The main advantage to mirroring via Sybase is the control of the mirroring is in the product that the mirroring is supporting. I've found it to work very well. With mirroring in Sybase, you basically have the options of dealing with the mirroring in ways that make sense for the DB application that's using the mirrored storage.
I'd recommend that you look at the controls on mirroring in Sybase (very easy to use and simple--particularly if you have Sybase Central) and compare those to what you have in HP-UX and see which looks easier to deal with. (My guess is that you'll find that Sybase is pretty simple to deal with and there may not be much advantage to doing it via the OS.)
One clear advantage to using Sybase mirroring is that there are specific instructions in the Sybase docs on how to recover/manage mirroring via Sybase. For instance, where would you find instructions on how to bring up Sybase if the primary side of the mirror of the master DB were lost? Well, Sybase explains what you need to do in their manuals IF SYBASE IS HANDLING THE MIRRORING. It may be less clear how to deal with things like that if you're having to make the conceptual jump from what the OS is doing to how Sybase is interacting with it all on your own (since neither the OS docs or Sybase is going to give you all the info you need).
Now, if you have a fancy disk subsystem that can handle the mirroring or higher-level RAID options (RAID 5, for example), you may not want to bother with Sybase or OS mirroring but just let the disk subsystem take care of it. HP-UX may be able to do this too and that may be a potential advantage of that approach (less space dedicated to achieving the mirror--on the other hand disk space can be pretty cheap now).
To sum up: I'd go with Sybase's mirroring unless there's a compelling reason (either speed or flexibility) in going with the OS's mirroring.
Good luck.
J M Craig
nsjmcraig@netscape.net
