CGroup Case 1 - I/O throttling

In the first use case we will see how we can manage the I/O Operations on the Disk devices. This is done by the blkio sub system available in the CGroups. The blkio sub system moderates I/O operations to the specified block devices.  In this example we will see how we can restrict the read operations performed on a drive

For this we use the “blkio.throttle.read_bps_device” parameter which specifies a upper limit on the number of read operations a device can perform. The rate of the read operations are specifired in bytes per second. The values for this accepts major, minor, and bytes_per_second.

Major& Minor - device types and node numbers specified in Linux

Bytes_per_second is the upper limit rate at which read operations can be performed.

Now lets block the read Operations on the device /dev/sda to 10MB. For this we need to first find the major and minor values for the device. These can be found by using the

[root@vx111a dev]# ls -l sd*
brw-rw---- 1 root disk 8, 0 Mar 15 14:24 sda
brw-rw---- 1 root disk 8, 1 Mar 15 14:24 sda1
brw-rw---- 1 root disk 8, 2 Mar 15 14:24 sda2
brw-rw---- 1 root disk 8, 3 Mar 15 14:24 sda3
brw-rw---- 1 root disk 8, 4 Mar 15 14:24 sda4
brw-rw---- 1 root disk 8, 5 Mar 15 14:24 sda5
brw-rw---- 1 root disk 8, 6 Mar 15 14:24 sda6
[Note: Major for /dev/sda1 is 8 and minor is 1]

OR

[root@vx111a ~]# cat /proc/partitions | grep sda

   8        0  488386584 sda
   8        1   81920000 sda1
   8        2   51200000 sda2
   8        3   51200000 sda3
   8        4          1 sda4
   8        5   10240000 sda5
   8        6   10240000 sda6

In this case for /sda, we have the major number as 8 and minor number as 0. Lets run the hdparm command first with out the cgroups and see what is the disk read rate for the drive  /dev/sda.

NOTE - Hdparm is the tool to use when it comes to tuning your hard disk or DVD drive, but it can also measure read speed, deliver valuable information about the device, change important drive settings, and even erase SSDs securely

[root@vx111a ~]# hdparm --direct -t /dev/sda
/dev/sda:
 Timing O_DIRECT disk reads: 368 MB in  3.00 seconds = 122.42 MB/sec

We can see that disk read rate is 122MB per second. Now we want to restrict the value to 10MB.

Now create a group in the /etc/cgconfig.conf file as

group limitIO{
    blkio {
              blkio.throttle.read_bps_device = "8:0   1048576";
    }
}

We have defined a limitIO group with taking the blkio subsystem.  Now lets configure the cgrules.conf files by adding the below line to the end of the file,

*:hdparm      blkio    limitIO/

This tells that operations performed by hdparm command needs to be added to blkio sub sytem and limited by the group limitIO.

Now restart both the services and run the lssubsys command to check the configuration,

[root@vx111a /]#lssubsys

cpuset:/
perf_event:/
hugetlb:/
blkio:/
blkio:/limitIO
memory:/
memory:/testOOM
net_cls:/

We can see the limitIO group is associated with the blkio subsystem.Once the serices are restarted run the hdparm again and see the values.

Testing

Now test the cgroup using the hdparm command as,

[root@vx111a ~]# hdparm --direct -t /dev/sda
/dev/sda:
 Timing O_DIRECT disk reads:   4 MB in  4.00 seconds = 1023.38 kB/sec​

We can see that the value is limited to 1MB which is under 10MB.

More to Come, Happy learning J