你的数据库服务器IO调度算法不对，难怪那么慢
之前的推文已经分享了数据库优化的方法，链接为https://mp.weixin.qq.com/s/6Atzk9UKPJRxxAs0nsKBXg 。其中操作系统部分介绍了IO调度算法的优化，本文将通过压力测试的方式来对比不同的调度算法下磁盘IO的表现。
1 准备工作
1.1 安装sysbench
本次采用sysbench进行压测，先安装sysbench，步骤如下：
curl -s https://packagecloud.io/install/repositories/akopytov/sysbench/script.rpm.sh | sudo bash
yum -y install sysbench
sysbench --version
1.2 准备测试文件
生成后续需要使用的测试文件，block大小为16k（MySQL DBA都懂的，哈哈），创建4个文件，合计20G
[root@mha1 ~]# sysbench fileio --file-num=4 --file-block-size=16384 --file-total-size=20G prepare
sysbench 1.0.20 (using bundled LuaJIT 2.1.0-beta2)
4 files, 5242880Kb each, 20480Mb total
Creating files for the test...
Extra file open flags: (none)
Creating file test_file.0
Creating file test_file.1
Creating file test_file.2
Creating file test_file.3
21474836480 bytes written in 47.94 seconds (427.24 MiB/sec).
1.3 准备测试表因为也要进行数据库读写方面的测试，因此需要先创建相关表及数据
[root@mha1 ~]# sysbench --db-driver=mysql --time=300 --threads=10 --report-interval=1 --mysql-host=127.0.0.1 --mysql-port=3306 --mysql-user=root --mysql-password=123456 --mysql-db=testdb --tables=20 --table_size=1000000 oltp_insert --db-ps-mode=disable prepare
sysbench 1.0.20 (using bundled LuaJIT 2.1.0-beta2)
Initializing worker threads...
Creating table 'sbtest3'...
Creating table 'sbtest2'...
Creating table 'sbtest5'...
Creating table 'sbtest6'...
Creating table 'sbtest8'...
Creating table 'sbtest7'...
Creating table 'sbtest4'...
Creating table 'sbtest1'...
Creating table 'sbtest10'...
Creating table 'sbtest9'...
Inserting 1000000 records into 'sbtest4'
Inserting 1000000 records into 'sbtest7'
Inserting 1000000 records into 'sbtest5'
Inserting 1000000 records into 'sbtest3'
Inserting 1000000 records into 'sbtest1'
Inserting 1000000 records into 'sbtest2'
Inserting 1000000 records into 'sbtest8'
Inserting 1000000 records into 'sbtest6'
Inserting 1000000 records into 'sbtest10'
Inserting 1000000 records into 'sbtest9'
Creating a secondary index on 'sbtest7'...
Creating a secondary index on 'sbtest10'...
Creating a secondary index on 'sbtest8'...
Creating a secondary index on 'sbtest5'...
Creating a secondary index on 'sbtest2'...
Creating a secondary index on 'sbtest9'...
Creating a secondary index on 'sbtest1'...
Creating table 'sbtest17'...
Creating a secondary index on 'sbtest3'...
Inserting 1000000 records into 'sbtest17'
Creating a secondary index on 'sbtest4'...
Creating a secondary index on 'sbtest6'...
Creating table 'sbtest20'...
Inserting 1000000 records into 'sbtest20'
Creating table 'sbtest18'...
Inserting 1000000 records into 'sbtest18'
Creating table 'sbtest15'...
Inserting 1000000 records into 'sbtest15'
Creating table 'sbtest19'...
Inserting 1000000 records into 'sbtest19'
Creating table 'sbtest14'...
Inserting 1000000 records into 'sbtest14'
Creating table 'sbtest11'...
Inserting 1000000 records into 'sbtest11'
Creating table 'sbtest13'...
Creating table 'sbtest12'...
Inserting 1000000 records into 'sbtest13'
Inserting 1000000 records into 'sbtest12'
Creating table 'sbtest16'...
Inserting 1000000 records into 'sbtest16'
Creating a secondary index on 'sbtest17'...
Creating a secondary index on 'sbtest20'...
Creating a secondary index on 'sbtest18'...
Creating a secondary index on 'sbtest19'...
Creating a secondary index on 'sbtest15'...
Creating a secondary index on 'sbtest11'...
Creating a secondary index on 'sbtest12'...
Creating a secondary index on 'sbtest13'...
Creating a secondary index on 'sbtest14'...
Creating a secondary index on 'sbtest16'...
2 查看支持的调度算法
本次磁盘为SSD硬盘，操作系统版本文Centos7.8 。下面将调度算法修改为三种不同的值来进行随机读与随机写的压力测试
本系统为Centos7.8,需要查看支持的IO调度算法，然后再进行修改测试。
[root@mha1 ~]# dmesg | grep -i scheduler
[ 4.885816] io scheduler noop registered
[ 4.885820] io scheduler deadline registered (default)
[ 4.885867] io scheduler cfq registered
[ 4.885870] io scheduler mq-deadline registered
[ 4.885872] io scheduler kyber registered
可见，在本系统中，默认的调度算法为 deadline。
也可以通过如下命令查看当前的调度算法，其中中括号里代表当前使用的调度算法。
[root@mha1 ~]# cat /sys/block/sda/queue/scheduler
noop [deadline] cfq
3 deadline算法
Deadline在机械盘的情况下对数据库环境(ORACLE RAC,MySQL等)是最好的选择。下面将进行随机写与随机读的压力测试
3.1 随机写
[root@mha1 ~]# sysbench fileio \
> --time=180 \
> --threads=24 \
> --file-total-size=20G \
> --file-test-mode=rndwr \
> --file-num=4 \
> --file-extra-flags=direct \
> --file-fsync-freq=0 \
> --file-block-size=16384 \
> run
sysbench 1.0.20 (using bundled LuaJIT 2.1.0-beta2)
Running the test with following options:
Number of threads: 24
Initializing random number generator from current time
Extra file open flags: directio
4 files, 5GiB each
20GiB total file size
Block size 16KiB
Number of IO requests: 0
Read/Write ratio for combined random IO test: 1.50
Calling fsync() at the end of test, Enabled.
Using synchronous I/O mode
Doing random write test
Initializing worker threads...
Threads started!
File operations:
reads/s: 0.00
writes/s: 6935.37
fsyncs/s: 0.53
Throughput:
read, MiB/s: 0.00
written, MiB/s: 108.37
General statistics:
total time: 180.0138s
total number of events: 1248484
Latency (ms):
min: 0.10
avg: 3.46
max: 107.39
95th percentile: 14.73
sum: 4317610.93
Threads fairness:
events (avg/stddev): 52020.1667/426.95
execution time (avg/stddev): 179.9005/0.01
随机写的iops为6935.37，磁盘写入速度是108.37MiB/s
3.2 随机读
[root@mha1 ~]# sysbench fileio \
> --time=180 \
> --threads=24 \
> --file-total-size=20G \
> --file-test-mode=rndrd \
> --file-num=4 \
> --file-extra-flags=direct \
> --file-fsync-freq=0 \
> --file-block-size=16384 \
> run
sysbench 1.0.20 (using bundled LuaJIT 2.1.0-beta2)
Running the test with following options:
Number of threads: 24
Initializing random number generator from current time
Extra file open flags: directio
4 files, 5GiB each
20GiB total file size
Block size 16KiB
Number of IO requests: 0
Read/Write ratio for combined random IO test: 1.50
Calling fsync() at the end of test, Enabled.
Using synchronous I/O mode
Doing random read test
Initializing worker threads...
Threads started!
File operations:
reads/s: 7956.88
writes/s: 0.00
fsyncs/s: 0.00
Throughput:
read, MiB/s: 124.33
written, MiB/s: 0.00
General statistics:
total time: 180.0075s
total number of events: 1432313
Latency (ms):
min: 0.10
avg: 3.01
max: 322.24
95th percentile: 5.47
sum: 4309094.67
Threads fairness:
events (avg/stddev): 59679.7083/2688.56
execution time (avg/stddev): 179.5456/0.18
随机读的iops为7956.88，磁盘读取速度是124.33MiB/s
3.3 测试数据库写
sysbench --db-driver=mysql --time=300 --threads=8 --report-interval=1 --mysql-host=127.0.0.1 --mysql-port=3306 --mysql-user=root --mysql-password=123456 --mysql-db=testdb --tables=10 --table_size=1000000 oltp_write_only --db-ps-mode=disable run
SQL statistics:
queries performed:
read: 0
write: 589934
other: 294968
total: 884902
transactions: 147483 (491.43 per sec.)
queries: 884902 (2948.62 per sec.)
ignored errors: 2 (0.01 per sec.)
reconnects: 0 (0.00 per sec.)
General statistics:
total time: 300.1050s
total number of events: 147483
Latency (ms):
min: 2.58
avg: 16.27
max: 2608.34
95th percentile: 35.59
sum: 2399415.58
Threads fairness:
events (avg/stddev): 18435.3750/90.33
execution time (avg/stddev): 299.9269/0.04
可见，随机写入的TPS为491.43 ,查询次数为2948.62
3.4 测试数据库读
sysbench --db-driver=mysql --time=300 --threads=8 --report-interval=1 --mysql-host=127.0.0.1 --mysql-port=3306 --mysql-user=root --mysql-password=123456 --mysql-db=testdb --tables=10 --table_size=1000000 oltp_read_only --db-ps-mode=disable run
SQL statistics:
queries performed:
read: 1651692
write: 0
other: 235956
total: 1887648
transactions: 117978 (393.13 per sec.)
queries: 1887648 (6290.13 per sec.)
ignored errors: 0 (0.00 per sec.)
reconnects: 0 (0.00 per sec.)
General statistics:
total time: 300.0949s
total number of events: 117978
Latency (ms):
min: 3.08
avg: 20.34
max: 170.48
95th percentile: 29.19
sum: 2399636.31
Threads fairness:
events (avg/stddev): 14747.2500/1513.84
execution time (avg/stddev): 299.9545/0.04
可见，随机读时的TPS为393.13 ,查询次数为6290.13
4 noop算法
4.1 修改为noop算法
noop称为电梯调度算法，是基于FIFO队列实现的。所有的请求都是先进先出的，因为SSD的随机读、随机写速度快，因此该算法适合SSD硬盘。
[root@mha1 ~]# echo 'noop' >/sys/block/sda/queue/scheduler
[root@mha1 ~]# cat /sys/block/sda/queue/scheduler
[noop] deadline cfq
4.2 随机写
[root@mha1 ~]# sysbench fileio \
> --time=180 \
> --threads=24 \
> --file-total-size=20G \
> --file-test-mode=rndwr \
> --file-num=4 \
> --file-extra-flags=direct \
> --file-fsync-freq=0 \
> --file-block-size=16384 \
> run
sysbench 1.0.20 (using bundled LuaJIT 2.1.0-beta2)
Running the test with following options:
Number of threads: 24
Initializing random number generator from current time
Extra file open flags: directio
4 files, 5GiB each
20GiB total file size
Block size 16KiB
Number of IO requests: 0
Read/Write ratio for combined random IO test: 1.50
Calling fsync() at the end of test, Enabled.
Using synchronous I/O mode
Doing random write test
Initializing worker threads...
Threads started!
File operations:
reads/s: 0.00
writes/s: 7057.60
fsyncs/s: 0.53
Throughput:
read, MiB/s: 0.00
written, MiB/s: 110.27
General statistics:
total time: 180.0136s
total number of events: 1270481
Latency (ms):
min: 0.10
avg: 3.40
max: 240.39
95th percentile: 14.46
sum: 4317435.99
Threads fairness:
events (avg/stddev): 52936.7083/487.57
execution time (avg/stddev): 179.8932/0.02
随机写的iops为7057.60，磁盘写入速度是110.27MiB/s
4.3 随机读
[root@mha1 ~]# sysbench fileio \
> --time=180 \
> --threads=24 \
> --file-total-size=20G \
> --file-test-mode=rndrd \
> --file-num=4 \
> --file-extra-flags=direct \
> --file-fsync-freq=0 \
> --file-block-size=16384 \
> run
sysbench 1.0.20 (using bundled LuaJIT 2.1.0-beta2)
Running the test with following options:
Number of threads: 24
Initializing random number generator from current time
Extra file open flags: directio
4 files, 5GiB each
20GiB total file size
Block size 16KiB
Number of IO requests: 0
Read/Write ratio for combined random IO test: 1.50
Calling fsync() at the end of test, Enabled.
Using synchronous I/O mode
Doing random read test
Initializing worker threads...
Threads started!
File operations:
reads/s: 8399.89
writes/s: 0.00
fsyncs/s: 0.00
Throughput:
read, MiB/s: 131.25
written, MiB/s: 0.00
General statistics:
total time: 180.0100s
total number of events: 1512081
Latency (ms):
min: 0.10
avg: 2.85
max: 315.77
95th percentile: 5.00
sum: 4312384.33
Threads fairness:
events (avg/stddev): 63003.3750/10086.77
execution time (avg/stddev): 179.6827/0.12
随机读的iops为8399.89，磁盘读取速度是131.25MiB/s
4.4 数据库写入
sysbench --db-driver=mysql --time=300 --threads=8 --report-interval=1 --mysql-host=127.0.0.1 --mysql-port=3306 --mysql-user=root --mysql-password=123456 --mysql-db=testdb --tables=10 --table_size=1000000 oltp_write_only --db-ps-mode=disable run
SQL statistics:
queries performed:
read: 0
write: 653457
other: 326730
total: 980187
transactions: 163364 (544.38 per sec.)
queries: 980187 (3266.28 per sec.)
ignored errors: 2 (0.01 per sec.)
reconnects: 0 (0.00 per sec.)
General statistics:
total time: 300.0903s
total number of events: 163364
Latency (ms):
min: 2.62
avg: 14.69
max: 220.12
95th percentile: 32.53
sum: 2399040.57
Threads fairness:
events (avg/stddev): 20420.5000/112.69
execution time (avg/stddev): 299.8801/0.04
可见，随机写入的TPS为 544.38 ,查询次数为3266.28
4.5 数据库只读
sysbench --db-driver=mysql --time=300 --threads=8 --report-interval=1 --mysql-host=127.0.0.1 --mysql-port=3306 --mysql-user=root --mysql-password=123456 --mysql-db=testdb --tables=10 --table_size=1000000 oltp_read_only --db-ps-mode=disable run
SQL statistics:
queries performed:
read: 1596364
write: 0
other: 228052
total: 1824416
transactions: 114026 (379.97 per sec.)
queries: 1824416 (6079.59 per sec.)
ignored errors: 0 (0.00 per sec.)
reconnects: 0 (0.00 per sec.)
General statistics:
total time: 300.0869s
total number of events: 114026
Latency (ms):
min: 3.08
avg: 21.04
max: 321.03
95th percentile: 31.37
sum: 2399600.56
Threads fairness:
events (avg/stddev): 14253.2500/1475.71
execution time (avg/stddev): 299.9501/0.02
可见，只读时的TPS为 379.97,查询次数为6079.59
5 cfq算法
5.1 修改为cfq算法
cfq称为绝对公平调度算法，为每个进程及线程单独创建一个队列来管理IO请求，起到每个进程和线程均匀分布IO的效果。此算法适用于通用服务器，centos6中为默认的IO调度算法。
[root@mha1 ~]# echo 'cfq' >/sys/block/sda/queue/scheduler
[root@mha1 ~]# cat /sys/block/sda/queue/scheduler
noop deadline [cfq]
5.2 随机写
[root@mha1 ~]# sysbench fileio \
> --time=180 \
> --threads=24 \
> --file-total-size=20G \
> --file-test-mode=rndwr \
> --file-num=4 \
> --file-extra-flags=direct \
> --file-fsync-freq=0 \
> --file-block-size=16384 \
> run
sysbench 1.0.20 (using bundled LuaJIT 2.1.0-beta2)
Running the test with following options:
Number of threads: 24
Initializing random number generator from current time
Extra file open flags: directio
4 files, 5GiB each
20GiB total file size
Block size 16KiB
Number of IO requests: 0
Read/Write ratio for combined random IO test: 1.50
Calling fsync() at the end of test, Enabled.
Using synchronous I/O mode
Doing random write test
Initializing worker threads...
Threads started!
File operations:
reads/s: 0.00
writes/s: 6614.37
fsyncs/s: 0.53
Throughput:
read, MiB/s: 0.00
written, MiB/s: 103.35
General statistics:
total time: 180.0118s
total number of events: 1190677
Latency (ms):
min: 0.10
avg: 3.63
max: 348.78
95th percentile: 15.27
sum: 4317092.54
Threads fairness:
events (avg/stddev): 49611.5417/517.80
execution time (avg/stddev): 179.8789/0.03
随机写的iops为6614.37，磁盘写入速度是103.35MiB/s
5.3 随机读
[root@mha1 ~]# sysbench fileio \
> --time=180 \
> --threads=24 \
> --file-total-size=20G \
> --file-test-mode=rndrd \
> --file-num=4 \
> --file-extra-flags=direct \
> --file-fsync-freq=0 \
> --file-block-size=16384 \
> run
sysbench 1.0.20 (using bundled LuaJIT 2.1.0-beta2)
Running the test with following options:
Number of threads: 24
Initializing random number generator from current time
Extra file open flags: directio
4 files, 5GiB each
20GiB total file size
Block size 16KiB
Number of IO requests: 0
Read/Write ratio for combined random IO test: 1.50
Calling fsync() at the end of test, Enabled.
Using synchronous I/O mode
Doing random read test
Initializing worker threads...
Threads started!
File operations:
reads/s: 7481.39
writes/s: 0.00
fsyncs/s: 0.00
Throughput:
read, MiB/s: 116.90
written, MiB/s: 0.00
General statistics:
total time: 180.0086s
total number of events: 1346731
Latency (ms):
min: 0.10
avg: 3.20
max: 374.49
95th percentile: 5.77
sum: 4312382.07
Threads fairness:
events (avg/stddev): 56113.7917/3058.00
execution time (avg/stddev): 179.6826/0.17
随机读的iops为7481.39，磁盘读取速度是116.90MiB/s
5.4 数据库写
sysbench --db-driver=mysql --time=300 --threads=8 --report-interval=1 --mysql-host=127.0.0.1 --mysql-port=3306 --mysql-user=root --mysql-password=123456 --mysql-db=testdb --tables=10 --table_size=1000000 oltp_write_only --db-ps-mode=disable run
SQL statistics:
queries performed:
read: 0
write: 598765
other: 299384
total: 898149
transactions: 149691 (498.54 per sec.)
queries: 898149 (2991.25 per sec.)
ignored errors: 2 (0.01 per sec.)
reconnects: 0 (0.00 per sec.)
General statistics:
total time: 300.2552s
total number of events: 149691
Latency (ms):
min: 2.55
avg: 16.02
max: 779.62
95th percentile: 35.59
sum: 2397311.08
Threads fairness:
events (avg/stddev): 18711.3750/132.24
execution time (avg/stddev): 299.6639/0.38
可见，随机写入的TPS为498.54 ,查询次数为2991.25
5.5 数据库读
sysbench --db-driver=mysql --time=300 --threads=8 --report-interval=1 --mysql-host=127.0.0.1 --mysql-port=3306 --mysql-user=root --mysql-password=123456 --mysql-db=testdb --tables=10 --table_size=1000000 oltp_read_only --db-ps-mode=disable run
SQL statistics:
queries performed:
read: 1448342
write: 0
other: 206906
total: 1655248
transactions: 103453 (344.66 per sec.)
queries: 1655248 (5514.58 per sec.)
ignored errors: 0 (0.00 per sec.)
reconnects: 0 (0.00 per sec.)
General statistics:
total time: 300.1562s
total number of events: 103453
Latency (ms):
min: 3.11
avg: 23.19
max: 222.31
95th percentile: 38.25
sum: 2399486.55
Threads fairness:
events (avg/stddev): 12931.6250/1278.72
execution time (avg/stddev): 299.9358/0.01
可见，只读时的TPS为 344.66,查询次数为5514.58
6 小结
根据测试结果对比一下三种调度算法的读写速度
| 算法 | IOPS | 磁盘写速度 | IOPS | 磁盘读速度 | oltp_write_only | oltp_read_only |
| deadline | 6935.37 | 118.37MiB/s | 7956.88 | 124.33MiB/s | TPS为491.43 ,查询次数为2948.62 | TPS为393.13 ,查询次数为6290.13 |
| noop | 7057.60 | 110.27MiB/s | 8399.89 | 131.25MiB/s | TPS为 544.38 ,查询次数为3266.28 | TPS为 379.97,查询次数为6079.59 |
| cfq | 6614.37 | 103.35MiB/s | 7481.39 | 116.90MiB/s | TPS为498.54 ,查询次数为2991.25 | TPS为 344.66,查询次数为5514.58 |
因为本次测试环境为SSD硬盘，因此，在此情况下建议选择noop磁盘IO调度算法，此结论也符合我们的预期。
特别注意：磁盘IO的调度算法还需要根据磁盘情况、数据库类型、数据库架构、业务场景（OLTP、OLAP等）等各种场景进行区分，不同的场景调度算法也要调整，不可一概而论。如果不确定的话，建议进行压测来判断，选择符合对应场景下最合适的算法。
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