Hi Tim,

Mirroring protects against media/hardware failures such as head crashes, which
OS or hardware mirroring can also do. However, there are other things that can
happen to a database volume, such as the partial page writes I mentioned in my
post.

If you set up ADSM to write to the database and/or recovery log sequentially,
it can protect against partial page writes being written to both mirrored
copies... provided that you are using ADSM mirroring. This is because the
software controls the mirroring, and it won't write to the mirror until the
first write is completed successfully.

However, the OS or hardware is not aware of what the software is doing. If you
use OS or hardware mirroring and a partial page write (or some other
corruption) occurs on one of the mirrors, the information will be written to
all mirrors. So ADSM mirroring is a bit more secure than OS or hardware
mirroring.

By the way, corruption other than partial page writes can occur: someone
decides to take an editor to a database volume, or the delete the volume.... OS
or hardware mirroring would propagate these changes, too, regardless of which
MIRRORWRITE method you use. I worked with someone once who, in trying to test
their ADSM recovery using mirroring, deliberately trashed one of their recovery
log volumes. They were using AIX mirroring, which propagated the corruption to
the mirror recovery log volume, and so were unable to restart their server.

Best regards,

Andy

Andy Raibeck
IBM Storage Systems Division
ADSM Client Development
e-mail: storman AT us.ibm DOT com

Please respond to ADSM-L AT VM.MARIST DOT EDU
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Subject: Re: Database backup procedures -Reply


Hi Andy:

What are the reasons for not using OS mirroring? (We will be
running on NT)  How about h/w mirroring?  What are the
advantages of using ADSM mirroring over these?

Thanks,

Tim Rushforth

>>> Andrew Raibeck <storman AT US.IBM DOT COM> 08/18/98
12:18am >>>

2) Make sure your mirrors are on separate physical devices, and
(ideally)
separate controllers. The idea is to physically isolate the mirrors
as much as
possible. Also, you didn't say what operating system your ADSM
server is
running on. Some operating systems, like AIX, have mirroring
capability built
in to them. Make sure you are using ADSM mirroring and not
operator system
mirroring.


Andy

Andy Raibeck
IBM Storage Systems Division
ADSM Client Development
e-mail: storman AT us.ibm DOT com