Re: WG: How to properly setup for snapshots

[Goffredo Baroncelli <kreijack@libero.it>]

On 12/21/20 7:26 PM, Andrei Borzenkov wrote:
> 21.12.2020 20:37, Roman Mamedov пишет:
[...]
> Having dedicated subvolume containing snapshots makes it easy to switch
> your root between subvolumes (either for roll back or transactional
> updates or whatever) and retain access to snapshots by simply mounting
> containing subvolume. Having them in subdirectory of your (root)
> subvolume means you can no more remove this subvolume without also
> destroying snapshots before, so you are stuck with it.
> 
> So it makes all sort of sense to think in advance and prepare dedicated
> subvolume for this purpose.

There is an asymmetry of the '/' subvolume; it can't be deleted, so it is not easy and cheap to swap it with its snapshot.

In my setup, I uses the '/' subvolume as container of all the other subvolumes, one of these is the root of my filesystem. Then each subvolume is mounted where it is needed:

$ findmnt  | egrep sdg3
/                                     /dev/sdg3[/@rootfs]       btrfs       rw,noatime,nodiratime,nossd,space_cache,subvolid=448,subvol=/@rootfs
├─/boot                               /dev/sdg3[/@boot]         btrfs       rw,noatime,nodiratime,nossd,space_cache,subvolid=258,subvol=/@boot
├─/var/btrfs                          /dev/sdg3                 btrfs       rw,noatime,nodiratime,nossd,space_cache,subvolid=5,subvol=/
├─/var/lib/machines/vm-sqlserver      /dev/sdg3[/@vm-sqlserver] btrfs       rw,noatime,nodiratime,nossd,space_cache,subvolid=454,subvol=/@vm-sqlserver
├─/var/lib/portables                  /dev/sdg3[/@portables]    btrfs       rw,noatime,nodiratime,nossd,space_cache,subvolid=282,subvol=/@portables
├─/var/log                            /dev/sdg3[/@log]          btrfs       rw,noatime,nodiratime,nossd,space_cache,subvolid=447,subvol=/@log

I mount the '/' subvolume under /var/btrfs, where i manage the subvolume. With this setup is easy to snapshot the root filesystem (/dev/sdg3[/@rootfs]) and mount the snapshot instead the old root. The other part of the filesystem which are better to not involve in the snapshot-rollback process (like /boot, /var/log...) are in a separate subvolumes which are mounted at boot time using /etc/fstab

I tough quite often that mkfs.btrfs should allow to create a "default" subvolume inside the '/'. It would simplify the management of the root filesystem in case if the user want to srollback the system to a previous snapshot.

BR

> 
>> not much of the "inside". Might as well just create a regular directory for
>> that -- and with less potential for confusion.
>>
>> * - keep in mind that "snapshot" and "subvolume" mean the same thing in Btrfs,
>>      the only difference being that "snapshot"-subvolume started its life as
>>      being a full copy(-on-write) of some other subvolume.
>>
> 


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