RE: Out of sync mirrors

Hi,
I am using linux 2.6.8 for my research on raid systems.

I need to have the disks of an array out of sync for a while.
I want to be able to stop a disk request queue (blk_stop_queue) and
start it sometime later. Meanwhile its mirrors should be serving r/w
as if this mirror was up.
Whenever the stopped mirror comes back it will serve all the requests
already sent to it.

I am queueing up the requests sent to all others while my mirror queue
is stopped.

The problem is that the other mirrors stop serving as well. Even if I
don't stop the queue, only delay its request a little bit, still the
others also slow down.

Question: how do I get raid to allow mirrors to be out of sync?

Obviously, I am missing something out in the code.

Any clues?

Thanks,
Ana Paula

Ana:

You are making this way too hard for yourself
If you want to stop the queue but let the RAID system continue
processing the queue, then just sending I/O from whatever program you
are using to generate the load.  This won't let you start up with a
partially unflushed queue, but if for whatever reason you need to
measure effects of pending I/Os and timing, then you can obtain a
reasonably good simulation by increasing the queue depth and repeating
the test.

As for A way to get RAID (I assume software RAID, a la the md driver),
to get out of sync is to "remove" a disk drive.  If you are afraid to
crack the case, then use a cheap USB enclosure for one of the disks and
then turn it off or unplug the USB connector.  You can also "remove" a
disk by sending it a command to go into a self-test, perform a low-level
format, send a mode select to change the block size, there are lots of
ways if you think outside of the box.

All of these "failure" scenarios will interact differently, just turning
a disk drive off creates a real failure, not a quasi-emulated more
graceful failure that may be easier for you to time and measure the
effects.

Good luck.  

David @ santools.com