[dm-devel] multipath prio_callout broke from 5.2 to 5.3

[Benjamin Marzinski]

On Thu, Apr 23, 2009 at 12:08:32PM -0600, Ty! Boyack wrote:
> This thread has been great information since I'm looking at the same type 
> of thing.  However it raises a couple of (slightly off-topic) questions for 
> me. 
> My recent upgrade to fedora 10 broke my prio_callout bash script just like 
> you described, but my getuid_callout (a bash script that calls udevadm, 
> grep, sed, and iscsi_id) runs just fine.  Are the two callouts handled 
> differently?

Fedora 10 uses the same method as upstream, priority callouts have been
replaced by priority modules. These are dynamic shared objects that get
loaded by multipath.  Because of this, multipath doesn't use a private
namespace, so it can use scripts without restrictions for the getuid_callout.

>
> Also, is there an easy way to know what tools are in the private namespace 
> already?  My prio_callout script calls two other binaries: /sbin/udevadm 
> and grep.  If I go to C-code, handling grep's functions myself is no 
> problem, but I'm not confident about re-implementing what udevadm does.  
> Can I assume that since /sbin/udevadm is in /sbin that it will be available 
> to call via exec()?  Or would I be right back where we are with the bash 
> scripting, as in having to include a dummy device as you described?

Sorry, the C code is necessary now.

> Finally, in my case I've got two redundant iscsi networks, one is 1GbE, and 
> the other is 10GbE.  In the past I've always had symetric paths, so I've 
> used round-robin/multibus.  But I want to focus traffic on the 10GbE path, 
> so I was looking at using the prio callout.  Is this even necessary?  Or 
> will round-robin/multibus take full advantage of both paths?  I can see 
> round-robin on that setup resulting in either around 11Gbps or 2 Gbps, 
> depending on whether the slower link becomes a limiting factor.  I'm just 
> wondering if I am making things unnecessarily complex by trying to set 
> priorities.

With round-robin, you will send half your IO to the slow path.  A
priority callout makes sense here.

> Thanks for all the help.
>
> -Ty!
>
> -- 
> -===========================-
>  Ty! Boyack
>  NREL Unix Network Manager
>  ty at nrel.colostate.edu
>  (970) 491-1186
> -===========================-
>
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