Make existing snapshots read-only?

Make existing snapshots read-only?

[Bruce Guenter]

[-- Attachment #1: Type: text/plain, Size: 228 bytes --]


Is there any way to mark existing snapshots as read-only? Making new
ones read-only is easy enough, but what about existing ones?

Thanks.

-- 
Bruce Guenter <bruce@untroubled.org>                http://untroubled.org/

[-- Attachment #2: Digital signature --]
[-- Type: application/pgp-signature, Size: 836 bytes --]

Re: Make existing snapshots read-only?

[Li Zefan]

On 2012/5/29 2:37, Bruce Guenter wrote:

> 
> Is there any way to mark existing snapshots as read-only? Making new
> ones read-only is easy enough, but what about existing ones?
> 


We have code in the kernel side, so what we need to do is to update btrfs-progs,
which is trivial.

Re: Make existing snapshots read-only?

[David Sterba]

On Tue, May 29, 2012 at 08:40:10AM +0800, Li Zefan wrote:
> > Is there any way to mark existing snapshots as read-only? Making new
> > ones read-only is easy enough, but what about existing ones?
> 
> We have code in the kernel side, so what we need to do is to update btrfs-progs,
> which is trivial.

Well, I don't like that it's even possible to turn a RO snapshot to a RW
one. What was the rationale behind this back then? Besides, I think that
it could break assumptions in the backref code.

If it's only a one-way operation from a regular subvol -> RO subvol,
this sounds reasonable to me. If the opposite direction is allowed, then
I'd not call it 'read-only' but "unwritable on-request".


david

Re: Make existing snapshots read-only?

[cwillu]

On Tue, May 29, 2012 at 5:18 AM, David Sterba <dave@jikos.cz> wrote:
> On Tue, May 29, 2012 at 08:40:10AM +0800, Li Zefan wrote:
>> > Is there any way to mark existing snapshots as read-only? Making new
>> > ones read-only is easy enough, but what about existing ones?
>>
>> We have code in the kernel side, so what we need to do is to update btrfs-progs,
>> which is trivial.
>
> Well, I don't like that it's even possible to turn a RO snapshot to a RW
> one. What was the rationale behind this back then? Besides, I think that
> it could break assumptions in the backref code.
>
> If it's only a one-way operation from a regular subvol -> RO subvol,
> this sounds reasonable to me. If the opposite direction is allowed, then
> I'd not call it 'read-only' but "unwritable on-request".

Is anyone actually expecting readonly-snapshots to be a worm
implementation?  And are they sane to expect it?

So long as the permissions required to change it are sane (admin
rights to change an arbitrary snapshot, possibly something like
write-permission on the mountpoint to change otherwise), I don't see
the gain.  It's not like root can't modify the disk directly, so
withholding an easy way to flip the readonly bit just strikes me as a
nuisance feature.

Re: Make existing snapshots read-only?

[Stephane Chazelas]

2012-05-28 12:37:00 -0600, Bruce Guenter:
> 
> Is there any way to mark existing snapshots as read-only? Making new
> ones read-only is easy enough, but what about existing ones?
[...]

you can always do

btrfs sub snap -r vol vol-ro
btrfs sub del vol
mv vol-ro vol

-- 
Stephane