[Bucardo-general] Replacing ontimecopy with cloning for b5

[David E. Wheeler]

On Oct 15, 2013, at 7:49 PM, Greg Sabino Mullane <greg at endpoint.com> wrote:

> On Tue, Oct 15, 2013 at 10:02:43AM -0700, David E. Wheeler wrote:
> ...
>> It will also need to deactivate any other kids associated with the tables 
>> to be copied, and then re-activate them when it’s done.
> 
> Yes, I've gone back and forth on this. First, it should only do so when 
> its copies of tables and dbgroups matches a sync exactly.

Which would be the case for the case where you ask it to `clone sync=foobar`.

> Second, I'm not sure there is a need to - the delta syncs should be very short, and I think MVCC will take care of everything.

Well, they may not be that short. What if I have tables with 1m rows I want to clone? Will take a while.

> The truncates may cause issue though. I'll think about this some more.

I think it would need some testing to see what happens in various situations. Like when someone does an insert while the truncate and copy is running.

> We would need a smooth way to handle the kids too - it may be enough to simply kill the kid, let clone slip in, and then the CTL resurrects the kid who gets in line behind the clone process?

That sounds like it might work.

>> Does onetimecopy even work in the 5.0 branch?
> 
> No.

Well in that case, I’ve reassigned this issue to you. :-)

 https://github.com/bucardo/bucardo/issues/37

> I should be more clear - the above scenario only happens when people 
> explicitly request it. In which case, caveat emptor. A use case for 
> that is a swap sync with alternating sequences that needs to be brought 
> online.
> 
> I have in mind that the CLI will complain if you try to clone a 
> multi-source sync, and demand you pick a "winner" or give it the 
> go ahead and do the above.
> 
> I'm also envisioning a pretty print out and final confirmation before 
> all clone events, as it is a pretty major dropped row "are you sure?" 
> moment.

Ah, okay, that makes much more sense.

Thanks!

David


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