5 Virtual storage 5.7 Host I/O caching Starting with version 3.2, VirtualBox can optionally disable the I/O caching that the host operat- ing system would otherwise perform on disk image files. Traditionally, VirtualBox has opened disk image files as normal files, which results in them being cached by the host operating system 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 never occurred virtual disks residing on remote iSCSI storage, which is the more common scenario in enterprise-class setups (see chapter 5.10, iSCSI servers, page 82). While buffering is a useful default setting for virtualizating a few machines on a desktop computer, there are some disadvantages to this approach: 1. 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 (power failure, host crash), the likelihood of data loss increases. 2. 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. 3. Physical memory is often wasted as guest operating systems 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, 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, 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, VirtualBox allows you to configure whether the host I/O cache is used for each I/O controller separately. Either uncheck the “Use host I/O cache” 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 controllername --hostiocache off See chapter 8.17, VBoxManage storagectl, page 121 for details. For the above reasons also, VirtualBox now uses SATA controllers by default for new virtual machines. 5.8 Limiting bandwidth for disk images Starting with version 4.0, VirtualBox allows for limiting the maximum bandwidth used for asyn- chronous I/O. Additionally it supports sharing limits through bandwidth groups for several im- ages. It is possible to have more than one such limit. 80
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