The virtual machine: disks, images and snapshots
What a virtual machine is made of (configuration, virtual disk, ISO image), fixed or dynamic allocation, copy-on-write snapshots, full or linked clones and templates.
- 25 cards
- 6 topics
- Free
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Each topic is weighted the way the exam weighs it.
- Concepts
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- Sort and link
- The numbers
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A few cards from the deck
Virtual machine: what is it?
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A computer simulated by a hypervisor, described by a configuration file (processors, memory, devices) and by one or more disk files.
Everything that defines it fits in files: it can be copied, archived or moved like any other data.
What does a snapshot hold?
- The frozen disk state; later writes go to a differencing disk
- A full, independent copy of the disk, stored on another medium
- Only the VM's RAM, without any of the disk's contents
- The list of files changed since the machine's last backup
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The frozen disk state; later writes go to a differencing disk
A snapshot copies nothing: it freezes the existing disk. That is what makes it instant, and why it must not be mistaken for an independent backup.
After a snapshot, the guest's writes go to a … disk, while the original disk becomes ….
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After a snapshot, the guest's writes go to a differencing disk, while the original disk becomes read-only.
Reverting to the snapshot means throwing the differencing disk away: the original disk stayed as it was.
Virtual disk: what is it?
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A file on the host that the guest sees as a hard disk; VDI, VMDK, QCOW2 and VHD are common formats.
VDI is VirtualBox's, VMDK is VMware's, QCOW2 is QEMU and KVM's, VHD is Microsoft's. An ISO image, on the other hand, acts as an optical drive to install a system.