Virtualization Pro

Oct 28 2009   6:32PM GMT

Restoring individual files from image-level backups of virtual machines

Eric Siebert Eric Siebert Profile: Eric Siebert

Virtual environments can change the way you back up your servers by providing an additional backup method where you back up the single large virtual disk file instead of the individual files inside the VM operating system. There is sometimes confusion, however, when using this method as if individual file restores are possible and how difficult it might be to restore them back to a VM.

There are two methods for backing up a VM, traditional backup methods that install an agent inside the OS and back it up file-by-file and image-level backups that back up the single virtual disk VMDK file. Image-level backups are usually done by backup applications that are designed to specifically back up virtual machines, like Veeam Backup and Replication. These applications use the snapshot feature that is built in to VMware to stop disk writes to the virtual disk so it can be safely backed up. Backup applications read the original virtual disk file which is now read-only as new disk writes get written to a newly created delta virtual disk file. Once the backup application has read all the blocks from the original virtual disk file the snapshot is committed, which takes the data from the delta virtual disk and writes it to the original virtual disk. Once this is complete the delta virtual disk is deleted.

In virtual environments it’s more efficient to do image-level backups, but they also offer more options when you need to do a restore. If you want to restore the whole VM to a previous state you can simply restore the whole virtual disk file. Or, if you want to restore individual files, you can easily do this also. When using a backup application for virtual environments the process for individual file restores in virtual machines is easy and straight-forward. For example, Veeam Backup and Replication can quickly mount the backed-up VM disk file from the backup location so it is available for browsing; the files that need to be restored can then be selected and copied back to the original VM, a process that takes only minutes.

VMware’s own VM backup product, VMware Data Recovery can also do individual file restores via a command-line application that mounts the virtual disk from a selected VM/restore point to a Windows drive letter so the files can be accessed and copied. Even if you use simple scripts to back up a VM by copying its disk to another storage location you can restore files easily enough. Virtualization makes this fairly simple as you can just add the backed-up virtual disk file to another helper VM as an additional disk, browse the files on that disk and copy and paste them back to the original VM.

As you can see restoring individual files to virtual machines is a simple and easy process when using a backup application designed to work with virtual machines. It may not be the same process that is used with physical servers and traditional methods, but it is definitely possible and not all that difficult. If anything virtualization makes backup and recovery easier by providing many unique alternative backup and recovery methods then are not possible with traditional methods.

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