| Technical background |
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This document provides a technical overview on VBoot. After reading this document, you should be able to get some ideas on how VBoot works internally. VBoot consists of three components, the virtual disk format, and VBoot Loader and the VBoot kernel drivers. Virtual StorageVBoot stores operating systems inside some kind of virtual storage, usually disk image files. VBoot supports all popular standard formats, and you can use tools from other vendors to manipulate them, in addition to those provided by VBoot. Disk image files (VHD, VMDK, VDI, Raw)Disk image files reside on the host system and are seen by the operating systems as hard disks of a certain geometry. When an operating system reads from or writes to a hard disk, VBoot redirects the request to the image file. When you create an image file, its capacity needs to be specified, which represents a fixed geometry of the virtual disk. It is therefore not possible to change the size of the virtual hard disk later. VBoot supports four variants of disk image files, which are the most popular formats:
Irrespective of the disk format, there are two options of how to create a disk image: fixed-size or dynamically expanding.
Note that the plain raw disk format must be allocated as ixed size. Differencing imagesSometimes, you may want to save disk changes to separate image files, this kind of disk images are called differencing images. VBoot fully supports this feature. A differencing image is a special disk image that only holds the differences to another image. 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 and process all write operations from the operating system 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; if not, VBoot looks into the parent. 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, VBoot 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, VBoot may need to look up all the parents in the chain until the sector in question is found. In all of these situations, from the point of view of the physical machine, the virtual hard disk behaves like any other disk. While the machine is running, there is a slight run-time I/O overhead because VBoot might need to look up sectors several times. This is not noticeable however since the tables with sector information are always kept in memory and can be looked up quickly. Differencing images are used in the following situations: 1. Snapshots. When you create a snapshot, VBoot “freezes” the image attached to the current running machine and creates a differencing image to the image. From the point of view of the machine, the virtual disks continue to operate as before, but all write operations go into the differencing image. Each time you create another snapshot, another differencing image is created and attached, forming a chain or tree. When you restore a snapshot – that is, if you want to go back to the exact disk state that was stored in the snapshot, 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; otherwise, some other differencing image descended from it.) This effectively restores the old disk state. If you later delete a snapshot in order to free disk space, one of the differencing images becomes obsolete. In this case, the differencing image of the disk cannot simply be deleted. Instead, VBoot 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. 2. Immutable images. When an image is booted as “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 machine is started, the immutable image has its differencing image thrown away, effectively resetting the virtual disk with every restart. To boot a virtual harddisk as immutable, you can simply append the keyword "immutable" to the vboot line inside grub.cfg file, for example: vboot harddisk=(hd0,1)/xp.vmdk immutable indicates xp.vmdk will be immutable, and all system changes, including files and registry entries will be thrown away after reboot. This way, you can browse to any websites without fear, any computer virus or spyware installed during the session will be automatically deleted after your machine is rebooted. VBoot LoaderVBoot Loader is is the first software program that runs when your computer starts. It is responsible for loading and transferring control to the operating system kernel software. VBoot Loader is modified from the popular GRUB2 boot loader, in particular, VBoot Loader supports loading operating system from virtual disk files in various disk formats, including VHD, VMDK, VDI, ISO and raw images. Features
Using Windows 7 Boot ManagerFor windows 7, you can actually use Windows 7 boot manager to boot Windows 7 VHD. Note only Windows 7 virtual disk file in VHD format can be booted this way. In all other cases, you will have to use VBoot boot manager. You can do the following to add Windows 7 vhd and boot via bootmgr: bcdedit /copy {default} /d "Windows 7 VBoot VHD" this will print out a uuid, used as input for below: bcdedit /set {guid} device vhd=[C:]\windows7.vhdbcdedit /set {guid} osdevice vhd=[C:]\windows7.vhd then you should be able to boot the VBoot VHD. VBoot command optionsTo use VBoot Loader, you need to insert a command into the grub.cfg configuration files. VBoot Loader passes these options to vboot virtual disk drivers inside the operating system. The VBoot virtual disk drivers utilize these options to perform the corresponding operations, such as mounting the disk files, taking a new snapshot, restoring to a snapshot and delete the child differencing disks, updating the grub.cfg file, etc. The full syntax of vboot command is as follows: vboot harddisk=disk_path cdrom=file_path floppy=file_path floppy_b=file_path boot=harddisk|cdrom|floppy parent_dir=folder_path take_snapshot=differencing_disk_path boot_entry=index config_file=grub_config_file new_menu_entry=title restore immutable recover_mbr pause Options:
VBoot Kernel DriversThe VBoot Loader lives outside of the virtual disk files, however in order to boot a virtual disk file, a kernel driver corresponding to the installed operating system must be present inside the virtual disk file:
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