| 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="iocaching">
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| 4 | <title>Host Input/Output Caching</title>
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| 5 |
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| 6 | <body>
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| 7 | <p>
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| 8 | <ph conkeyref="vbox-conkeyref-phrases/product-name"/> can optionally disable the I/O caching that the
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| 9 | host OS would otherwise perform on disk image files.
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| 10 | </p>
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| 11 | <p>
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| 12 | Traditionally, <ph conkeyref="vbox-conkeyref-phrases/product-name"/> has opened disk image files as
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| 13 | normal files, which results in them being cached by the host OS
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| 14 | like any other file. The main advantage of this is speed: when the
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| 15 | guest OS writes to disk and the host OS cache uses delayed
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| 16 | writing, the write operation can be reported as completed to the
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| 17 | guest OS quickly while the host OS can perform the operation
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| 18 | asynchronously. Also, when you start a VM a second time and have
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| 19 | enough memory available for the OS to use for caching, large parts
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| 20 | of the virtual disk may be in system memory, and the VM can access
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| 21 | the data much faster.
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| 22 | </p>
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| 23 | <p>
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| 24 | Note that this applies only to image files. Buffering does not
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| 25 | occur for virtual disks residing on remote iSCSI storage, which is
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| 26 | the more common scenario in enterprise-class setups. See
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| 27 | <xref href="storage-iscsi.dita#storage-iscsi"/>.
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| 28 | </p>
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| 29 | <p>
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| 30 | While buffering is a useful default setting for virtualizing a few
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| 31 | machines on a desktop computer, there are some disadvantages to
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| 32 | this approach:
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| 33 | </p>
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| 34 | <ul>
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| 35 | <li>
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| 36 | <p>
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| 37 | Delayed writing through the host OS cache is less secure. When
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| 38 | the guest OS writes data, it considers the data written even
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| 39 | though it has not yet arrived on a physical disk. If for some
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| 40 | reason the write does not happen, such as power failure or
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| 41 | host crash, the likelihood of data loss increases.
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| 42 | </p>
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| 43 | </li>
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| 44 | <li>
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| 45 | <p>
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| 46 | Disk image files tend to be very large. Caching them can
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| 47 | therefore quickly use up the entire host OS cache. Depending
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| 48 | on the efficiency of the host OS caching, this may slow down
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| 49 | the host immensely, especially if several VMs run at the same
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| 50 | time. For example, on Linux hosts, host caching may result in
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| 51 | Linux delaying all writes until the host cache is nearly full
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| 52 | and then writing out all these changes at once, possibly
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| 53 | stalling VM execution for minutes. This can result in I/O
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| 54 | errors in the guest as I/O requests time out there.
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| 55 | </p>
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| 56 | </li>
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| 57 | <li>
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| 58 | <p>
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| 59 | Physical memory is often wasted as guest OSes typically have
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| 60 | their own I/O caches, which may result in the data being
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| 61 | cached twice, in both the guest and the host caches, for
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| 62 | little effect.
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| 63 | </p>
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| 64 | </li>
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| 65 | </ul>
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| 66 | <p>
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| 67 | If you decide to disable host I/O caching for the above reasons,
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| 68 | <ph conkeyref="vbox-conkeyref-phrases/product-name"/> uses its own small cache to buffer writes, but no
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| 69 | read caching since this is typically already performed by the
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| 70 | guest OS. In addition, <ph conkeyref="vbox-conkeyref-phrases/product-name"/> fully supports asynchronous
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| 71 | I/O for its virtual SATA, SCSI, and SAS controllers through
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| 72 | multiple I/O threads.
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| 73 | </p>
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| 74 | <p>
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| 75 | Since asynchronous I/O is not supported by IDE controllers, for
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| 76 | performance reasons, you may want to leave host caching enabled
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| 77 | for your VM's virtual IDE controllers.
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| 78 | </p>
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| 79 | <p>
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| 80 | For this reason, <ph conkeyref="vbox-conkeyref-phrases/product-name"/> enables you to configure whether
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| 81 | the host I/O cache is used for each I/O controller separately.
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| 82 | Either select the <b outputclass="bold">Use Host I/O
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| 83 | Cache</b> check box in the
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| 84 | <b outputclass="bold">Storage</b> settings for a given
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| 85 | virtual storage controller, or use the following
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| 86 | <userinput>VBoxManage</userinput> command to disable the host I/O
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| 87 | cache for a virtual storage controller:
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| 88 | </p>
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| 89 | <pre xml:space="preserve">VBoxManage storagectl "VM name" --name <controllername> --hostiocache off</pre>
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| 90 | <p>
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| 91 | See <xref href="vboxmanage-storagectl.dita"/>.
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| 92 | </p>
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| 93 | <p>
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| 94 | For the above reasons, <ph conkeyref="vbox-conkeyref-phrases/product-name"/> uses SATA controllers by
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| 95 | default for new virtual machines.
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| 96 | </p>
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| 97 | </body>
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| 98 |
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| 99 | </topic>
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