3.5. Managing Disk I/O Parameters¶

3.5.1. Configuring Priority Levels for Virtual Machines and Containers¶

In Virtuozzo Hybrid Server, you can configure the disk I/O (input/output) priority level of virtual machines and containers. The higher the I/O priority level, the more time the virtual machine or container will get for its disk I/O activities as compared to the other virtual machines and containers on the hardware node. By default, any virtual machine or container on the hardware node has the I/O priority level set to 4. However, you can change the current I/O priority level in the range from 0 to 7 using the --ioprio option of the prlctl set command. For example, you can issue the following command to set the I/O priority of the container MyCT and the virtual machine MyVM to 6:

# prlctl set MyCT --ioprio 6
# prlctl set MyVM --ioprio 6

To check the I/O priority level currently applied to the container MyCT and the virtual machine MyVM, you can execute the following commands:

• For container MyCT:

  # grep IOPRIO /etc/vz/conf/fbb30afa-e770-4081-9d9e-6b9c262eb091.conf
  IOPRIO="6"
  

• For the virtual machine MyVM:

  # prlctl list MyVM --info | grep ioprio
  cpu cpus=2 VT-x accl=high mode=32 ioprio=6 iolimit='0'
  

3.5.2. Limiting Disk I/O Bandwidth¶

In Virtuozzo Hybrid Server, you can configure the bandwidth virtual machines and containers are allowed to use for their disk input and output (I/O) operations. Limiting the disk I/O bandwidth can help you prevent the situations when high disk activities in one virtual machine or container (generated, for example, by transferring huge amounts of data to/from the virtual machine or container) can slow down the performance of other virtual machines and containers on the hardware node.

By default, the I/O bandwidth limit for all newly created virtual machines and containers is set to 0, which means that no limits are applied. To limit the disk I/O bandwidth for a virtual machine or container, use the --iolimit option of the prlctl set command. For example, to set the I/O bandwidth limit for the container MyCT to 10 MB/s:

# prlctl set MyCT --iolimit 10

By default, the limit is set in megabytes per second. In addition, you can use the following suffixes to indicate other measurement units:

• G sets the limit in gigabytes per second (1G).

• K sets the limit in kilobytes per second (10K).

• B sets the limit in bytes per second (10B).

To check that the I/O speed limit has been successfully applied to the container MyCT, use the prlctl list command:

# prlctl list MyCT -o iolimit
IOLIMIT
10485760

At any time, you can remove the I/O bandwidth limit set for container MyCT by running this command:

# prlctl set MyCT --iolimit 0

3.5.3. Limiting the Number of I/O Operations per Second¶

In Virtuozzo Hybrid Server, you can limit the maximum number of disk input and output operations per second virtual machines and containers are allowed to perform (known as the IOPS limit). You may consider setting the IOPS limit for virtual machines and containers with high disk activities to ensure that they do not affect the performance of other virtual machines and containers on the Node.

By default, IOPS is not limited for newly created virtual machines and containers. To set the IOPS limit, you can use the --iopslimit option of the prlctl set command. For example, to allow the container MyCT and the virtual machine MyVM to perform no more than 100 disk I/O operations per second, you can run the following commands:

# prlctl set MyCT --iopslimit 100
# prlctl set MyVM --iopslimit 100

To ensure that the IOPS limit has been successfully applied, use the prlctl list -i command. For example:

# prlctl list MyCT -i | grep iopslimit
<...> iopslimit=100

At any time, you can remove the set IOPS limits by running this command:

# prlctl set MyCT --iopslimit 0
# prlctl set MyVM --iopslimit 0

3.5.4. Viewing Disk I/O Statistics¶

In Virtuozzo Hybrid Server, you can view disk input and output (I/O) statistics for all processes on the host. To do this:

1. Run the vztop utility.

2. Press F2 or S to switch to the Setup menu.

3. In the Setup column, choose Columns.

4. In Available Columns, choose from the following parameters to add to the output (Active Columns):

   Parameter	Description	Column
   RBYTES	Number of bytes read for the process.	IO_RBYTES
   WBYTES	Number of bytes written for the process.	IO_WBYTES
   IO_READ_RATE	Process read rate, in bytes per second.	DISK READ
   IO_WRITE_RATE	Process write rate, in bytes per second.	DISK WRITE
   IO_RATE	Process total I/O rate, in bytes per second.	DISK R/W
   IO_PRIORITY	Process I/O priority.	IO

   To add a parameter, select it and press F5 or Enter. To remove a parameter from Active Columns, select it and press F9.

5. When you finish managing columns, press F10 to save the changes and view the output.

3.5.5. Setting I/O Limits for Backup and Migration Operations¶

Backup and migration operations with containers and virtual machines can generate a high I/O load on the server, thus reducing the performance of other virtual environments or the server itself. You can avoid such situations by setting I/O limits for these operations.

To set an I/O limit, do the following:

1. In the /etc/vz/vz.conf global configuration file, locate the following section:

   # VZ Tools limits
   <...>
   # Uncomment next line to specify required disk IO bandwidth in Bps (10485760 - 10MBps)
   # VZ_TOOLS_IOLIMIT=10485760
   

2. Uncomment the VZ_TOOLS_IOLIMIT parameter, and set the I/O limit for backup and migration operations in bytes per second.

3. Save the file.

When setting I/O limits, pay attention to the following:

• VZ_TOOLS_IOLIMIT is a global parameter that has effect on all virtual environments on the server.

• The VZ_TOOLS_IOLIMIT parameter controls the I/O load only for backup and migration operations. Restore operations are not limited.

• Simultaneous operations do not share the limit and are limited separately.

• For migration, only the limit set on the source server applies, while the limit set on the destination server is ignored.

3.5.6. Improving Disk I/O Performance for Virtual Machines¶

In case of VM virtual disks mapped to QCOW2 disk images, the disk space is only allocated when it is actually needed by the VM and the cache size depends on how much space is allocated. In Virtuozzo Hybrid Server, you can significantly improve disk I/O performance by means of cache policies applied to VM disks based on their allocated space size.

The default cache policy is used for typical VM workloads and disk sizes up to 1TB. If a guest needs continuous access to an entire disk that is larger than 1TB, the default cache size will not be sufficient and will reduce disk I/O perfomance. In this case, you can increase cache size to speed up disk access as follows:

1. In the /etc/libvirt/qemu.conf configuration file, uncomment and set the qcow2_cache_policy parameter to disk.

2. Restart the libvirtd daemon to apply the changes.

   # systemctl restart libvirtd
   

The disk policy will provide a cache large enough to handle the entire disk workload.