Re: Default to read-only on snapshot creation and have a flag if snapshot should be writable (was: [PATCH 0/5] btrfs: Readonly snapshots)

[C Anthony Risinger <anthony@extof.me>]

On Nov 29, 2010, at 3:48 PM, Andrey Kuzmin <andrey.v.kuzmin@gmail.com>
wrote:

> I'm not sure why zfs came up, they don't own the term :). As to
> zfs/overhead topic, I doubt there's any difference between clone and
> writable shapshot (there should be none, of course, it's just two
> different names for the same concept).
>
> Regards,
> Andrey
>
>
>
>
> On Tue, Nov 30, 2010 at 12:43 AM, Mike Fedyk <mfedyk@mikefedyk.com>
> wrote:
>> On Mon, Nov 29, 2010 at 1:31 PM, Andrey Kuzmin
>> <andrey.v.kuzmin@gmail.com> wrote:
>>> This may sound excessive as any new concept introduction that late
>>> in
>>> development, but readonly/writable snapshots could be further
>>> differentiated by naming the latter clones. This way end-user would
>>> naturally perceive snapsot as read-only PIT fs image, while clone
>>> would naturally refer to (writable) head fork.
>>>
>>
>> I'm not sure we want to take all of the terminology that zfs uses as
>> it may also bring the percieved drawbacks as well.  Isn't there some
>> additional overhead for a zfs clone compared to a snapshot?  I'm not
>> very familiar with zfs so that's why I ask.
>>
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I don't like the idea of readonly by default, or further changes to
terminology, for several reasons:

) readonly by default offers no real enhancement whatsoever other than
breaking _anything_ that's written right now
) btrfs readonly is not even really readonly; as superuser could
simply flip a flag to enable writes, readonly merely prevents
accidental writes or misbehaving apps... ie. protecting you from
yourself
) backups are the simple/obvious use case; I personally use btrfs
heavily for LXC containers, in which case nearly every single snapshot
is intended to be writable -- usually cloning a template into a new
domain
) I also use an initramfs hook to provide system rollbacks, also
writable; the hook also provides multiple versions of the "branch"...
all writable
) adding new terms is not a good idea imo; I've already spewed out
many sentences explaining the difference between subvolumes and
snapshots, ie. that there is none... adding another term only adds to
this problem; they each describe the same thing, but differentiate
based on origin or current state, neither of which actually describe
what it _is_-- a new named pointer to a tree, like a git branch -- a
subvolume.

I think a better solution/compromise would be to leave snapshots
writeable by default, since that's more true to what's happening
internally anyway, but maybe introduce a mount option controlling the
default action for that mount point.

C Anthony [mobile]