linux磁盘顺序写、随机写的方法

一、前言

● 随机写会导致磁头不停地换道，造成效率的极大降低；顺序写磁头几乎不用换道，或者换道的时间很短
● 本文来讨论一下两者具体的差别以及相应的内核调用

二、环境准备

组件	版本
os	ubuntu 16.04.4 lts
fio	2.2.10

三、fio介绍

通过fio测试，能够反映在读写中的状态，我们需要重点关注fio的输出报告中的几个关键指标：
slat ：是指从 i/o 提交到实际执行 i/o 的时长（submission latency）
clat ：是指从 i/o 提交到 i/o 完成的时长（completion latency）
lat ：指的是从 fio 创建 i/o 到 i/o 完成的总时长
bw ：吞吐量
iops ：每秒 i/o 的次数

四、同步写测试

（1）同步随机写

主要采用fio作为测试工具，为了能够看到系统调用，使用strace工具，命令看起来是这样：

先来测试一个随机写

strace -f -tt -o /tmp/randwrite.log -d fio -name=randwrite -rw=randwrite \
-direct=1 -bs=4k -size=1g -numjobs=1 -group_reporting -filename=/tmp/test.db

提取关键信息

root@wilson-ubuntu:~# strace -f -tt -o /tmp/randwrite.log -d fio -name=randwrite -rw=randwrite \
> -direct=1 -bs=4k -size=1g -numjobs=1 -group_reporting -filename=/tmp/test.db
randwrite: (g=0): rw=randwrite, bs=4k-4k/4k-4k/4k-4k, ioengine=sync, iodepth=1
fio-2.2.10
starting 1 process
...
randwrite: (groupid=0, jobs=1): err= 0: pid=26882: wed aug 14 10:39:02 2019
 write: io=1024.0mb, bw=52526kb/s, iops=13131, runt= 19963msec
 clat (usec): min=42, max=18620, avg=56.15, stdev=164.79
  lat (usec): min=42, max=18620, avg=56.39, stdev=164.79
...
 bw (kb /s): min=50648, max=55208, per=99.96%, avg=52506.03, stdev=1055.83
...

run status group 0 (all jobs):
 write: io=1024.0mb, aggrb=52525kb/s, minb=52525kb/s, maxb=52525kb/s, mint=19963msec, maxt=19963msec

disk stats (read/write):
...
 sda: ios=0/262177, merge=0/25, ticks=0/7500, in_queue=7476, util=36.05%

列出了我们需要重点关注的信息：

 （1）clat ，平均时长56ms左右

 （2）lat ，平均时长56ms左右

 （3）bw ，吞吐量，大概在52m左右

再来看内核调用信息：

root@wilson-ubuntu:~# more /tmp/randwrite.log
...
26882 10:38:41.919904 lseek(3, 665198592, seek_set) = 665198592
26882 10:38:41.919920 write(3, "\220\240@\6\371\341\277>\0\200\36\31\0\0\0\0\202\2\7\320\343\6h\26p\340\277\370\330\30e\30"..., 4096) = 4096
26882 10:38:41.919969 lseek(3, 4313088, seek_set) = 4313088
26882 10:38:41.919985 write(3, "\220\240@\6\371\341\277>\0\200\36\31\0\0\0\0\202\2\7\320\343\6h\26p\340\277\370\330\30e\30"..., 4096) = 4096
26882 10:38:41.920032 lseek(3, 455880704, seek_set) = 455880704
26882 10:38:41.920048 write(3, "\220\240@\6\371\341\277>\0\200\36\31\0\0\0\0\202\2\7\320\343\6h\26p\340\277\370\330\30e\30"..., 4096) = 4096
26882 10:38:41.920096 lseek(3, 338862080, seek_set) = 338862080
26882 10:38:41.920112 write(3, "\220\240@\6\371\341\277>\0\2402\24\0\0\0\0\202\2\7\320\343\6h\26p\340\277\370\330\30e\30"..., 4096) = 4096
26882 10:38:41.920161 lseek(3, 739086336, seek_set) = 739086336
26882 10:38:41.920177 write(3, "\220\240@\6\371\341\277>\0\2402\24\0\0\0\0\202\2\7\320\343\6h\26p\340\277\370\330\30e\30"..., 4096) = 4096
26882 10:38:41.920229 lseek(3, 848175104, seek_set) = 848175104
26882 10:38:41.920245 write(3, "\220\240@\6\371\341\277>\0\2402\24\0\0\0\0\202\2\7\320\343\6h\26p\340\277\370\330\30e\30"..., 4096) = 4096
26882 10:38:41.920296 lseek(3, 1060147200, seek_set) = 1060147200
26882 10:38:41.920312 write(3, "\220\240@\6\371\341\277>\0\2402\24\0\0\0\0\202\2\7\320\343\6h\26p\340\277\370\330\30e\30"..., 4096) = 4096
26882 10:38:41.920362 lseek(3, 863690752, seek_set) = 863690752
26882 10:38:41.920377 write(3, "\220\240@\6\371\341\277>\0\2402\24\0\0\0\0\202\2\7\320\343\6h\26p\340\277\370\330\30e\30"..., 4096) = 4096
26882 10:38:41.920428 lseek(3, 279457792, seek_set) = 279457792
26882 10:38:41.920444 write(3, "\220\240@\6\371\341\277>\0\2402\24\0\0\0\0\202\2\7\320\343\6h\26p\340\277\370\330\30e\30"..., 4096) = 4096
26882 10:38:41.920492 lseek(3, 271794176, seek_set) = 271794176
26882 10:38:41.920508 write(3, "\220\240@\6\371\341\277>\0\2402\24\0\0\0\0\202\2\7\320\343\6h\26p\340\277\370\330\30e\30"..., 4096) = 4096
26882 10:38:41.920558 lseek(3, 1067864064, seek_set) = 1067864064
26882 10:38:41.920573 write(3, "\220\240@\6\371\341\277>\0\2402\24\0\0\0\0\202\2\7\320\343\6h\26p\340\277\370\330\30e\30"..., 4096) = 4096
...

随机读每一次写入之前都要通过lseek去定位当前的文件偏移量

同步顺序写

用刚才的方法来测试顺序写

root@wilson-ubuntu:~# strace -f -tt -o /tmp/write.log -d fio -name=write -rw=write \
-direct=1 -bs=4k -size=1g -numjobs=1 -group_reporting -filename=/tmp/test.db
write: (g=0): rw=write, bs=4k-4k/4k-4k/4k-4k, ioengine=sync, iodepth=1
fio-2.2.10
starting 1 process
jobs: 1 (f=1): [w(1)] [100.0% done] [0kb/70432kb/0kb /s] [0/17.7k/0 iops] [eta 00m:00s]
write: (groupid=0, jobs=1): err= 0: pid=27005: wed aug 14 10:53:02 2019
 write: io=1024.0mb, bw=70238kb/s, iops=17559, runt= 14929msec
 clat (usec): min=43, max=7464, avg=55.95, stdev=56.24
  lat (usec): min=43, max=7465, avg=56.15, stdev=56.25
...
 bw (kb /s): min=67304, max=72008, per=99.98%, avg=70225.38, stdev=1266.88
...

run status group 0 (all jobs):
 write: io=1024.0mb, aggrb=70237kb/s, minb=70237kb/s, maxb=70237kb/s, mint=14929msec, maxt=14929msec

disk stats (read/write):
...
 sda: ios=0/262162, merge=0/10, ticks=0/6948, in_queue=6932, util=46.49%

可以看到：

 吞吐量提升至70m左右

再来看一下内核调用：

root@wilson-ubuntu:~# more /tmp/write.log
...
27046 10:54:28.194508 write(3, "\0\0\23\0\0\0\0\0\0\300\16\0\0\0\0\0\0\360\t\0\0\0\0\0\0\320\17\0\0\0\0\0"..., 4096) = 4096
27046 10:54:28.194568 write(3, "\0\0\23\0\0\0\0\0\0\300\16\0\0\0\0\0\0\220\24\0\0\0\0\0\0\320\17\0\0\0\0\0"..., 4096) = 4096
27046 10:54:28.194627 write(3, "\0\0\23\0\0\0\0\0\0\300\16\0\0\0\0\0\0\220\24\0\0\0\0\0\0\320\17\0\0\0\0\0"..., 4096) = 4096
27046 10:54:28.194687 write(3, "\0\0\23\0\0\0\0\0\0\300\16\0\0\0\0\0\0\220\24\0\0\0\0\0\0\320\17\0\0\0\0\0"..., 4096) = 4096
27046 10:54:28.194747 write(3, "\0\0\23\0\0\0\0\0\0\300\16\0\0\0\0\0\0\220\24\0\0\0\0\0\0\320\17\0\0\0\0\0"..., 4096) = 4096
27046 10:54:28.194807 write(3, "\0\0\23\0\0\0\0\0\0\300\16\0\0\0\0\0\0\220\24\0\0\0\0\0\0\320\17\0\0\0\0\0"..., 4096) = 4096
27046 10:54:28.194868 write(3, "\0\0\23\0\0\0\0\0\0\300\16\0\0\0\0\0\0\220\24\0\0\0\0\0\0\320\17\0\0\0\0\0"..., 4096) = 4096
27046 10:54:28.194928 write(3, "\0\0\23\0\0\0\0\0\0\300\16\0\0\0\0\0\0\220\24\0\0\0\0\0\0\320\17\0\0\0\0\0"..., 4096) = 4096
27046 10:54:28.194988 write(3, "\0\0\23\0\0\0\0\0\0\300\16\0\0\0\0\0\0\220\24\0\0\0\0\0\0\320\17\0\0\0\0\0"..., 4096) = 4096
27046 10:54:28.195049 write(3, "\0\0\23\0\0\0\0\0\0\300\16\0\0\0\0\0\0\220\24\0\0\0\0\0\0\320\17\0\0\0\0\0"..., 4096) = 4096
27046 10:54:28.195110 write(3, "\0\0\23\0\0\0\0\0\0\300\16\0\0\0\0\0\0\220\24\0\0\0\0\0\0\320\17\0\0\0\0\0"..., 4096) = 4096
27046 10:54:28.195197 write(3, "\0\0\23\0\0\0\0\0\0\300\16\0\0\0\0\0\0\220\24\0\0\0\0\0\0\320\17\0\0\0\0\0"..., 4096) = 4096
27046 10:54:28.195262 write(3, "\0\0\23\0\0\0\0\0\0\300\16\0\0\0\0\0\0\220\24\0\0\0\0\0\0\320\17\0\0\0\0\0"..., 4096) = 4096
27046 10:54:28.195330 write(3, "\0\0\23\0\0\0\0\0\0\300\16\0\0\0\0\0\0\220\24\0\0\0\0\0\0\320\17\0\0\0\0\0"..., 4096) = 4096
27046 10:54:28.195426 write(3, "\0\0\23\0\0\0\0\0\0\300\16\0\0\0\0\0\0\220\24\0\0\0\0\0\0\320\17\0\0\0\0\0"..., 4096) = 4096
27046 10:54:28.195497 write(3, "\0\0\23\0\0\0\0\0\0\300\16\0\0\0\0\0\0\220\24\0\0\0\0\0\0\320\17\0\0\0\0\0"..., 4096) = 4096
27046 10:54:28.195567 write(3, "\0\0\23\0\0\0\0\0\0\300\16\0\0\0\0\0\0\220\24\0\0\0\0\0\0\320\17\0\0\0\0\0"..., 4096) = 4096
27046 10:54:28.195637 write(3, "\0\0\23\0\0\0\0\0\0\300\16\0\0\0\0\0\0\220\24\0\0\0\0\0\0\320\17\0\0\0\0\0"..., 4096) = 4096
27046 10:54:28.195704 write(3, "\0\0\23\0\0\0\0\0\0\300\16\0\0\0\0\0\0\220\24\0\0\0\0\0\0\320\17\0\0\0\0\0"..., 4096) = 4096
27046 10:54:28.195757 write(3, "\0\0\23\0\0\0\0\0\0\300\16\0\0\0\0\0\0\220\24\0\0\0\0\0\0\320\17\0\0\0\0\0"..., 4096) = 4096
27046 10:54:28.195807 write(3, "\0\0\23\0\0\0\0\0\0\300\16\0\0\0\0\0\0\220\24\0\0\0\0\0\0\320\17\0\0\0\0\0"..., 4096) = 4096
27046 10:54:28.195859 write(3, "\0\0\23\0\0\0\0\0\0\300\16\0\0\0\0\0\0\220\24\0\0\0\0\0\0\320\17\0\0\0\0\0"..., 4096) = 4096
27046 10:54:28.195910 write(3, "\0\0\23\0\0\0\0\0\0\300\16\0\0\0\0\0\0\220\24\0\0\0\0\0\0\320\17\0\0\0\0\0"..., 4096) = 4096
27046 10:54:28.195961 write(3, "\0\0\23\0\0\0\0\0\0\300\16\0\0\0\0\0\0\220\24\0\0\0\0\0\0\320\17\0\0\0\0\0"..., 4096) = 4096
27046 10:54:28.196012 write(3, "\0\0\23\0\0\0\0\0\0\300\16\0\0\0\0\0\0\220\24\0\0\0\0\0\0\320\17\0\0\0\0\0"..., 4096) = 4096
27046 10:54:28.196062 write(3, "\0\0\23\0\0\0\0\0\0\300\16\0\0\0\0\0\0\220\24\0\0\0\0\0\0\320\17\0\0\0\0\0"..., 4096) = 4096
27046 10:54:28.196112 write(3, "\0\0\23\0\0\0\0\0\0\300\16\0\0\0\0\0\0 \26\0\0\0\0\0\0\320\17\0\0\0\0\0"..., 4096) = 4096
27046 10:54:28.196162 write(3, "\0\0\23\0\0\0\0\0\0\300\16\0\0\0\0\0\0 \26\0\0\0\0\0\0\320\17\0\0\0\0\0"..., 4096) = 4096
27046 10:54:28.196213 write(3, "\0\0\23\0\0\0\0\0\0\300\16\0\0\0\0\0\0 \26\0\0\0\0\0\0\320\17\0\0\0\0\0"..., 4096) = 4096
27046 10:54:28.196265 write(3, "\0\0\23\0\0\0\0\0\0\300\16\0\0\0\0\0\0 \26\0\0\0\0\0\0\320\17\0\0\0\0\0"..., 4096) = 4096
27046 10:54:28.196314 write(3, "\0\0\23\0\0\0\0\0\0\300\16\0\0\0\0\0\0 \26\0\0\0\0\0\0\320\17\0\0\0\0\0"..., 4096) = 4096
27046 10:54:28.196363 write(3, "\0\0\23\0\0\0\0\0\0\300\16\0\0\0\0\0\0 \26\0\0\0\0\0\0\320\17\0\0\0\0\0"..., 4096) = 4096
27046 10:54:28.196414 write(3, "\0\0\23\0\0\0\0\0\0\300\16\0\0\0\0\0\0 \26\0\0\0\0\0\0\320\17\0\0\0\0\0"..., 4096) = 4096
27046 10:54:28.196472 write(3, "\0\0\23\0\0\0\0\0\0\300\16\0\0\0\0\0\0 \26\0\0\0\0\0\0\320\17\0\0\0\0\0"..., 4096) = 4096
27046 10:54:28.196524 write(3, "\0\0\23\0\0\0\0\0\0\300\16\0\0\0\0\0\0 \26\0\0\0\0\0\0\320\17\0\0\0\0\0"..., 4096) = 4096
27046 10:54:28.196573 write(3, "\0\0\23\0\0\0\0\0\0\300\16\0\0\0\0\0\0 \26\0\0\0\0\0\0\320\17\0\0\0\0\0"..., 4096) = 4096
...

由于顺序读，不需要反复定位文件偏移量，所以能够专注于写操作

五、slat指标

从上面的测试，我们在fio的测试报告中，并没有发现slat的身影，那是由于上述都是同步操作，对同步 i/o 来说，由于 i/o 提交和 i/o 完成是一个动作，所以 slat 实际上就是 i/o 完成的时间

异步顺序写，将同步顺序写的命令添加-ioengine=libaio：

root@wilson-ubuntu:~# fio -name=write -rw=write -ioengine=libaio -direct=1 -bs=4k -size=1g -numjobs=1 -group_reporting -filename=/tmp/test.db
write: (g=0): rw=write, bs=4k-4k/4k-4k/4k-4k, ioengine=libaio, iodepth=1
fio-2.2.10
starting 1 process
jobs: 1 (f=1): [w(1)] [100.0% done] [0kb/119.3mb/0kb /s] [0/30.6k/0 iops] [eta 00m:00s]
write: (groupid=0, jobs=1): err= 0: pid=27258: wed aug 14 11:14:36 2019
 write: io=1024.0mb, bw=120443kb/s, iops=30110, runt= 8706msec
 slat (usec): min=3, max=70, avg= 4.31, stdev= 1.56
 clat (usec): min=0, max=8967, avg=28.13, stdev=55.68
  lat (usec): min=22, max=8976, avg=32.53, stdev=55.72
...
 bw (kb /s): min=118480, max=122880, per=100.00%, avg=120467.29, stdev=1525.68
...

run status group 0 (all jobs):
 write: io=1024.0mb, aggrb=120442kb/s, minb=120442kb/s, maxb=120442kb/s, mint=8706msec, maxt=8706msec

disk stats (read/write):
...
 sda: ios=0/262147, merge=0/1, ticks=0/6576, in_queue=6568, util=74.32%

可以看到，slat指标出现，lat 近似等于 slat + clat 之和（avg平均值）；并且换成异步io之后，吞吐量得到了极大的提升，120m左右

六、总结

● fio应该作为磁盘的baseline工具，拿到机器（物理机或者云机器）都应该第一时间对机器的磁盘做一个基线测试，做到心中有数
● 本文所有的测试，都是绕开了缓存，在实际应用中需要将缓存的影响考虑进去

以上所述是小编给大家介绍的linux磁盘顺序写、随机写,希望对大家有所帮助