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DAVID BROOK wrote:
It might well be my lack of understanding of the cacheing process, but
as several applications use Delayed writes and the operating systems
themselves use cacheing, we are having difficulty seeing how consistency
can be guaranteed if there are any files on the system opened for writing.
My guess would be that Bacula uses the OS's read functions, so its up to
the OS to return the data, even the part that is still in the write
cache. Alternatively, the OS could flush the cache before reading the data.
As far as we can see, for 100% certainty of data consistency, nothing
else should be running on the client - is this the case?
That is definitely true. You can never guarantee consistency on a moving
target. There are ways to reduce chances of backing up an inconsistent
state (for Windows there's St Bernards Open File Manager with its
caching/snapshotting algorythms, for Linux you can try to snapshot stuff
while your app is up, hoping that there were no changes at the moment
you took the snapshot) but if you really need to be 100% sure, my advice
would be to shut down the applications whose data you're backing up.
To reduce downtime on Linux systems, you can shut down the applications,
snapshot the volume with LVM (provided you use LVM volumes of course),
start up your apps again and run the backup on the LVM snapshot in the
background.
Also note, that the Bacula-FD cannot back up open files on Windows (it
will simply skip them). You will need St Bernards Open File Manager for
that.
Hope that helps.
Greetings,
Michel
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