深入理解 Linux磁碟順序寫、隨機寫

一、前言

● 隨機寫會導致磁頭不停地換道，造成效率的極大降低；順序寫磁頭幾乎不用換道，或者換道的時間很短
● 本文來討論一下兩者具體的差別以及相應的核心呼叫

二、環境準備

三、fio介紹

通過fio測試，能夠反映在讀寫中的狀態，我們需要重點關注fio的輸出報告中的幾個關鍵指標：
slat ：是指從 I/O 提交到實際執行 I/O 的時長（Submission latency）
clat ：是指從 I/O 提交到 I/O 完成的時長（Completion latency）
lat ：指的是從 fio 建立 I/O 到 I/O 完成的總時長
bw ：吞吐量
iops ：每秒 I/O 的次數

四、同步寫測試

（1）同步隨機寫

主要採用fio作為測試工具，為了能夠看到系統呼叫，使用strace工具，命令看起來是這樣：

先來測試一個隨機寫

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

提取關鍵資訊

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%

列出了我們需要重點關注的資訊：
（1）clat ，平均時長56ms左右
（2）lat ，平均時長56ms左右
（3）bw ，吞吐量，大概在52M左右

再來看核心呼叫資訊：

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, "220240@6371341277>2003631202273203436H26P34027737033030e30"..., 4096) = 4096
26882 10:38:41.919969 lseek(3, 4313088, SEEK_SET) = 4313088
26882 10:38:41.919985 write(3, "220240@6371341277>2003631202273203436H26P34027737033030e30"..., 4096) = 4096
26882 10:38:41.920032 lseek(3, 455880704, SEEK_SET) = 455880704
26882 10:38:41.920048 write(3, "220240@6371341277>2003631202273203436H26P34027737033030e30"..., 4096) = 4096
26882 10:38:41.920096 lseek(3, 338862080, SEEK_SET) = 338862080
26882 10:38:41.920112 write(3, "220240@6371341277>240224202273203436H26P34027737033030e30"..., 4096) = 4096
26882 10:38:41.920161 lseek(3, 739086336, SEEK_SET) = 739086336
26882 10:38:41.920177 write(3, "220240@6371341277>240224202273203436H26P34027737033030e30"..., 4096) = 4096
26882 10:38:41.920229 lseek(3, 848175104, SEEK_SET) = 848175104
26882 10:38:41.920245 write(3, "220240@6371341277>240224202273203436H26P34027737033030e30"..., 4096) = 4096
26882 10:38:41.920296 lseek(3, 1060147200, SEEK_SET) = 1060147200
26882 10:38:41.920312 write(3, "220240@6371341277>240224202273203436H26P34027737033030e30"..., 4096) = 4096
26882 10:38:41.920362 lseek(3, 863690752, SEEK_SET) = 863690752
26882 10:38:41.920377 write(3, "220240@6371341277>240224202273203436H26P34027737033030e30"..., 4096) = 4096
26882 10:38:41.920428 lseek(3, 279457792, SEEK_SET) = 279457792
26882 10:38:41.920444 write(3, "220240@6371341277>240224202273203436H26P34027737033030e30"..., 4096) = 4096
26882 10:38:41.920492 lseek(3, 271794176, SEEK_SET) = 271794176
26882 10:38:41.920508 write(3, "220240@6371341277>240224202273203436H26P34027737033030e30"..., 4096) = 4096
26882 10:38:41.920558 lseek(3, 1067864064, SEEK_SET) = 1067864064
26882 10:38:41.920573 write(3, "220240@6371341277>240224202273203436H26P34027737033030e30"..., 4096) = 4096
...

隨機讀每一次寫入之前都要通過lseek去定位當前的檔案偏移量

同步順序寫

用剛才的方法來測試順序寫

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

再來看一下核心呼叫：

root@wilson-ubuntu:~# more /tmp/write.log
...
27046 10:54:28.194508 write(3, "2330016360t32017"..., 4096) = 4096
27046 10:54:28.194568 write(3, "23300162202432017"..., 4096) = 4096
27046 10:54:28.194627 write(3, "23300162202432017"..., 4096) = 4096
27046 10:54:28.194687 write(3, "23300162202432017"..., 4096) = 4096
27046 10:54:28.194747 write(3, "23300162202432017"..., 4096) = 4096
27046 10:54:28.194807 write(3, "23300162202432017"..., 4096) = 4096
27046 10:54:28.194868 write(3, "23300162202432017"..., 4096) = 4096
27046 10:54:28.194928 write(3, "23300162202432017"..., 4096) = 4096
27046 10:54:28.194988 write(3, "23300162202432017"..., 4096) = 4096
27046 10:54:28.195049 write(3, "23300162202432017"..., 4096) = 4096
27046 10:54:28.195110 write(3, "23300162202432017"..., 4096) = 4096
27046 10:54:28.195197 write(3, "23300162202432017"..., 4096) = 4096
27046 10:54:28.195262 write(3, "23300162202432017"..., 4096) = 4096
27046 10:54:28.195330 write(3, "23300162202432017"..., 4096) = 4096
27046 10:54:28.195426 write(3, "23300162202432017"..., 4096) = 4096
27046 10:54:28.195497 write(3, "23300162202432017"..., 4096) = 4096
27046 10:54:28.195567 write(3, "23300162202432017"..., 4096) = 4096
27046 10:54:28.195637 write(3, "23300162202432017"..., 4096) = 4096
27046 10:54:28.195704 write(3, "23300162202432017"..., 4096) = 4096
27046 10:54:28.195757 write(3, "23300162202432017"..., 4096) = 4096
27046 10:54:28.195807 write(3, "23300162202432017"..., 4096) = 4096
27046 10:54:28.195859 write(3, "23300162202432017"..., 4096) = 4096
27046 10:54:28.195910 write(3, "23300162202432017"..., 4096) = 4096
27046 10:54:28.195961 write(3, "23300162202432017"..., 4096) = 4096
27046 10:54:28.196012 write(3, "23300162202432017"..., 4096) = 4096
27046 10:54:28.196062 write(3, "23300162202432017"..., 4096) = 4096
27046 10:54:28.196112 write(3, "2330016 2632017"..., 4096) = 4096
27046 10:54:28.196162 write(3, "2330016 2632017"..., 4096) = 4096
27046 10:54:28.196213 write(3, "2330016 2632017"..., 4096) = 4096
27046 10:54:28.196265 write(3, "2330016 2632017"..., 4096) = 4096
27046 10:54:28.196314 write(3, "2330016 2632017"..., 4096) = 4096
27046 10:54:28.196363 write(3, "2330016 2632017"..., 4096) = 4096
27046 10:54:28.196414 write(3, "2330016 2632017"..., 4096) = 4096
27046 10:54:28.196472 write(3, "2330016 2632017"..., 4096) = 4096
27046 10:54:28.196524 write(3, "2330016 2632017"..., 4096) = 4096
27046 10:54:28.196573 write(3, "2330016 2632017"..., 4096) = 4096
...

由於順序讀，不需要反複定位檔案偏移量，所以能夠專注於寫操作

五、slat指標

從上面的測試，我們在fio的測試報告中，並沒有發現slat的身影，那是由於上述都是同步操作，對同步 I/O 來說，由於 I/O 提交和 I/O 完成是一個動作，所以 slat 實際上就是 I/O 完成的時間

非同步順序寫，將同步順序寫的命令新增-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指標出現，lat 近似等於 slat + clat 之和（avg平均值）；並且換成非同步io之後，吞吐量得到了極大的提升，120M左右

六、總結

● fio應該作為磁碟的baseline工具，拿到機器（物理機或者雲機器）都應該第一時間對機器的磁碟做一個基線測試，做到心中有數
● 本文所有的測試，都是繞開了快取，在實際應用中需要將快取的影響考慮進去

至此，本文結束