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Re: [dm-devel] Multipath Questions



On Wed, Jan 14, 2004 at 01:36:49PM -0600, Kevin Corry wrote:
> Hi,
> 
> 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.

The idea is that you may have multiple classes of device paths, and
wish to load balance between one group in priority to another.  I
don't know if this reflects real life in any way, is there anyone on
the list with more multipath experience who could tell me if this is a
sane scenario ?  Originally I planned to put the priority groups in
the path selectors, but it quickly became apparent that every path
selector would be duplicating this, so it was lifted into dm-mpath.c.
I don't have any changes planned for it, so it'll either stay as it is
or prove worthless and be removed completely at some point.

> 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.

schedule_timeout() ?

- Joe



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