5.7. Host Input/Output Caching

Oracle VM VirtualBox can optionally disable the I/O caching that the host OS would otherwise perform on disk image files.

Traditionally, Oracle VM VirtualBox has opened disk image files as normal files, which results in them being cached by the host OS like any other file. The main advantage of this is speed: when the guest OS writes to disk and the host OS cache uses delayed writing, the write operation can be reported as completed to the guest OS quickly while the host OS can perform the operation asynchronously. Also, when you start a VM a second time and have enough memory available for the OS to use for caching, large parts of the virtual disk may be in system memory, and the VM can access the data much faster.

Note that this applies only to image files. Buffering does not occur for virtual disks residing on remote iSCSI storage, which is the more common scenario in enterprise-class setups. See Section 5.10, “iSCSI Servers”.

While buffering is a useful default setting for virtualizing a few machines on a desktop computer, there are some disadvantages to this approach:

  • Delayed writing through the host OS cache is less secure. When the guest OS writes data, it considers the data written even though it has not yet arrived on a physical disk. If for some reason the write does not happen, such as power failure or host crash, the likelihood of data loss increases.

  • Disk image files tend to be very large. Caching them can therefore quickly use up the entire host OS cache. Depending on the efficiency of the host OS caching, this may slow down the host immensely, especially if several VMs run at the same time. For example, on Linux hosts, host caching may result in Linux delaying all writes until the host cache is nearly full and then writing out all these changes at once, possibly stalling VM execution for minutes. This can result in I/O errors in the guest as I/O requests time out there.

  • Physical memory is often wasted as guest OSes typically have their own I/O caches, which may result in the data being cached twice, in both the guest and the host caches, for little effect.

If you decide to disable host I/O caching for the above reasons, Oracle VM VirtualBox uses its own small cache to buffer writes, but no read caching since this is typically already performed by the guest OS. In addition, Oracle VM VirtualBox fully supports asynchronous I/O for its virtual SATA, SCSI, and SAS controllers through multiple I/O threads.

Since asynchronous I/O is not supported by IDE controllers, for performance reasons, you may want to leave host caching enabled for your VM's virtual IDE controllers.

For this reason, Oracle VM VirtualBox enables you to configure whether the host I/O cache is used for each I/O controller separately. Either select the Use Host I/O Cache check box in the Storage settings for a given virtual storage controller, or use the following VBoxManage command to disable the host I/O cache for a virtual storage controller:

VBoxManage storagectl "VM name" --name <controllername> --hostiocache off

See Section 7.18, “VBoxManage storagectl”.

For the above reasons, Oracle VM VirtualBox uses SATA controllers by default for new virtual machines.