Simulating Drive Failure on Mirrored OS drive

[andy liebman <andyliebman@aol.com>]

Various documents on Mirrored OS Drives suggest you can simulate the 
failure of one of your mirrored pairs by marking one or more partitions 
as "failed" with mdadm.

Indeed when I mark a drive as "failed", if I have the mdadm monitoring 
daemon running, I get an email warning me that my RAIDS are degraded.

Also, I can reboot with only one drive plugged in and the OS comes up 
fine (albeit degraded).

This gives me a REASONABLE degree of confidence that the Mirrored 
Partitions will continue working okay if one drive should fail. However, 
I would like to run a more definitive test.

I tried simply unplugging one drive from its power and from its SATA 
connector. The OS didn't like that at all. My KDE session kept running, 
but I could no longer open any new terminals. I couldn't become root in 
an existing terminal that was already running. And I couldn't SSH into 
the machine.

It was like I had an OS running on only a 1/4th of a cylinder. I 
couldn't even make the OS cleanly shutdown or reboot.

I know that simply unplugging a drive is not the same as a drive failing 
or timing out. But is there a more realistic way to simulate a failure 
so that I can know that the mirror will work when it's needed?

Andy Liebman