Re: how do versioning filesystems take snapshot of opened files?

["Xin Zhao" <uszhaoxin@gmail.com>]

OK. From discussion above, can we reach a conclusion: from the
application perspective, it is very hard, if not impossible, to take a
transactional consistent snapshot without the help from applications?

Chris, you mentioned that "Many different applications support some
form of pausing in order to facilitate live backups. " Can you provide
some examples? I mean popular apps.

Finally, if we back up a little bit, say, we don't care the
transaction level consistency ( a transaction that open/close many
times), but we want a open/close consistency in snapshots. That is, a
file in a snapshot must be in a single version, but it can be in a
middle state of a transaction. Can we do that? Pausing apps itself
does not solve this problem, because a file could be already opened
and in the middle of write. As I mentioned earlier, some systems can
backup old data every time new data is written, but I suspect that
this will impact the system performance quite a bit. Any idea about
that?

Thanks.



On 7/3/07, Chris Mason <chris.mason@oracle.com> wrote:
> On Tue, 3 Jul 2007 12:31:49 -0400
> "Xin Zhao" <uszhaoxin@gmail.com> wrote:
>
> > That's a good point!
> >
> > But this sounds hopeless to take a real consistent snapshot from app
> > perspective unless you shutdown the computer. Right?
>
> Many different applications support some form of pausing in order
> to facilitate live backups.  You just have to keep it all in mind when
> designing the total backup solution.
>
> -chris
>