Linux high IOwait is a common Linux performance issue. Today we will look at what iowait means and what contributes to this problem. Hope this can give you more ideas about high IOwait issue.
What is IOwait?
IO wait is related to the CPU resource on the server.
The iowait column on top command output shows the percentage of time that the processor was waiting for I/O to complete. It indicates that the system is waiting on disk or network IO. Because the system is waiting on those resources, it can not fully utilize the CPU.
Learn more about what is IOwait here.
I/O Wait is a problem that requires the use of advanced tools to debug, and of course there are many advanced uses of the basic tools.
Determine if an I/O problem is causing the system to slow
Verify that the system is slow due to I/O we can use multiple commands, but the simplest is the UNIX command top.
代码语言：txt复制[[email protected] ~]# top
top - 15:19:26 up 6:10, 4 users, load average: 0.00, 0.01, 0.05
Tasks: 147 total, 1 running, 146 sleeping, 0 stopped, 0 zombie
%Cpu(s): 0.0 us, 0.3 sy, 0.0 ni, 99.7 id, 96.0 wa, 0.0 hi, 0.0 si, 0.0 st
KiB Mem : 999936 total, 121588 free, 328672 used, 549676 buff/cache
KiB Swap: 2097148 total, 2095792 free, 1356 used. 450460 avail Mem From the CPU line we can see the percentage of CPU wasted on I/O wait; the higher the number, the more CPU resources are waiting for I/O permissions
代码语言：txt复制wa -- iowait
Amount of time the CPU has been waiting for I/O to complete.check this post to understand CPU Usage us sy wa hi in Linux Top
Find which disk is being written
The top command above explains I/O wait from a whole, but does not indicate which disk is affected, and to know which disk is causing the problem, we use another command Iostat command. Check this post Troubleshooting Disk Issues In Linux for more.
代码语言：txt复制[[email protected] ~]# iostat -x 2 5
Linux 3.10.0-514.el7.x86_64 (localhost.localdomain) 2017年03月03日 _x86_64_ (1 CPU)
avg-cpu: %user %nice %system %iowait %steal %idle
0.34 0.00 0.31 0.01 0.00 99.33
Device: rrqm/s wrqm/s r/s w/s rkB/s wkB/s avgrq-sz avgqu-sz await r_await w_await svctm %util
sda 0.00 0.05 1.16 0.17 39.00 17.38 84.60 0.00 2.17 0.87 11.14 0.65 111.41
scd0 0.00 0.00 0.00 0.00 0.00 0.00 8.00 0.00 0.64 0.64 0.00 0.64 0.00
dm-0 0.00 0.00 1.10 0.20 37.85 17.21 84.71 0.00 2.43 0.90 10.88 0.66 0.09
dm-1 0.00 0.00 0.01 0.02 0.07 0.08 9.70 0.00 1.42 0.27 2.05 0.09 0.00In the example above, Iostat will be updated every 2 seconds, printing 5 times of information, and-X's option is to print out the extended information
The first Iostat report prints the statistics after the last boot of the system, which means that in most cases the first printed information should be ignored, and the remaining reports are based on the time of the previous interval. For example, this command will be printed 5 times, the second report is a statistic from the first report, the third time is based on the second, and so on
In the above example, the%utilized of SDA is 111.41%, which is a good indication that a process is being written to the SDA disk.
In addition to%utilized, we can get richer resources from iostat, such as read/write requests per millisecond (rrqm/s & wrqm/s)), read and write per second (r/s & w/s), and of course more. In the example above, our project seems to be reading and writing very much information. This is very useful for us to find the appropriate process.
Find processes that cause high I/O wait
代码语言：txt复制[[email protected] ~]# iotop
Total DISK READ : 0.00 B/s | Total DISK WRITE : 0.00 B/s
Actual DISK READ: 0.00 B/s | Actual DISK WRITE: 0.00 B/s
TID PRIO USER DISK READ DISK WRITE SWAPIN IO> COMMAND
1028 be/4 root 0.00 B/s 0.00 B/s 0.00 % 0.00 % sshdThe simplest way to find the culprit is to use Linux Iotop to check disk IO usage Per process , by looking at iotop statistics, we can easily guide sshd as the culprit
Although Iotop is a very powerful tool and is easy to use, it is not installed by default on all Linux operating systems.
Find which file caused the i/owait
The lsof command can show all the files that a process opens, or all the processes that open a file. From this list, we can find out exactly what files are written, depending on the size of the file and the specific data of the IO file in/proc
We can use the-p <pid> method to reduce the output, PID is the specific process
代码语言：txt复制[[email protected] ~]# lsof -p 1028
COMMAND PID USER FD TYPE DEVICE SIZE/OFF NODE NAME
sshd 1028 root cwd DIR 253,0 233 64 /
sshd 1028 root rtd DIR 253,0 233 64 /
sshd 1028 root txt REG 253,0 819640 2393730 /usr/sbin/sshd
sshd 1028 root mem REG 253,0 61752 180464 /usr/lib64/libnss_files-2.17.so
sshd 1028 root mem REG 253,0 43928 180476 /usr/lib64/librt-2.17.so
sshd 1028 root mem REG 253,0 15688 269136 /usr/lib64/libkeyutils.so.1.5
sshd 1028 root mem REG 253,0 62744 482870 /usr/lib64/libkrb5support.so.0.1
sshd 1028 root mem REG 253,0 11384 180425 /usr/lib64/libfreebl3.so
sshd 1028 root mem REG 253,0 143352 180472 /usr/lib64/libpthread-2.17.so
sshd 1028 root mem REG 253,0 251784 202440 /usr/lib64/libnspr4.so
sshd 1028 root mem REG 253,0 20016 202441 /usr/lib64/libplc4.so
sshd 1028 root mem REG 253,0 15768 202442 /usr/lib64/libplds4.so
sshd 1028 root mem REG 253,0 182056 202443 /usr/lib64/libnssutil3.so
sshd 1028 root mem REG 253,0 1220240 650074 /usr/lib64/libnss3.so
sshd 1028 root mem REG 253,0 164048 650076 /usr/lib64/libsmime3.so
sshd 1028 root mem REG 253,0 276752 650077 /usr/lib64/libssl3.so
sshd 1028 root mem REG 253,0 121296 269112 /usr/lib64/libsasl2.so.3.0.0
sshd 1028 root mem REG 253,0 398264 202404 /usr/lib64/libpcre.so.1.2.0
sshd 1028 root mem REG 253,0 2116736 180446 /usr/lib64/libc-2.17.so
sshd 1028 root mem REG 253,0 15848 202439 /usr/lib64/libcom_err.so.2.1
sshd 1028 root mem REG 253,0 202568 482862 /usr/lib64/libk5crypto.so.3.1
sshd 1028 root mem REG 253,0 959008 482868 /usr/lib64/libkrb5.so.3.3
sshd 1028 root mem REG 253,0 324888 482858 /usr/lib64/libgssapi_krb5.so.2.2
sshd 1028 root mem REG 253,0 110632 180474 /usr/lib64/libresolv-2.17.so
sshd 1028 root mem REG 253,0 40640 180450 /usr/lib64/libcrypt-2.17.so
sshd 1028 root mem REG 253,0 113152 180456 /usr/lib64/libnsl-2.17.so
sshd 1028 root mem REG 253,0 90664 202424 /usr/lib64/libz.so.1.2.7
sshd 1028 root mem REG 253,0 14432 186432 /usr/lib64/libutil-2.17.so
sshd 1028 root mem REG 253,0 61872 766946 /usr/lib64/liblber-2.4.so.2.10.3
sshd 1028 root mem REG 253,0 344280 766948 /usr/lib64/libldap-2.4.so.2.10.3
sshd 1028 root mem REG 253,0 19344 180452 /usr/lib64/libdl-2.17.so
sshd 1028 root mem REG 253,0 2025472 482880 /usr/lib64/libcrypto.so.1.0.1e
sshd 1028 root mem REG 253,0 23968 202508 /usr/lib64/libcap-ng.so.0.0.0
sshd 1028 root mem REG 253,0 155744 202421 /usr/lib64/libselinux.so.1
sshd 1028 root mem REG 253,0 61672 539049 /usr/lib64/libpam.so.0.83.1
sshd 1028 root mem REG 253,0 122936 202512 /usr/lib64/libaudit.so.1.0.0
sshd 1028 root mem REG 253,0 42520 298848 /usr/lib64/libwrap.so.0.7.6
sshd 1028 root mem REG 253,0 11328 568388 /usr/lib64/libfipscheck.so.1.2.1
sshd 1028 root mem REG 253,0 155064 180439 /usr/lib64/ld-2.17.so
sshd 1028 root 0u CHR 1,3 0t0 5930 /dev/null
sshd 1028 root 1u CHR 1,3 0t0 5930 /dev/null
sshd 1028 root 2u CHR 1,3 0t0 5930 /dev/null
sshd 1028 root 3u IPv4 21185 0t0 TCP *:ssh (LISTEN)
sshd 1028 root 4u IPv6 21194 0t0 TCP *:ssh (LISTEN)In order to further confirm that these files are read and written frequently, we can view them with the following command
代码语言：txt复制[[email protected] ~] # df / tmp
File system 1K-block used available used% mount point
/ dev / mapper / cl-root 17811456 3981928 13829528 23% /From the results of the above command, we can determine that/tmp is the root of our environment's logical disk
代码语言：txt复制[[email protected] ~]# pvdisplay
--- Physical volume ---
PV Name /dev/sda2
VG Name cl
PV Size 19.00 GiB / not usable 3.00 MiB
Allocatable yes (but full)
PE Size 4.00 MiB
Total PE 4863
Free PE 0
Allocated PE 4863
PV UUID 4QfaOy-DNSO-niK1-ayn2-K6AY-WZMy-9Nd2ItOver Pvdisplay we can see that/dev/sda2 is actually the specific disk we used to create the logical disk. With the above information we can safely say that the result of lsof is the file we are looking for
Reference:
Troubleshooting high iowait linux
Sort Disk Read Write in Linux Iotop Command
Debugging High CPU Utilization For a Process on Linux
How to Check Disk usage Disk utilization on Linux
What is High IOwait on Linux
Using Linux Iotop to check disk IO usage Per process
How to Check if a Disk is Busy in Linux
Troubleshooting Disk Issues In Linux
