[systemd-devel] Merging journal logs from btrfs snapshots

[Chris Murphy]

On Jan 16, 2014, at 1:58 PM, Kai Krakow <hurikhan77 at gmail.com> wrote:

> Chris Murphy <lists at colorremedies.com> schrieb:
> 
>> Due to anti-magic, a recent update horribly broke the system's ability to
>> do further updates. This is resolved by regression to a prior Btrfs
>> snapshot, once updated it works fine. But that's a two week old snapshot.
>> I don't need the broken rootfs but I want to keep the journal for those
>> two weeks.
>> 
>> Is this a reasonable want or need and if so how to merge the logs? Between
>> the two snapshots there are several like named files in
>> /var/log/journal/<machine-id>.
> 
> I'd recommend to place /var/log/journal on a subvolume so it is not affected 
> by snapshotting. You can do separate snapshots for it (tho I cannot imagine 
> why you would want to do it). That way you get a snapshot "protection" for 
> these files, too, and you are free to roll back the rest of the system 
> without affecting this subvolume.

Aha, good idea. So then I mount the subvol at /var/log/journal? Is there any risk of journald writing to rootfs /var/log/journal before the subvolume is mounted? Or is the flush to persistent storage sufficiently delayed as to not be a concern?

Chris Murphy