Steffen Heil wrote:

>
> I asked about copying using windows valume shadow copies.
> These are frozen, they cannot be changed by the database server.
> (The database server can modify it, but that's another instance.)

The problem is getting a consistent copy of the file for use as the
initial volume shadow. This is what Firebird does when it wants to
start a new shadow of itself. First, it marks itself as being copied.
Then it starts copying from page 0. Before the engine writes a page to
the database (not the copy, but the database) it checks to see if its
copying itself. If it is, it checks the page number. If the number is
higher than the current copy point (the page that is currently being
copied) it just writes the page to the database. If the number is lower
or the same, it writes the page both to the database and to the shadow copy.

>
> I always wondered, why the concept of volume shadow copies are so difficult
> to understand for database people.
> Windows volume shadow copies are basically snapshots of a filesystem with
> transaction support. Once opened a volume shadow transaction to a folder,
> the content YOU SEE cannot change anymore. As it is with transactions in a
> database ...

I think that the problem is with the definition of "transaction
support". I know what I mean by a transaction - it's a set of
operations that succeed or fail as a group and which are not visible to
other processes until they succeed. I'm not sure what that means in a
file system.

>