Memory usage 编辑

Two components contribute to the memory footprint of the Citrix Hypervisor server. First, the memory consumed by the Xen hypervisor itself. Second, there is the memory consumed by the Control Domain of the host. Also known as ‘Domain0’, or ‘dom0’, the control domain is a secure, privileged Linux VM that runs the Citrix Hypervisor management toolstack (XAPI). Besides providing Citrix Hypervisor management functions, the control domain also runs the driver stack that provides user created VM access to physical devices.

Control domain memory

The amount of memory allocated to the control domain is adjusted automatically and is based on the amount of physical memory on the physical host. By default, Citrix Hypervisor allocates 1 GiB plus 5% of the total physical memory to the control domain, up to an initial maximum of 8 GiB.

Note:

The amount reported in the Citrix Hypervisor section in Citrix Hypervisor Center includes the memory used by the control domain (dom0), the Xen hypervisor itself, and the crash kernel. Therefore, the amount of memory reported in Citrix Hypervisor Center can exceed these values. The amount of memory used by the hypervisor is larger for hosts using more memory.

Change the amount of memory allocated to the control domain

You can change the amount of memory allocated to dom0 by using Citrix Hypervisor Center or by using the command line. If you increase the amount of memory allocated to the control domain beyond the amount allocated by default, this action results in less memory being available to VMs.

You might need to increase the amount of memory assigned to the control domain of a Citrix Hypervisor server in the following cases:

• You are running many VMs on the server
• You are using PVS-Accelerator
• You are using read caching

The amount of memory to allocate to the control domain depends on your environment and the requirements of your VMs.

You can monitor the following metrics to judge whether the amount of control domain memory is appropriate for your environment and what effects any changes you make have:

• Swap activity: If the control domain is swapping, increase the control domain memory.
• Tapdisk mode: You can monitor whether your tapdisks are in low-memory mode from within the Citrix Hypervisor Center Performance tab for the server. Select Actions > New Graph and choose the Tapdisks in low memory mode graph. If a tapdisk is in low-memory mode, increase the control domain memory.
• Pagecache pressure: Use the top command to monitor the buff/cache metric. If this number becomes too low, you might want to increase the control domain memory.

Changing the dom0 memory by using Citrix Hypervisor Center

For information about changing the dom0 memory by using Citrix Hypervisor Center, see Changing the Control Domain Memory in the Citrix Hypervisor Center documentation.

Note:

You cannot use Citrix Hypervisor Center to reduce dom0 memory below the value that was initially set during Citrix Hypervisor installation. To make this change you must use the command line.

Changing the dom0 memory by using the command line

Note:

On hosts with smaller memory (less than 16 GiB), you might want to reduce the memory allocated to the Control Domain to lower than the installation default value. You can use the command line to make this change. However, we recommend that you do not reduce the dom0 memory below 1 GiB and that you do this operation under the guidance of the Support Team.

1. On the Citrix Hypervisor server, open a local shell and log on as root.

2. Type the following:

   /opt/xensource/libexec/xen-cmdline --set-xen dom0_mem=<nn>M,max:<nn>M
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