On Tue, Dec 8, 2020 at 5:53 AM Sreyan Chakravarty <sreyan32@xxxxxxxxx> wrote: > > On Mon, Dec 7, 2020 at 3:43 PM Chris Murphy <lists@xxxxxxxxxxxxxxxxx> wrote: >> >> >> This isn't a great location for a swapfile on btrfs because you can't ever snapshot /. > > > You never said why it was not possible to snapshot /. > > Why is that ? I referred you to man 5 btrfs about a list of limitations of swapfiles on btrfs 10 days ago. https://lists.fedoraproject.org/archives/list/users@xxxxxxxxxxxxxxxxxxxxxxx/message/XF7YO27N2XPWGLPMPUN76VFT3RW2KFYS/ It's important to know all the limitations. Most people will find them a non-issue. But... Like, where should the swapfile go? I think maybe /var/swap could be a nested subvolume, i.e. actually create the subvolume in /var/. That's pretty straightforward, and is compatible with rpm-ostree systems where root is read-only. The catch is if there's any kind of rollback of sysroot (on the subvolume typically named "root") because if /var/swap is a subvolume, then the snapshot of the "root" subvolume on / will stop at /var/swap. That's exactly what we want, to avoid snapshotting it. But then on rollback of "root", /var/swap is just an empty directory. So we need some intelligent handling up front, or at the back end to find that older/pre-rollback /var/swap and move that subvolume into the new position. Ick. Right? However, it might be better to have a "swap" subvolume along with "root" and "home" in the top-level of the file system. And add it to fstab or maybe we can create a native systemd mount unit. Importantly we have to have complete certainty that this mount happens before swapon, or it'll fail. So we can't have races either. Now both issues are fixed. It won't be snapshot if we snapshot "root". But if we rollback a "root" snapshot, then the same "swap" subvolume always mounts into the proper position at /var/swap with the same swapfile. But that automatic solution is also kinda hidden. So no matter what there's some kind of tradeoff. And that's the long answer for why it's not the default yet, and also not even documented how to do a post-install setup of swap. But it's on my todo. -- Chris Murphy _______________________________________________ users mailing list -- users@xxxxxxxxxxxxxxxxxxxxxxx To unsubscribe send an email to users-leave@xxxxxxxxxxxxxxxxxxxxxxx Fedora Code of Conduct: https://docs.fedoraproject.org/en-US/project/code-of-conduct/ List Guidelines: https://fedoraproject.org/wiki/Mailing_list_guidelines List Archives: https://lists.fedoraproject.org/archives/list/users@xxxxxxxxxxxxxxxxxxxxxxx
