With VirtualBox , it's possible to emulate many types of hard drives and make them appear as conventional hard drives or SSDs if you wish. In particular, to avoid data loss. To add an IDE hard drive, the principle is the same.
Click the small hard disk icon to the right of : IDE Controller , and then click : Create a new disk. If you have already mounted an old computer, you probably know that the hard drives had to be connected respecting a system of master and slave. Just for info, it was done through jumpers. These are small white plastic blocks that had to be plugged in according to :.
This is the most common method, described in Section 5. Similarly, how do I create a virtual hard drive in VirtualBox? You will see the screen on the right. For Memory, choose a suitable value usually more than MB.
As shown on the right, choose "create a virtual hard disk". VDI is the native format of VirtualBox. HDD is a format for Parallels. The VHD file represents a physical hard disk and can be used by a virtual machine as if it were a physical hard disk.
What is virtual disk image? A virtual disk image VDI is the image of a virtual hard disk or the logical disk associated with a virtual machine. How do I open virtual disk image? If there are more than one partition in the vdi file, PowerISO will list all partitions, you need select a partition from the list to continue. What is virtual disk?
A virtual disk is a file or set of files that appears as a physical disk drive to a guest operating system. Because the virtual disks are stored as files on the host machine or a remote computer, you can move them easily to a new location on the same computer or to a different computer.
How does a virtual hard disk work? Virtual hard disk VHD is a disk image file format for storing the complete contents of a hard drive.
The disk image, sometimes called a virtual machine, replicates an existing hard drive and includes all data and structural elements. A differencing image by itself is useless, it must always refer to another image. The differencing image is then typically referred to as a child , which holds the differences to its parent. When a differencing image is active, it receives all write operations from the virtual machine instead of its parent.
The differencing image only contains the sectors of the virtual hard disk that have changed since the differencing image was created. When the machine reads a sector from such a virtual hard disk, it looks into the differencing image first. If the sector is present, it is returned from there. In other words, the parent becomes read-only. It is never written to again, but it is read from if a sector has not changed. Differencing images can be chained.
If another differencing image is created for a virtual disk that already has a differencing image, then it becomes a grandchild of the original parent.
The first differencing image then becomes read-only as well, and write operations only go to the second-level differencing image. When reading from the virtual disk, Oracle VM VirtualBox needs to look into the second differencing image first, then into the first if the sector was not found, and then into the original image. There can be an unlimited number of differencing images, and each image can have more than one child.
As a result, the differencing images can form a complex tree with parents, siblings, and children, depending on how complex your machine configuration is.
Write operations always go to the one active differencing image that is attached to the machine, and for read operations, Oracle VM VirtualBox may need to look up all the parents in the chain until the sector in question is found. You can view such a tree in the Virtual Media Manager. In all of these situations, from the point of view of the virtual machine, the virtual hard disk behaves like any other disk. This is not noticeable however since the tables with sector information are always kept in memory and can be looked up quickly.
When you create a snapshot, as explained in the previous section, Oracle VM VirtualBox freezes the images attached to the virtual machine and creates differencing images for each image that is not in write-through mode. From the point of view of the virtual machine, the virtual disks continue to operate before, but all write operations go into the differencing images. Each time you create another snapshot, for each hard disk attachment, another differencing image is created and attached, forming a chain or tree.
In the above screenshot, you see that the original disk image is now attached to a snapshot, representing the state of the disk when the snapshot was taken.
If you restore a snapshot, and want to go back to the exact machine state that was stored in the snapshot, the following happens:. Oracle VM VirtualBox copies the virtual machine settings that were copied into the snapshot back to the virtual machine. As a result, if you have made changes to the machine configuration since taking the snapshot, they are undone.
If the snapshot was taken while the machine was running, it contains a saved machine state, and that state is restored as well. After restoring the snapshot, the machine will then be in Saved state and resume execution from there when it is next started.
Otherwise the machine will be in Powered Off state and do a full boot. For each disk image attached to the machine, the differencing image holding all the write operations since the current snapshot was taken is thrown away, and the original parent image is made active again. If you restored the root snapshot, then this will be the root disk image for each attachment. Otherwise, some other differencing image descended from it. This effectively restores the old machine state.
If you later delete a snapshot in order to free disk space, for each disk attachment, one of the differencing images becomes obsolete. In this case, the differencing image of the disk attachment cannot simply be deleted. Instead, Oracle VM VirtualBox needs to look at each sector of the differencing image and needs to copy it back into its parent. This is called "merging" images and can be a potentially lengthy process, depending on how large the differencing image is.
It can also temporarily need a considerable amount of extra disk space, before the differencing image obsoleted by the merge operation is deleted. Immutable images. When an image is switched to immutable mode, a differencing image is created as well. As with snapshots, the parent image then becomes read-only, and the differencing image receives all the write operations.
Every time the virtual machine is started, all the immutable images which are attached to it have their respective differencing image thrown away, effectively resetting the virtual machine's virtual disk with every restart.
You can duplicate hard disk image files on the same host to quickly produce a second virtual machine with the same OS setup. However, you should only make copies of virtual disk images using the utility supplied with Oracle VM VirtualBox. If you do accidentally try to reimport a disk image which you copied normally, you can make a second copy using the VBoxManage clonevm command and import that instead.
Note that Linux distributions identify the boot hard disk from the ID of the drive. So if you clone a disk image and try to boot the copied image the guest might not be able to determine its own boot disk as the UUID changed.
The disk ID looks like the following:. 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. 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. Additionally it supports sharing limits through bandwidth groups for several images. It is possible to have more than one such limit. Limits are configured using VBoxManage. The example below creates a bandwidth group named Limit, sets the limit to 20 MB per second, and assigns the group to the attached disks of the VM:.
All disks in a group share the bandwidth limit, meaning that in the example above the bandwidth of both images combined can never exceed 20 MBps. However, if one disk does not require bandwidth the other can use the remaining bandwidth of its group.
The limits for each group can be changed while the VM is running, with changes being picked up immediately. The example below changes the limit for the group created in the example above to 10 MBps:. The medium configuration is changeable at runtime. You can select between the following options to provide the medium data:. Host Drive defines that the guest can read from the medium in the host drive. Image file gives the guest read-only access to the data in the image.
This is typically an ISO file. Empty means a drive without an inserted medium. Changing between the above, or changing a medium in the host drive that is accessed by a machine, or changing an image file will signal a medium change to the guest OS. The guest OS can then react to the change, for example by starting an installation program. Medium changes can be prevented by the guest, and Oracle VM VirtualBox reflects that by locking the host drive if appropriate.
You can force a medium removal in such situations by using the VirtualBox Manager or the VBoxManage command line tool. The guest OS can issue error messages, just like on real hardware, and guest applications may misbehave. Use this with caution. This is to prevent hardware detection from being triggered in the guest OS every time the configuration is changed.
Depending on the host hardware, this may potentially enable the following things to work:. To enable host drive passthrough you can use the --passthrough option of the VBoxManage storageattach command. Even if passthrough is enabled, unsafe commands, such as updating the drive firmware, will be blocked. Video CD formats are never supported, not even in passthrough mode, and cannot be played from a virtual machine.
On Oracle Solaris hosts, passthrough requires running Oracle VM VirtualBox with real root permissions due to security measures enforced by the host. Especially with the advent of Gigabit Ethernet, it has become affordable to attach iSCSI storage servers simply as remote hard disks to a computer network.
Use this utility to mount, view, and optionally modify the disk image contents. When using vboximg-mount , ensure that the following conditions apply:. The disk image is not being used by any other systems, such as by guest VMs.
Raw access using FUSE is preferred over direct loopback mounting of virtual disk images, because it is snapshot aware. It can selectively merge disk differencing images in an exposed virtual hard disk, providing historical or up-to-date representations of the virtual disk contents.
Also, you can view partition information for a disk image. The vboximg-mount command includes experimental read-only access to file systems inside a VM disk image. This feature enables you to extract some files from the disk image without starting the VM and without requiring third-party file system drivers on the host system. Use the --help option to view information about the vboximg-mount command usage.
The complete command reference is described in Section 8.
0コメント