Re: How does Suse do live filesystem revert with btrfs?

[Duncan <1i5t5.duncan@cox.net>]

Marc MERLIN posted on Sat, 03 May 2014 17:52:57 -0700 as excerpted:

> (more questions I'm asking myself while writing my talk slides)
> 
> I know Suse uses btrfs to roll back filesystem changes.
> 
> So I understand how you can take a snapshot before making a change, but
> not how you revert to that snapshot without rebooting or using rsync,
> 
> How do you do a pivot-root like mountpoint swap to an older snapshot,
> especially if you have filehandles opened on the current snapshot?
> 
> Is that what Suse manages, or are they doing something simpler?

While I don't have any OpenSuSE specific knowledge on this, I strongly 
suspect their solution is more along the select-the-root-snapshot-to-roll-
back-to-from-the-initramfs/initrd line.

Consider, they do the snapshot, then the upgrade.  In-use files won't be 
entirely removed and the upgrade actually activated for them until a 
reboot or at least an application restart[1] for all those running apps 
in ordered to free their in-use files, anyway.  At that point, if the 
user finds something broke, they've just rebooted[1], so rebooting[1] to 
select the pre-upgrade rootfs snapshot won't be too big a deal, since 
they've already disrupted the normal high level session and have just 
attempted a reload in ordered to discover the breakage, in the first 
place.

IOW, for the rootfs and main system, anyway, the rollback technology is a 
great step up from not having that snapshot to rollback to in the first 
place, but it's /not/ /magic/; if a rollback is needed, they almost 
certainly will need to reboot[1] and from there select the rootfs 
snapshot to rollback to, in ordered to mount it and accomplish that 
rollback.

---
[1] Reboot:  Or possibly dipped to single user mode, and/or to the 
initramfs, which they'd need to reload and switch-root into for the 
purpose, but systemd is doing just that sort of thing these days in 
ordered to properly unmount rootfs after upgrades before shutdown as it's 
a step safer than the old style remount read-only, and implementing a 
snapshot selector and remount of the rootfs in that initr* instead of 
dropping all the way to a full reboot is only a small step from there.

-- 
Duncan - List replies preferred.   No HTML msgs.
"Every nonfree program has a lord, a master --
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