Xen users - Mar 2008 - xen, ha, ms-sql -- are my goals reasonable?

I want to combine Xen and Linux ha tools to keep four or five virtual
servers up reliably.

We''d have two physical servers, connected with DRBD and Heartbeat.  On
the primary server, we''d be running Xen, and about 4 or 5 guest OS
instances.  At least one of them would be a windows server running
MS-SQL.

I''m not sure how db engines deal with I/O stuff, and how that xen
layer would impact it.

Let''s say the db engine in the guest OS writes something to what it
thinks is the disk.  Does the SQL engine have the ability to write
data to the disk in a way that it knows it won''t be cached?  Does the
file system itself take care of this?  Is that what journaling does?

If it writes it to the disk, which is really a file that lives in
Dom0, will that get written out to the server''s physical disk right
away?  Or does Xen do caching to try to eliminate the I/O bottleneck
that you get with virtual systems?  Is it different if you use LVM, or
a native partition?

And in the real world, does DRBD keep things in sync quickly enough to
make this work?

Let''s say that I have an app that''s doing lots of insertions
into a db
engine.  It''s pounding on the db.  And I pull the plug on the primary
physical server.  Heartbeat would move the secondary to primary, and
it would fire up the guest OSs using their disk files, or partitions,
or whatever.  And after windows server boots, the sql engine would
come up, and the db would be accessible again.

Would I really not have lost any data?

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