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This article mainly introduces "what is the difference between Linux system disk sequential writing and random writing". In daily operation, I believe that many people have doubts about what is the difference between Linux system disk sequential writing and random writing. Xiaobian consulted all kinds of materials and sorted out simple and easy-to-use operation methods. I hope it will be helpful to answer the doubts of "what is the difference between Linux system disk sequential writing and random writing"! Next, please follow the editor to study!

I. Preface

● random writing will lead to continuous channel change of magnetic heads, resulting in a great reduction in efficiency; sequential write heads rarely need to change channels, or the time to change channels is very short ● this article discusses the specific differences between the two and the corresponding kernel calls

II. Environmental preparation

module

Version

OS Ubuntu 16.04.4 LTS fio 2.2.10

III. Introduction to fio

Through the fio test, the status can be reflected in reading and writing, we need to focus on several key indicators in the fio output report: slat: refers to the time from the submission to the actual execution of the fio (Submission latency) clat: refers to the time from the submission of the fio to the completion of the lat (Completion latency): refers to the total time from the creation of the fio to the completion of the bw: throughput iops: the number of times per second

Fourth, synchronous write test

(1) synchronous random writing

Fio is mainly used as the test tool, in order to see the system calls, use the strace tool, the command looks like this:

Let's test a random write first.

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

Extract key information

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.dbrandwrite: (grub0): rw=randwrite, bs=4K-4K/4K-4K/4K-4K, ioengine=sync, iodepth=1fio-2.2.10Starting 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=19963msecDisk stats (read/write):. Sda: ios=0/262177, merge=0/25, ticks=0/7500, in_queue=7476, util=36.05%

List the information we need to focus on: (1) clat, the average duration 56ms is about (2) lat, the average duration 56ms is about (3) bw, and the throughput is about 52m.

Let's take a look at the kernel call information:

Root@wilson-ubuntu:~# more / tmp/randwrite.log...26882 10 lseek 38 lseek (3, 665198592, SEEK_SET) = 66519859226882 10 lseek 41.919920 write (3, "\ 220\ 240@\ 6\ 371\ 341\ 277 >\ 0\ 200\ 31\ 0\ 202\ 2\ 7\ 320\ 343\ 6H\ 26P\ 340\ 370\ 330\ 30e\ 30"... 4096) = 409626882 10 38 Freight 41.919969 lseek (3,4313088) SEEK_SET) = 431308826882 10lseek 38 lseek 41.919985 write (3, "\ 220\ 240@\ 6\ 371\ 341\ 277 >\ 0\ 200\ 36\ 0\ 0\ 0\ 0202\ 7\ 320\ 343\ 6H\ 340\ 277\ 370\ 330\ 30e\ 30"..., 4096) = 409626882 10 Switzerland 38 41.920032 write (3,455880704, SEEK_SET) = 45588070426882 10140 38 write (3) "\ 220\ 240@\ 6\ 371\ 341\ 277 >\ 0200\ 36\ 31\ 0\ 0\ 0\ 0202\ 2\ 7\ 320\ 343\ 6H\ 26p\ 340\ 277\ 370\ 330\ 30e\ 30", 4096) = 409626882 10VRV 3841.920096 lseek (3, 338862080, SEEK_SET) = 33886208026882 1038 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) = 409626882 10VRV 38RV 41.920161 lseek (3, 739086336, SEEK_SET) = 73908633626882 10RV 3841.920177 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) = 409626882 1038 lseek 41.920229 write (3, 848175104, SEEK_SET) = 84817510426882 1038 write 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) = 409626882 10Rich 38RH 41.920296 lseek (3, 1060147200, SEEK_SET) = 106014720026882 10 3841.920312 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) = 409626882 10VRV 38RV 41.920362 lseek (3, 863690752, SEEK_SET) = 86369075226882 10140 3841.920377 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) = 409626882 10RV 38RV 41.920428 lseek (3, 279457792, SEEK_SET) = 27945779226882 10RN 3841.920444 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) = 409626882 1038 lseek 41.920492 write (3, 271794176, SEEK_SET) = 27179417626882 10 3841.920508 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) = 409626882 10VRV 38RV 41.920558 lseek (3, 1067864064, SEEK_SET) = 106786406426882 10 38R41.920573 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.

Lseek is used to locate the current file offset before each random read.

Synchronous sequential write

Use the previous method to test the sequential write

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.dbwrite: (grub0): rw=write, bs=4K-4K/4K-4K/4K-4K, ioengine=sync Iodepth=1fio-2.2.10Starting 1 processJobs: 1 (fission 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: 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=14929msecDisk stats (read/write):. Sda: ios=0/262162, merge=0/10, ticks=0/6948, in_queue=6932, util=46.49%

It can be seen that the throughput has increased to about 70m.

Let's take a look at the kernel calls:

Root@wilson-ubuntu:~# more / tmp/write.log...27046 10 write 54 write 28.194508 write (3, "\ 0\ 0\ 23\ 0\ 0\ 0\ 0\ 0\ 0\ 0\ 0\ 300\ 16\ 0\ 0\ 0\ 0\ 0\ 0\ 0360\ t\ 0\ 0\ 0\ 0\ 0\ 0320\ 17\ 0\ 0\ 0\ 0\ 0..., 4096) = 409627046 10write 5428 194568 (3) "\ 0\ 0\ 23\ 0\ 0\ 0\ 0\ 0\ 0\ 300\ 16\ 0\ 0\ 0\ 0\ 0\ 0\ 0220\ 24\ 0\ 0\ 0\ 0\ 0320\ 17\ 0\ 0\ 0\ 0\ 0", 4096) = 409627046 1054virtual 28.194627 write (3) "\ 0\ 0\ 23\ 0\ 0\ 0\ 0\ 0\ 0\ 300\ 16\ 0\ 0\ 0\ 0\ 0\ 0\ 0\ 0220\ 24\ 0\ 0\ 0\ 0\ 0320\ 17\ 0\ 0\ 0\ 0\ 0", 4096) = 409627046 1054virtual 28.194687 write (3) "\ 0\ 0\ 23\ 0\ 0\ 0\ 0\ 0\ 0\ 300\ 16\ 0\ 0\ 0\ 0\ 0\ 0\ 0\ 0220\ 24\ 0\ 0\ 0\ 0\ 0320\ 17\ 0\ 0\ 0\ 0\ 0", 4096) = 409627046 1054virtual 28.194747 write (3) "\ 0\ 0\ 23\ 0\ 0\ 0\ 0\ 0\ 0\ 300\ 16\ 0\ 0\ 0\ 0\ 0\ 0\ 0\ 0220\ 24\ 0\ 0\ 0\ 0\ 0320\ 17\ 0\ 0\ 0\ 0\ 0", 4096) = 409627046 1054virtual 28.194807 write (3 "\ 0\ 0\ 23\ 0\ 0\ 0\ 0\ 0\ 0\ 300\ 16\ 0\ 0\ 0\ 0\ 0\ 0\ 0\ 0220\ 24\ 0\ 0\ 0\ 0\ 0320\ 17\ 0\ 0\ 0\ 0\ 0", 4096) = 409627046 1054virtual 28.194868 write (3) "\ 0\ 0\ 23\ 0\ 0\ 0\ 0\ 0\ 0\ 300\ 16\ 0\ 0\ 0\ 0\ 0\ 0\ 0220\ 24\ 0\ 0\ 0\ 0\ 0320\ 17\ 0\ 0\ 0\ 0\ 0", 4096) = 409627046 1054virtual 28.194928 write (3) "\ 0\ 0\ 23\ 0\ 0\ 0\ 0\ 0\ 0\ 300\ 16\ 0\ 0\ 0\ 0\ 0\ 0\ 0220\ 24\ 0\ 0\ 0\ 0\ 0320\ 17\ 0\ 0\ 0\ 0\ 0", 4096) = 409627046 1054virtual 28.194988 write (3) "\ 0\ 0\ 23\ 0\ 0\ 0\ 0\ 0\ 0\ 300\ 16\ 0\ 0\ 0\ 0\ 0\ 0\ 0\ 0220\ 24\ 0\ 0\ 0\ 0\ 0320\ 17\ 0\ 0\ 0\ 0\ 0", 4096) = 409627046 1054virtual 28.195049 write (3) "\ 0\ 0\ 23\ 0\ 0\ 0\ 0\ 0\ 0\ 300\ 16\ 0\ 0\ 0\ 0\ 0\ 0\ 0\ 0220\ 24\ 0\ 0\ 0\ 0\ 0320\ 17\ 0\ 0\ 0\ 0\ 0", 4096) = 409627046 1054virtual 28.195110 write (3) "\ 0\ 0\ 23\ 0\ 0\ 0\ 0\ 0\ 0\ 300\ 16\ 0\ 0\ 0\ 0\ 0\ 0\ 0220\ 24\ 0\ 0\ 0\ 0\ 0320\ 17\ 0\ 0\ 0\ 0\ 0", 4096) = 409627046 1054virtual 28.195197 write (3) "\ 0\ 0\ 23\ 0\ 0\ 0\ 0\ 0\ 0\ 300\ 16\ 0\ 0\ 0\ 0\ 0\ 0\ 0\ 0220\ 24\ 0\ 0\ 0\ 0\ 0320\ 17\ 0\ 0\ 0\ 0\ 0", 4096) = 409627046 1054virtual 28.195262 write (3) "\ 0\ 0\ 23\ 0\ 0\ 0\ 0\ 0\ 0\ 300\ 16\ 0\ 0\ 0\ 0\ 0\ 0\ 0\ 0220\ 24\ 0\ 0\ 0\ 0\ 0320\ 17\ 0\ 0\ 0\ 0\ 0", 4096) = 409627046 1054virtual 28.195330 write (3) "\ 0\ 0\ 23\ 0\ 0\ 0\ 0\ 0\ 0\ 300\ 16\ 0\ 0\ 0\ 0\ 0\ 0\ 0220\ 24\ 0\ 0\ 0\ 0\ 0320\ 17\ 0\ 0\ 0\ 0\ 0", 4096) = 409627046 1054virtual 28.195426 write (3) "\ 0\ 0\ 23\ 0\ 0\ 0\ 0\ 0\ 0\ 300\ 16\ 0\ 0\ 0\ 0\ 0\ 0\ 0220\ 24\ 0\ 0\ 0\ 0\ 0320\ 17\ 0\ 0\ 0\ 0\ 0", 4096) = 409627046 1054virtual 28.195497 write (3) "\ 0\ 0\ 23\ 0\ 0\ 0\ 0\ 0\ 0\ 300\ 16\ 0\ 0\ 0\ 0\ 0\ 0\ 0220\ 24\ 0\ 0\ 0\ 0\ 0320\ 17\ 0\ 0\ 0\ 0\ 0", 4096) = 409627046 1054virtual 28.195567 write (3) "\ 0\ 0\ 23\ 0\ 0\ 0\ 0\ 0\ 0\ 300\ 16\ 0\ 0\ 0\ 0\ 0\ 0\ 0\ 0220\ 24\ 0\ 0\ 0\ 0\ 0\ 0\ 320\ 17\ 0\ 0\ 0\ 0\ 0", 4096) = 409627046 1054v 28.195637 write (3) "\ 0\ 0\ 23\ 0\ 0\ 0\ 0\ 0\ 0\ 300\ 16\ 0\ 0\ 0\ 0\ 0\ 0\ 0\ 0220\ 24\ 0\ 0\ 0\ 0\ 0320\ 17\ 0\ 0\ 0\ 0\ 0", 4096) = 409627046 1054virtual 28.195704 write (3) "\ 0\ 0\ 23\ 0\ 0\ 0\ 0\ 0\ 0\ 300\ 16\ 0\ 0\ 0\ 0\ 0\ 0\ 0220\ 24\ 0\ 0\ 0\ 0\ 0320\ 17\ 0\ 0\ 0\ 0\ 0", 4096) = 409627046 1054virtual 28.195757 write (3) "\ 0\ 0\ 23\ 0\ 0\ 0\ 0\ 0\ 0\ 300\ 16\ 0\ 0\ 0\ 0\ 0\ 0\ 0220\ 24\ 0\ 0\ 0\ 0\ 0320\ 17\ 0\ 0\ 0\ 0\ 0", 4096) = 409627046 1054virtual 28.195807 write (3) "\ 0\ 0\ 23\ 0\ 0\ 0\ 0\ 0\ 0\ 300\ 16\ 0\ 0\ 0\ 0\ 0\ 0\ 0\ 0220\ 24\ 0\ 0\ 0\ 0\ 0320\ 17\ 0\ 0\ 0\ 0\ 0", 4096) = 409627046 1054virtual 28.195859 write (3) "\ 0\ 0\ 23\ 0\ 0\ 0\ 0\ 0\ 0\ 300\ 16\ 0\ 0\ 0\ 0\ 0\ 0\ 0\ 0220\ 24\ 0\ 0\ 0\ 0\ 0320\ 17\ 0\ 0\ 0\ 0\ 0", 4096) = 409627046 1054virtual 28.195910 write (3) "\ 0\ 0\ 23\ 0\ 0\ 0\ 0\ 0\ 0\ 300\ 16\ 0\ 0\ 0\ 0\ 0\ 0\ 0\ 0220\ 24\ 0\ 0\ 0\ 0\ 0320\ 17\ 0\ 0\ 0\ 0\ 0", 4096) = 409627046 1054virtual 28.195961 write (3) "\ 0\ 0\ 23\ 0\ 0\ 0\ 0\ 0\ 0\ 300\ 16\ 0\ 0\ 0\ 0\ 0\ 0\ 0\ 0220\ 24\ 0\ 0\ 0\ 0\ 0320\ 17\ 0\ 0\ 0\ 0\ 0", 4096) = 409627046 1054virtual 28.196012 write (3) "\ 0\ 0\ 23\ 0\ 0\ 0\ 0\ 0\ 0\ 300\ 16\ 0\ 0\ 0\ 0\ 0\ 0\ 0\ 0220\ 24\ 0\ 0\ 0\ 0\ 0320\ 17\ 0\ 0\ 0\ 0\ 0", 4096) = 409627046 1054virtual 28.196062 write (3) "\ 0\ 0\ 23\ 0\ 0\ 0\ 0\ 0\ 0\ 300\ 16\ 0\ 0\ 0\ 0\ 0\ 0\ 0\ 0220\ 24\ 0\ 0\ 0\ 0\ 0320\ 17\ 0\ 0\ 0\ 0\ 0", 4096) = 409627046 1054virtual 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\ 0\ 20\ 17\ 0\ 0\ 0\ 0\ 0", 4096) = 409627046 1054virtual 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\ 0\ 20\ 17\ 0\ 0\ 0\ 0\ 0", 4096) = 409627046 1054 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\ 20\ 17\ 0\ 0\ 0\ 0\ 0", 4096) = 409627046 1054 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\ 0\ 320\ 17\ 0\ 0\ 0\ 0\ 0", 4096) = 409627046 1054virtual 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\ 0\ 20\ 17\ 0\ 0\ 0\ 0\ 0", 4096) = 409627046 1054virtual 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\ 20\ 17\ 0\ 0\ 0\ 0\ 0", 4096) = 409627046 1054virtual 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\ 0\ 320\ 17\ 0\ 0\ 0\ 0\ 0", 4096) = 409627046 1054 virtual 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\ 0 320\ 17\ 0\ 0\ 0\ 0\ 0", 4096) = 409627046 1054virtual 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\ 0\ 320\ 17\ 0\ 0\ 0\ 0\ 0", 4096) = 409627046 1054 write 28.196573 "\ 0\ 0\ 23\ 0\ 0\ 0\ 0\ 0\ 0\ 300\ 16\ 0\ 0\ 0\ 0\ 0\ 26\ 0\ 0\ 0\ 0\ 0", 4096) = 4096.

Because of sequential reading, there is no need to repeatedly locate the file offset, so you can focus on the write operation.

5. Slat indicators

From the above test, we did not find slat in the test report of fio. That is because the above operations are all synchronous operations. For synchronous Imax O, since the submission and completion of Imax O are the same action, slat is actually the time it takes for it to complete.

Write asynchronously, adding commands written in synchronous order to-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.dbwrite: (gumb0): rw=write, bs=4K-4K/4K-4K/4K-4K, ioengine=libaio, iodepth=1fio-2.2.10Starting 1 processJobs: 1 (fission 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=8706msecDisk stats (read/write):. Sda: ios=0/262147, merge=0/1, ticks=0/6576, in_queue=6568, util=74.32%

It can be seen that the slat index appears, and lat is approximately equal to the sum of slat + clat (avg average), and after being replaced by asynchronous io, the throughput has been greatly improved, about 120m.

At this point, the study on "what is the difference between disk sequential writing and random writing in Linux system" is over. I hope to be able to solve your doubts. The collocation of theory and practice can better help you learn, go and try it! If you want to continue to learn more related knowledge, please continue to follow the website, the editor will continue to work hard to bring you more practical articles!

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