Re: btrfs snapshots (of root file system)?
On Mon, 2023-10-02 at 21:33 +0200, Anders Andersson wrote:
> On Sun, Oct 1, 2023 at 7:20 AM hw <hw@adminart.net> wrote:
> >
> > Hi,
>
> Hello! I'm not going into much detail but maybe I can guide you to
> better be able to find what you want.
Hi, like I said, my idea was to take a snapshot of the root file
system so that I could roll back after software
updates. Unfortunately, that won't work because there won't be
snapshots of the MBR and of the UEFI partition, and using clonezilla
to copy the volume didn't seem to be a very idea because it's on
hardware RAID. I ended up replacing the disks because I wanted to do
that anyway and reinstalled.
Now I have the problem that during the installation, /home was created
as a subvolume, and it can not be deleted. I want /home to be a
normal directory because it's mount point for another volume. I
resorted to renaming it to /nohome so I could create the /home
directory, and I still can't delete it. I might have to reinstall
again to get rid of it. Btrfs still says the path to the subvolume is
home:
# btrfs subvol list /
ID 256 gen 61 top level 5 path home
ID 257 gen 1286 top level 5 path root
ID 258 gen 238 top level 257 path var/lib/machines
When I try to delete it, btrfs either says the parameter is invalid or
that it's not a subvolume, even when I try to delete it using the ID.
I don't have the exact error message now because /home has to mounted
to write emails, and I'm not gona try to delete it before unmounting
it.
So how do you delete a subvolume? Why isn't the path adjusted when
renaming it? That must be somehow buggy.
> > with btrfs, how do I make a snapshot of the root file system? The
> > purpose is to update software and being able to go back to a previous
> > state if necessary.
> >
> > There doesn't seem to be a command to create snapshots but only
> > subvolumes? How does a subvolume turn into a snapshot? (The root
> > file system is, of course, not on a subvolume.)
>
> Everything in btrfs is a suvolume, including the root (aka
> "top-level").
Well, that is nonsense. Something that's on top isn't on a lower
level.
> A snapshot is merely a subvolume created *from* another
> subvolume, and can optionally be read-only.
>
> You can take a snapshot of the top level subvolume by doing "btrfs sub
> snap / @foo". Documentation for creating both empty subvolumes and
> snapshots are in btrfs-subvolume.
Isn't that 'btrfs subvolume snapshot ...'? And I would have created
the snapshot on the other volume because the other volume is larger.
>From what I've been reading that should be no problem.
> > How do I merge snapshots? IIRC, when you remove a ZFS snapshot, the
> > older state is merged to the state the snapshot is in. Apparently
> > btrfs can only delete snapshots --- and it seems like a bad idea to
> > delete the root file system. How would I boot from it when it's been
> > deleted?
>
> I don't think you can merge snapshots the way you describe. I don't
> see how it could be atomic?
Why would something be atomic? Think of how ZFS does it. IIRC, you either
'destroy' a snapshot, which is misleading because the snapshot isn't
destroyed but what the snapshot was made of is being merged with the
snapshot such that its origin disappears and what you get is the
current version as the result of the merge. Or you 'rollback' to
something a snapshot was made of and you get that. Both happens in
the same place. That also is so confusing that I never really did
anything with snapshots.
Btrfs seems to suck in that when you create a snapshot --- which can't be
merged --- and, which is very important, it doesn't happen in place:
If you want to roll back to a previous version, you somehow need to
replace the current version with the old version, and that leaves you
with the question of how you put the old version into the newer
version after you deleted the newer one.
Not being able to do it in place makes me wonder what snapshots are
supposed to be for. They're pretty useless when you can't just roll
back, and they are no substitue for backups. If you have to make them
on the same volume, that's a very severe limitation. So what's the
point?
Or maybe I got it all wrong. It's confusing.
> You can however move subvolumes around freely, create new snapshots
> at will, and select the boot subvolume either by the "subvol=<path>"
> mount option or by setting the default subvolume (btrfs sub
> set-default).
So I can create a snapshot on a different volume? I haven't tried.
There's no subvolume for /boot but an EFI partition. IIRC that's part
of the reason why you can not use btrfs for boot volumes without
hardware RAID. When one of the disks fails, you're screwed if the
'right' one is the one that failed.
> > Can I make a snapshot on a different volume? The manpage doesn't say
> > that the destination of a subvolume must be on the same volume, and in
> > any case, I should be able to do that.
>
> You can create snapshots anywhere *within the same filesystem*. To be
> fair, the manpage says "A BTRFS subvolume is a part of filesystem" and
> "A snapshot is also subvolume". It could be more clear.
Well, in that case, btrfs suffers the same bad limitation as LVM does.
It shouldn't be a problem to make a snapshot whereever you need to.
And why are snapshots involved with subvolumes? I don't care about
subvolumes, and putting another layer in between makes things only
more confusing. And when there subvolumes, why are we still supposed
to partition the storage media?
Too bad that we can't reasonably use ZFS ...
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