On Wed, 2020-11-11 at 01:04 -0700, Chris Murphy wrote: > On Thu, Nov 5, 2020, 10:03 AM Patrick O'Callaghan <pocallag...@gmail.com> > wrote: > > > The swapon(8) man page says: > > > > The swap file implementation in the kernel expects to be able to > > write > > to the file directly, without the assistance of the filesystem. This > > is a > > problem on files with holes or on copy-on-write files on filesystems > > like Btrfs. > > > > As I'm getting OOM errors when I try to run a VM, it looks like I need > > a swap file or partition. I'd prefer to use part of my (large) SSD for > > this, but currently it's entirely formatted as BTRFS. Do I need to > > resize the BTRFS partition rather than using a swap file? > > > > > There's several options depending on the workload. > > swapfile on Btrfs performs the same as a swap partition in my testing. The > main difference other than the limitations in 'man 5 btrfs' is an > additional set of steps are needed to make it possible to do hibernation. > > The swapfile must not be snapshot. If it is, COW applies and the swapfile > can no longer be activated. There's several ways to avoid this if you want > to snapshot root (but not the swapfile). Create a /swap subvolume, or > /var/swap subvolume. Since btrfa snapshots are not recursive, either of > these prevents a snapshot of root from snapshotting the swapfile. > > if you aren't doing any snapshotting at all then it doesn't matter you can > probably make the swapfile most anywhere. > > I've got some work to do to figure out if there's a more elegant way to do > it. And of course, encryption and SELinux implications. > > The longer story is to automatically create the swap files on demand > dynamically in the proper location. And in that case possibly use zswap, > instead of swap on zram. > > It's a bit esoteric, but there is a way to track sysfs memory.stat for page > faults. And the zram driver tracks some statistics that could be helpful in > figuring out situations where zram is getting full of seldom used dirty > pages that are just taking up memory. In those kinds of workloads it's > probably more beneficial to use conventional disk-based swap. That's > because disk space swap fully evicts the page, freeing up all of that > memory, whereas with the zram based swap we are still consuming some > memory, in effect it's a partial eviction. > > One thing to watch out for with new installs, it's possible we will see > cases where there's average or below average RAM and heavy Firefox usage. > And it could be easier to go below the low water threshold on zram-based > swap, leading to early home issuing SIGTERM to a Firefox tab. This does get > logged. > > I think the first recommendation is to create a custom zram-generator > configuration and bump the size of the zram device from 50% ram to 75% ram. > While 100% is normally OK, it might be a use case better off with disk > based swap. It just depends on the workload. > > Most users should be ok with the defaults. It's what I'm using most of the > time for over one year now. But I'm always on the lookout for any issues.
Thanks. For the moment I'm not doing hibernation (it doesn't work properly when you have a VM guest with GPU passthrough, basically because the GPU state is not preserved) but I'll keep your notes on file if I ever change my mind. poc _______________________________________________ users mailing list -- users@lists.fedoraproject.org To unsubscribe send an email to users-le...@lists.fedoraproject.org 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@lists.fedoraproject.org
