Secure virtual disk deletion - is your data safe?

Everyone knows the dangers of old hard disks being discarded with sensitive data still on them, but what about virtual disks? With so many virtual machine hosting services cropping up of late (hi!), have you ever wondered what happens to your data when you delete your virtual machine?

Usually your machine’s ‘partition’ is just a small part of a larger disk array; the partition is deleted and the space returned for the pool to be used by another virtual machine. This means, the next time someone buys a virtual machine with the same host, some of the blocks that made up your filesystem could end up making up their filesystem. The metadata will be wiped clean when the filesystem is formatted of course, so they won’t just see your files listed, but the blocks can still contain your data. It depends on how they’re managing their disks.

Homework: go buy a virtual machine somewhere and pipe the contents of your new disk through the strings command and look out for anything that isn’t yours (ssh root@newmachine "dd if=/dev/sda1 bs=1M | strings"). Extra credit if you don’t get thrown off your new host on the first day for maxing out the disk IO :)

So, you’re probably careful and securely wipe your sensitive data before you leave, phew. But disk space is virtualised too. The blocks that make up your disk might not all be in order or even all be on the same disk. With snapshots, your data may exist in duplicate too that you can’t even access. And what about if you bought extra disk space, then removed it?

At Brightbox we use Linux’s LVM implementation to manage disk space and these are problems we have to deal with and we take it seriously. All virtual machine disks are wiped at the block level when the machine is deleted or when a new machine is created. The only corner case we’re likely to run into is if a disk image is extended into space that had previously been used as a snapshot or as a disk that was shrunk. Luckily we don’t currently offer snapshots or disk shrinking but it’ll be something we’ll probably offer at some point, so we’ll have to address it then.

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