql优化完成后可以进行io的继续观察，看io是否有下降
六、代码分析我们可以使用pstack进行跟踪线程号，获取当前的线程堆栈信息。切记pstack会调用gdb进行debug调试

shell> pstack 74770 >/tmp/74770.pstack Thread 1 (process 74770): #0ha_innobase::general_fetch (this=0xea654228, buf=0xea662028 "\212t\317\030\002", direction=1, match_mode=0) at /builds/naiwei.fang/percona-server/storage/innobase/handler/ha_innodb.cc:11159 #10x0000000000d9913c in handler::ha_rnd_next (this=0xea654228, buf=0xea662028 "\212t\317\030\002") at /build s/naiwei.fang/percona-server/sql/handler.cc:3173 #20x0000000000f77db0 in TableScanIterator::Read (this=0xd256d5e8) at /builds/naiwei.fang/percona-server/sql/ row_iterator.h:208 #30x000000000124c714 in WriteRowsToChunks (xxhash_seed=899339, write_to_build_chunk=true, write_rows_with_nu ll_in_join_key=false, join_key_buffer=0xd01fdb98, tables_to_get_rowid_for=0, chunks=0xd01fdb58, join_condition s=..., tables=..., iterator=0xd256d5e8, thd=0xdb888000) at /builds/naiwei.fang/percona-server/sql/hash_join_it erator.cc:282 #4HashJoinIterator::BuildHashTable (this=this@entry=0xd01fd028) at /builds/naiwei.fang/percona-server/sql/ha sh_join_iterator.cc:495 #50x000000000124c8ac in Init (this=0xd01fd028) at /builds/naiwei.fang/percona-server/sql/hash_join_iterator. cc:203 #6HashJoinIterator::Init (this=0xd01fd028) at /builds/naiwei.fang/percona-server/sql/hash_join_iterator.cc:1 45 #70x00000000010eca14 in Query_expression::ExecuteIteratorQuery (this=0xdec3a8b8, thd=thd@entry=0xdb888000) a t /builds/naiwei.fang/percona-server/sql/sql_union.cc:1224 #80x00000000010ecccc in Query_expression::execute (this=this@entry=0xdec3a8b8, thd=thd@entry=0xdb888000) at /builds/naiwei.fang/percona-server/sql/sql_union.cc:1284 #90x0000000001083db0 in Sql_cmd_dml::execute_inner (this=0xd256bcb0, thd=0xdb888000) at /builds/naiwei.fang/ percona-server/sql/sql_select.cc:791 #10 0x000000000108cac8 in Sql_cmd_dml::execute (this=0xd256bcb0, thd=0xdb888000) at /builds/naiwei.fang/percon a-server/sql/sql_select.cc:575 #11 0x00000000010384e8 in mysql_execute_command (thd=thd@entry=0xdb888000, first_level=first_level@entry=true) at /builds/naiwei.fang/percona-server/sql/sql_parse.cc:4677 #12 0x000000000103b314 in dispatch_sql_command (thd=thd@entry=0xdb888000, parser_state=parser_state@entry=0xff f7bd4735b0, update_userstat=update_userstat@entry=false) at /builds/naiwei.fang/percona-server/sql/sql_parse.c c:5273 #13 0x000000000103ccf0 in dispatch_command (thd=thd@entry=0xdb888000, com_data=https://www.it610.com/article/0xffffb467c4d0, com_data@entry= 0xfff7bd474640, command=COM_QUERY) at /builds/naiwei.fang/percona-server/sql/sql_parse.cc:1938 #14 0x000000000103da40 in do_command (thd=thd@entry=0xdb888000) at /builds/naiwei.fang/percona-server/sql/sql_ parse.cc:1386 #15 0x0000000001152ca8 in handle_connection (arg=arg@entry=0xda53ab10) at /builds/naiwei.fang/percona-server/s ql/conn_handler/connection_handler_per_thread.cc:307 #16 0x00000000022bc3ec in pfs_spawn_thread (arg=) at /builds/naiwei.fang/percona-server/storage /perfschema/pfs.cc:2899 #17 0x0000ffffb43c7c48 in start_thread () from /lib64/libpthread.so.0 #18 0x0000ffffb3c0f600 in thread_start () from /lib64/libc.so.6

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