Tag Archives: backup

This old CBT bug that puts your backups at risk!

Recently an old, old CBT (Changed Block Tracking) bug has been discovered. This bug exists since ESX 4.x! As a reminder, CBT keeps track of the changed blocks of a virtual machine disk, which enables incremental backups of virtual disks. As far as I know, all software vendors rely on CBT to provide virtual machine backup, so that’s a big issue!

The problem appears when virtual disks are extended past specific disk sizes: 128GB, 256GB, 512GB or 1024GB. When this happens, CBT transmits incorrect information and backups become corrupt. The very bad part is that the bug is completely silent and you won’t notice until you (try to) restore the virtual machine.

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