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back-end maximum active paths

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The VPLEX best practices recommend a maximum of 4 active paths to a back-end LUN:

VPLEX recommends a maximum of 4 active paths per director to a given LUN (Optimal). This is considered optimal because each director will load balance across the four active paths to the storage volume.

Is there a more detailed reasoning behind this recommendation? What happens if for example there are 8 active paths to a LUN? What would the downside be?

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GBson
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Re: back-end maximum active paths

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Having 8 active paths would be overkill. Vplex uses the Adaptive PP policy which assigns IOs based on an algorithm taking into account path load and logical device priority. During a path failure PP marks the failed path for testing so it is no longer sending IO down it. it also prioritises the testing of the remaining  paths to avoid common shared path failures.  The quicker these paths can be tested the quicker IO's can be sent down them.  If there are more than the recommended paths (say 8 paths) then the application may experience time-out issues while waiting for the PP to complete its testing and algorithm calculations.

http://uk.emc.com/collateral/software/white-papers/h8180-powerpath-load-balancing-failover-wp.pdf

checkout this link - page 23 (Proactive path testing on failures)

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GBson
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Re: back-end maximum active paths

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Having 8 active paths would be overkill. Vplex uses the Adaptive PP policy which assigns IOs based on an algorithm taking into account path load and logical device priority. During a path failure PP marks the failed path for testing so it is no longer sending IO down it. it also prioritises the testing of the remaining  paths to avoid common shared path failures.  The quicker these paths can be tested the quicker IO's can be sent down them.  If there are more than the recommended paths (say 8 paths) then the application may experience time-out issues while waiting for the PP to complete its testing and algorithm calculations.

http://uk.emc.com/collateral/software/white-papers/h8180-powerpath-load-balancing-failover-wp.pdf

checkout this link - page 23 (Proactive path testing on failures)

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Re: back-end maximum active paths

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Hi GBson,

Just a slight correction on what you said, or maybe I misinterpreted it - VPLEX in its back-end code uses a simplistic round-robin policy.  (VPLEX internally does not use PP as it's multipathing software say in a director.  The VPLEX firmware manages the allocation of I/O to the multiple paths.)

8 back-end paths (or any value greater than 4) has shown in the past to potentially cause problems, primarily when you get into a situation where multiple paths fail simultaneously.  VPLEX will attempt to re-issue the same I/O down multiple paths, and it may have to simply wait for the I/O to timeout (10 seconds is the default) down each new path. Meanwhile, while that's happening, the host continues to wait and it's possible that the host experiences quite a long delay.  Limiting the paths limits the total time that an I/O get retried.

Front-end (so host to VPLEX FE ports), yes, we recommend using ADaptive policy for PowerPath.

Gary

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