PrepareBackup for every Writer is called prior to Freeze for any Writer. So,
if you've got three Writers, it'll go:
PrepareBackup (1)
PrepareBackup (2)
PrepareBackup (3)
Freeze (1)
Freeze (2)
Freeze (3)
The PrepareBackup phase allows asynchronous backup preparation to occur.
Like for Exchange, Exchange initiates a log flush and transfer. For NTFS,
the disks buffers are written to disk, but the Writer doesn't wait for the
I/O to complete.
It also allows a Writer to say "NO" to taking a backup right now, without
going through the overhead of a full Freeze. For example, Exchange will say
"no" during PrepareBackup if the last Exchange backup was an incremental
streaming backup. You can't mix streaming and VSS backups in Exchange. You
also can't have two backups on-going using the same Writer. This is enforced
by the Writer.
So, you may have an Exchange backup going on and a system-state backup going
on - that's perfectly OK. But you'll be prevented from having two system
state backups at the same time or having two backups of an Exchange storage
group at the same time.
In general, since I/O is still allowed between PrepareBackup and Freeze, it
gives the system an opportunity to quiesce.
[That's a technical term, I know Shookie will have to look it up.]
And thanks. :-)
Regards,
Michael B. Smith, MCITP:SA,EMA/MCSE/Exchange MVP
My blog: http://TheEssentialExchange.com/blogs/michael
I'll be at TEC'2009! http://www.tec2009.com/vegas/index.php
-----Original Message-----
From: Ben Scott [mailto:mailvor...@gmail.com]
Sent: Thursday, January 15, 2009 10:13 AM
To: MS-Exchange Admin Issues
Subject: Re: VSS and Exchange - The Basics
On Wed, Jan 14, 2009 at 11:04 AM, Michael B. Smith
<mich...@theessentialexchange.com> wrote:
> ... how VSS works with Exchange ...
>
>
<http://theessentialexchange.com/blogs/michael/archive/2009/01/14/volume-sha
dow-copy-services-vss-and-exchange-the-basics.aspx>
Excellent article. I've been meaning to learn more about VSS for
years now. Thank you for writing and posting that!
Also, I had no trouble reaching your site. :)
Question: To me, it seems like PrepareBackup and Freeze are
redundant. PrepareBackup just seems to say "We'll be calling Freeze
next" but doesn't actually do anything for snapshot purposes. Freeze
is where the Writer has to commit. The Provider doesn't proceed until
Freeze returns success, so it will wait. So what's the point of
PrepareBackup? Is there a delay or callback between PrepareBackup and
Freeze, to give different Writers a chance to prepare asynchronously,
and then tell the Provider they're ready to Freeze? That might be a
win if some Writers take longer than others to prepare. Or is this
just one of those architectural mysteries?
-- Ben
~ Ninja Email Security with Cloudmark Spam Engine Gets Image Spam ~
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