[dm-devel] [PATCH v2] dm pref-path: provides preferred path load balance policy

[Hannes Reinecke]

On 01/22/2016 02:31 PM, Ravikanth Nalla wrote:
> v2:
>   - changes merged with latest mainline and functionality re-verified.
>   - performed additional tests to illustrate performance benefits of
>     using this feature in certain configuration.
> 
> In a dm multipath environment, providing end user with an option of
> selecting preferred path for an I/O in the SAN based on path speed,
> health status and user preference is found to be useful. This allows
> a user to select a reliable path over flakey/bad paths thereby
> achieving higher I/O success rate. The specific scenario in which
> it is found to be useful is where a user has a need to eliminate
> the paths experiencing frequent I/O errors due to SAN failures and
> use the best performing path for I/O whenever it is available.
> 
> Another scenario where it is found to be useful is in providing
> option for user to select a high speed path (say 16GB/8GB FC)
> over alternative low speed paths (4GB/2GB FC).
> 
> A new dm path selector kernel loadable module named "dm_pref_path"
> is introduced to handle preferred path load balance policy
> (pref-path) operations. The key operations of this policy is to
> select and return user specified path from the current discovered
> online/ healthy paths. If the user specified path do not exist in
> the online/ healthy paths list due to path being currently in
> failed state or user has mentioned wrong device information, it
> will fall back to round-robin policy, where all the online/ healthy
> paths are given equal preference.
> 
> Functionality provided in this module is verified on wide variety
> of servers ( with 2 CPU sockets, 4 CPU sockets and 8 CPU sockets).
> Additionally in some specific multipathing configurations involving
> varied path speeds, proposed preferred path policy provided some
> performance improvements over existing round-robin and service-time
> load balance policies.
> 
Shouldn't service-time provide similar functionality?
After all, for all scenarios described above the preferred path
would have a lower service time, so they should be selected
automatically, no?

Cheers,

Hannes
-- 
Dr. Hannes Reinecke		   Teamlead Storage & Networking
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