| 1 | <?xml version='1.0' encoding='UTF-8'?>
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| 2 | <!DOCTYPE topic PUBLIC "-//OASIS//DTD DITA Topic//EN" "topic.dtd">
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| 3 | <topic xml:lang="en-us" id="guestadd-balloon">
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| 4 | <title>Memory Ballooning</title>
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| 5 |
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| 6 | <body>
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| 7 | <p>
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| 8 | The Guest Additions can change the amount of host memory that a
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| 9 | VM uses, while the machine is running. Because of how this is
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| 10 | implemented, this feature is called <i>memory
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| 11 | ballooning</i>.
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| 12 | </p>
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| 13 | <note>
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| 14 | <ul>
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| 15 | <li>
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| 16 | <p>
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| 17 | <ph conkeyref="vbox-conkeyref-phrases/product-name"/> supports memory ballooning only on 64-bit
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| 18 | hosts. It is not supported on macOS hosts.
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| 19 | </p>
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| 20 | </li>
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| 21 | <li>
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| 22 | <p>
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| 23 | Memory ballooning does not work well with large pages
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| 24 | enabled. To turn off large pages support for a VM, run
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| 25 | <userinput>VBoxManage modifyvm <varname>vmname</varname> --large-pages
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| 26 | off</userinput>
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| 27 | </p>
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| 28 | </li>
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| 29 | </ul>
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| 30 | </note>
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| 31 | <p>
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| 32 | Normally, to change the amount of memory allocated to a virtual
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| 33 | machine, you have to shut down the virtual machine entirely and
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| 34 | modify its settings. With memory ballooning, memory that was
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| 35 | allocated for a virtual machine can be given to another virtual
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| 36 | machine without having to shut the machine down.
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| 37 | </p>
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| 38 | <p>
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| 39 | When memory ballooning is requested, the <ph conkeyref="vbox-conkeyref-phrases/product-name"/> Guest
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| 40 | Additions, which run inside the guest, allocate physical memory
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| 41 | from the guest operating system on the kernel level and lock
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| 42 | this memory down in the guest. This ensures that the guest will
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| 43 | not use that memory any longer. No guest applications can
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| 44 | allocate it, and the guest kernel will not use it either.
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| 45 | <ph conkeyref="vbox-conkeyref-phrases/product-name"/> can then reuse this memory and give it to another
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| 46 | virtual machine.
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| 47 | </p>
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| 48 | <p>
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| 49 | The memory made available through the ballooning mechanism is
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| 50 | only available for reuse by <ph conkeyref="vbox-conkeyref-phrases/product-name"/>. It is
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| 51 | <i>not</i> returned as free memory to the host.
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| 52 | Requesting balloon memory from a running guest will therefore
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| 53 | not increase the amount of free, unallocated memory on the host.
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| 54 | Effectively, memory ballooning is therefore a memory
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| 55 | overcommitment mechanism for multiple virtual machines while
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| 56 | they are running. This can be useful to temporarily start
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| 57 | another machine, or in more complicated environments, for
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| 58 | sophisticated memory management of many virtual machines that
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| 59 | may be running in parallel depending on how memory is used by
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| 60 | the guests.
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| 61 | </p>
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| 62 | <p>
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| 63 | At this time, memory ballooning is only supported through
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| 64 | <userinput>VBoxManage</userinput>. Use the following command to
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| 65 | increase or decrease the size of the memory balloon within a
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| 66 | running virtual machine that has Guest Additions installed:
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| 67 | </p>
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| 68 | <pre xml:space="preserve">VBoxManage controlvm "VM name" guestmemoryballoon n</pre>
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| 69 | <p> where <varname>VM name</varname> is the name or UUID of the virtual machine in question and <varname>n</varname> is the amount of memory to allocate from the guest in megabytes. See <xref href="../cli_topics/vboxmanage-controlvm.dita"/>.</p>
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| 70 | <p>
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| 71 | You can also set a default balloon that will automatically be
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| 72 | requested from the VM every time after it has started up with
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| 73 | the following command:
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| 74 | </p>
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| 75 | <pre xml:space="preserve">VBoxManage modifyvm "VM name" --guest-memory-balloon n</pre>
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| 76 | <p> By default, no balloon memory is allocated. This is a VM setting, like other <userinput>modifyvm</userinput> settings, and therefore can only be set while the machine is shut down. See <xref href="../cli_topics/vboxmanage-modifyvm.dita"/>. </p>
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| 77 | </body>
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| 78 |
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| 79 | </topic>
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