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[dm-devel] Multipath Questions


I just had a couple questions about the overall design and direction of the DM 
multipath module.

1. Priority Groups.

DM-multipath allows paths to be placed in groups, with each group assigned a 
priority (0 to 1024). What is the intended purpose for these groups? In the 
current code, if a device has multiple groups, only the lowest-priority group 
is used. I'm assuming this will change, but I just wanted to get an idea of 
how the group priority will affect path selection.

2. Multipath Daemon

The daemon's job is to resubmit failed I/Os to a different path, and to 
periodically test each of the paths to see if they've failed or recovered 
from failure. However, currently the daemon only wakes up on the error path 
for normal I/Os. This means a path could be marked faulty, but never 
reactivated if the path gets fixed.

Obviously the daemon needs a way to wake-up periodically so it can run it's 
test I/Os. The comments in dm-daemon.h say the worker function should return 
a "hint" as to when they should be woken up, but currently this return value 
is unused. I'm trying to look into how to implement this functionality in the 
dm-daemon code, but I figured I'd ask here and see if anyone has already come 
up with a plan for this. Once we get the multipath daemon to do its periodic 
path-testing, I think we'll have a working prototype.

Kevin Corry
kevcorry us ibm com

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