What is image-level backup?
Image-level backup captures a whole system, including the operating system, applications, configuration and data, as a block-level image of its disks. The restore point represents the complete machine at one moment and can be used to rebuild it in full or to extract individual files.
How does image-level backup differ from file-level backup?
File-level backup walks the file system and copies files one at a time. Image-level backup reads disk blocks beneath the file system and has no need to open individual files.
| Aspect | Image-level backup | File-level backup |
|---|---|---|
| Unit captured | Disk blocks of the whole system | Selected files and folders |
| Full system restore | Yes, directly | Requires reinstalling the system first |
| Speed on many small files | Fast, unaffected by file count | Slower as file counts grow |
| Selective scope | Whole disks, with exclusions | Precise control per folder |
| Typical use | Virtual machines, servers | File shares, user data, NAS |
How is an image-level backup taken?
For virtual machines, the backup software asks the hypervisor for a snapshot and reads the virtual disks through a backup proxy, using changed block tracking to limit later runs to changed blocks. For physical machines, an agent inside the system uses a volume snapshot and a tracking driver to do the same. Adding application awareness makes the image consistent for databases.
What can be restored from an image?
An image supports several kinds of restore. The entire machine can be restored to its original place or to different hardware or a different hypervisor. Individual virtual disks can be restored. The image can be mounted to pull out files, which is file-level restore. And with instant recovery the machine can be started directly from the backup storage while the data is copied back in the background.
How does image-level backup affect storage?
Images include everything on the disk, so the first full is as large as the used space of the system. Later runs store only changed blocks. Operating system files are near-identical across machines, which makes images respond well to deduplication. Instant recovery places an extra demand on the repository, because it reads the image with random access while the machine runs from it.
Frequently asked questions
Is an image-level backup the same as a snapshot?
No. A snapshot is a point-in-time view kept on the same storage as the original. An image-level backup is a separate copy stored in a backup repository.
Can single files be restored from an image-level backup?
Yes. Backup software mounts the image and lets files and folders be copied out of it.
Does image-level backup include unused disk space?
Usually not. Backup software skips blocks the file system reports as unused, and often swap and temporary files as well.