On Sun, 4 Apr 2010, Brad wrote:
I had always thought that with mpxio, it load-balances IO request
across your storage ports but this article
http://christianbilien.wordpress.com/2007/03/23/storage-array-bottlenecks/
has got me thinking its not true.
"The available bandwidth is 2 or 4Gb/s (200 or 400MB/s – FC frames
are 10 bytes long -) per port. As load balancing software
(Powerpath, MPXIO, DMP, etc.) are most of the times used both for
redundancy and load balancing, I/Os coming from a host can take
advantage of an aggregated bandwidth of two ports. However, reads
can use only one path, but writes are duplicated, i.e. a host write
ends up as one write on each host port. "
Is this true?
This text seems strange and wrong since duplicating writes would
result in duplicate writes to disks, which could cause corruption if
the ordering was not perfectly preserved. Depending on the storage
array capabilities, MPXIO could use different strategies. A common
strategy is active/standby on a per-LUN level. Even with
active/standby, effective load sharing is possible if the storage
array can be told to assign preference between a LUN and a port.
That is what I have done with my own setup. 1/2 the LUNs have a
preference for each port so that with all paths functional, the FC
traffic is similar for each FC link.
--
Bob Friesenhahn
bfrie...@simple.dallas.tx.us, http://www.simplesystems.org/users/bfriesen/
GraphicsMagick Maintainer, http://www.GraphicsMagick.org/
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