BTRFS's copy-on-write and VirtualBox disk images

markus-n · 03-21-2015, 04:24 AM

I've ordered a new harddisk and I'm thinking about formatting it as BTRFS. The feature I like most about BTRFS are snapshots. As far as I understand, the snapshots are possible because BTRFS does copy-on-write.
But I also use VirtualBox with several virtual machines. What happens if a machine is started and regularly writes to it's disk image ? Will BTRFS copy all the large file for every small update ?
I think I can go without snapshots for the disk images. Regular backups would be sufficient for them.

syg00 · 03-22-2015, 12:38 AM

I see Arch listed in your profile - see the wiki entry for btrfs. It specifically recommends disabling CoW for VMs.
As for the unit of write, I'd reckon a block (16k by default), but I haven't checked. If a block is updated, the snap will be updated to contain the "before-update" instance - so yes it can grow over time.