Linux磁盘顺序写、随机写

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

---

fio 介绍

• 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

---

同步写测试

同步随机写

• 使用strace工具查看系统调用

  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%
  

• 重点关注的信息
  • clat: 平均时长56ms左右
  • lat: 平均时长56ms左右
  • bw: 吞吐量，大概在52M左右
• 内核调用信息，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

  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
  ...
  

同步顺序写

• 测试顺序写

  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
  

• 可以看到，吞吐量提升至70M左右

  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%
  

• 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
  ...
  

• 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

---

异步顺序写

• 前面 fio 的测试报告中没有发现slat，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
• 异步顺序写测试命令

  # 同步顺序写的命令添加 -ioengine=libaio
  fio -name=write -rw=write -ioengine=libaio -direct=1 -bs=4k -size=1G -numjobs=1  -group_reporting -filename=/tmp/test.db
  

• 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

  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%
  

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来源：https://my.oschina.net/colben/blog/3099614