El dom, 6 dic 2020 a las 22:52, Chris Murphy (<li...@colorremedies.com>) escribió:
> On Sun, Dec 6, 2020 at 3:54 PM Sergio Belkin <seb...@gmail.com> wrote: > > > > Hi, > > Let's say that I have 2 subvolumes for both / and /home. What happen > with a snapshot of /home if I run before : > > chattr +C /home/jdoe/VMs ? > > If /home/jdoe/VMs is a directory, then it and its contents are > included in the snapshot. And it means nodatacow extents have become > shared extents. Writes to shared extents are always cow, even if > marked as nocow. So the initial VM "overwrite" to a shared extent will > be cow to a new extent that is exclusive, not shared. Subsequent > writes to that same exclusive extent will be nocow, i.e. an overwrite. > > This same principle applies to reflink copies of any file, whether on > Btrfs or XFS. A reflink copy also causes extents to become shared > extents. > > My suggestion is to put the VM images in their own subvolume. i.e. > from ~/ just do: > > btrfs sub create VMs > > Since Btrfs snapshots are not recursive [1] and instead stop at > subvolume boundaries, a snapshot of ~/ will not snapshot ~/VMs. > > [1] https://github.com/kdave/btrfs-progs/tree/master/libbtrfsutil > libbtrfsutil provies a C API and python bindings making it possible to > do recursive snapshot creation and deletion; but this is not exposed > in the user space tools for a number of reasons, mainly it's > potentially confusing and risky since btrfs subvolumes and snapshots > can be created anywhere without any meaningful limits beyond what the > user institutes. Another advantage of libbtrfsutil is it provides an > _fd variant of subvolume/snapshot creation and deletion, meaning there > doesn't need to be a "line of sight" path to the source or destination > via the mounted file system. Ergo, in effect creating "hidden" > subvolumes and snapshots; "hidden" as in they're not in the normal > mounted file system path. A user can of course always mount the > top-level of the file system and see all such subvolumes. > > > -- > Chris Murphy > Thanks Chris, nice explanation -- -- Sergio Belkin LPIC-2 Certified - http://www.lpi.org
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