On 4/13/19 5:40 PM, Peter Wiersig wrote: > Peter Wiersig <pe...@friesenpeter.de> writes: >> >> I would be pissed if my OS removes snapshots I might or might not need >> in the future. That's a release critical bug in my eyes. Yeah, I know >> Microsoft and Apple do that automatically if your capacity runs out, but >> that's also why I don't recommend them at all. > > Ok, I checked https://en.opensuse.org/Portal:Snapper and they do support > LVM and ext4, and they have a bullet of auto-removing old snapshots. I > hope they did it right, perhaps I need to make a new test drive with the > latest release. > > Snapshots on ZFS can't be zero cost, so you need to account for them > there, too. > > Peter >
ZFS Snapshots are nearly zero cost to create the snapshot, since ZFS (and likewise BTRFS) are copy-on-write file systems. What it does is records the deltas after the snapshots. This is a good thinkg as it saves on disk space. For example: You create a random 10 MB file, and take a snapshot. You then alter the 5 tail MBs and add five more MBs at the tail. You are left with: Pre-Snapshot | 5MB chunk #1 | 5MB chunk #2 | - total 10MBs Post-Snapshot | 5MB chunk #1 | 5MB chunk #3 | 5MB chunk #4 | - total 15MBs Actual disk usage will be 20 MBs, since the 5MB chunk #1 is only recorded on disk once, not twice. Here is a good talk on the subject by Michael Lucas, one of the premier experts on ZFS. Its worth noting that a lot of the concepts apply to BTRFS to varying degrees: https://www.youtube.com/watch?v=x9A0dX2WqW8 -Matt
