[Zope] zope backups

[Paul Winkler]

On Mon, May 24, 2004 at 02:59:06PM -0400, Jake wrote:
> Hmm...
> 
> I have used it on a Data.fs from 300 -> 800 MB.
> 
> What is to say the copy won't go bad if it gets written to at the time of
> the copy?

This is my understanding:

At least on unix, cp simply starts reading at the front
of the file and reads bytes until EOF.

Zope only appends to Data.fs.  (It has been mentioned that
it is *possible* to twiddle stuff in the middle of a filestorage,
but the core Zope code is not supposed to do this, and I've never
heard an example of any add-on product that does so either.)

So, if Zope writes some data while you're copying, it
doesn't matter - one of two things happen:

1) cp hits EOF before the new data is flushed to disk.
Your new data doesn't make it into the backup, but the backup
is perfectly good otherwise... yay transactions :-)

2) cp isn't up to the end yet. So the new data will be
copied before cp gets to EOF.

Either way, you're fine.

rsync is different because it actually analyzes the file for
differences and this can get confused if the file
changes while it's working. Apparently this is more likely
to be a problem as the file gets larger.

-- 

Paul Winkler
http://www.slinkp.com