path Selection Policies

[Mohammad1982]

Hi All,

Can anyone tell me why VMware recommends fixed policy for Active active storage and MRU for active passive?

[MauroBonder · Accepted solution]

Check if this kb answer your question http://kb.vmware.com/selfservice/microsites/search.do?language=en_US&cmd=displayKC&externalId=101134...

Please, don't forget the awarding points for "helpful" and/or "correct" answers. 

Mauro Bonder - Moderator

[Mohammad1982]

The KB article doesnot mention " why". It explains what it is. Could you please tell me why I have to use MRU for A/P array and fixed for A/A array

[MauroBonder]

http://www.vmware.com/pdf/vsphere4/r40/vsp_40_san_cfg.pdf

when  there is a fiber board by accessing a storage controller, depending on  the storage when a failover is just changing the storage controller  where there will be access the LUN.

The Fixed can also be used to make manual balancing through the storage, requesting a lun for each storage.

Please, don't forget the awarding points for "helpful" and/or "correct" answers. 

Mauro Bonder - Moderator

[AndreTheGiant]

Why it's simple...

For a active/standby contollers the LUN migration is an expensive function (usually its is called tresspass and may be notified by the storage) and require TIME... this mean that I/O operations are delayed for some seconds (time depends by the controller type).

To avoid this problem this operation must be used only in case of failure.

Fixed option, instead define a fixed path (possible different for each LUN or at least balanced) and this path must be used for performance aspect.

Andre

[ShannonZ]

I have similar question about Fixed policy for A/A array. If using Fixed policy for A/A array, only one path is active for I/O, I don't think it fully utilizes A/A array's great performance, since for A/A array, all ports on two controllers can go through I/O accessing one LUN in the array. Fixed policy doesn't provide load balancing actually.

So why not RR policy is the better choice for A/A array? Does RR policy really realize load balancing for A/A array?  or so far, we can only actually direct I/O through one path because we are still using SCSI-2 protocal in our storage multipathing feature?

Looking forward to your reply.

Thanks,

Shannon