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

[Chris Mason <chris.mason@oracle.com>]

On Tue, 3 Jul 2007 13:15:06 -0400
"Xin Zhao" <uszhaoxin@gmail.com> wrote:

> 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?

You definitely need help from the applications.  They define what a
transaction is.

> 
> 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.

Oracle, db2, mysql, ldap, postgres, sleepycat databases...just search
for online backup and most programs that involve something
transactional have a way to do it.

> 
> 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?
> 

This depends on the transaction engine in your filesystem.  None of the
existing linux filesystems have a way to start a transaction when the
file opens and finish it when the file closes, or a way to roll back
individual operations that have happened inside a given transaction.

It certainly could be done, but it would also introduce a great deal of
complexity to the FS.

-chris