On Fri, May 19, 2017, at 11:27 AM, Theodore Ts'o wrote: > > One of the things that came up when Darrick and I discussed this on > the weekly ext4 developer's conference call was our mutual wonderment > that none of the userspace tools implemented a reboot by created a > tmpfs chroot, pivoting into the chroot, and then unmounting all of the > remaining file systems. On general purpose systems we have a tmpfs chroot already: the initramfs. Although IIRC, systemd will only switch back to it on shutdown I think only if you have a root storage daemon enabled: https://www.freedesktop.org/wiki/Software/systemd/RootStorageDaemons/ That said I'd like to focus on the harder case: supporting powerloss/system lockup on single-partition systems. IMO, the shutdown case is just a special variant of that where the user asked nicely for the system to halt =) (See also https://en.wikipedia.org/wiki/Crash-only_software) I was thinking about this a bit, and I think if userspace tools (like ostree) *delayed* their updates to /boot until shutdown, then we could ensure that on powerloss, the system is unchanged. (In a traditional dpkg/rpm scenario where you only have one userspace root, you'd end up with old kernel + new rootfs, but that's exactly the problem ostree solves) That narrows the problem down to keeping `/boot` consistent at shutdown time. AIUI, a problem here is that XFS doesn't flush the journal on `syncfs`, only on unmount? And from what I can tell, even the `XFS_IOC_FREEZE` ioctl won't do that either. So as far as I can see, a userspace API to ensure the journal is flushed on a mounted filesystem is going to be necessary for the general case. I don't have a strong opinion on whether or not that's `syncfs()` - if it's e.g. a `XFS_IOC_FREEZE` `_THAW` pair that seems OK to me too.
