Cambex Bus Adapter Manual De Usuario

 
The basic idea of multipathing is fairly simple. Access a disk on 
one of two paths. If the path fails, move I/O activity to the other 
path. We build on that basis by adding more factors into the 
configuration and seeing what effect they have.  
 
A path is defined as an end-to-end connection through a SAN, 
ignoring any intermediate routing points. A complex SAN may 
have multiple connections from an HBA to a storage device, but 
this is invisible to the software. The combination of one HBA to 
one target is one path, regardless of how it is routed.  

 
The effect of multiple LUNs on a RAID controller is that we 
can choose to distribute them so as to balance the I/O across 
multiple paths. The choice can be made in one of two ways. If 
one RAID controller has preference over the other, we call this 
"controller preference". If the selection is made in the host  
software we call this "target preference". A third type of

 

preference is to lock a LUN to a particular HBA, called "HBA 
preference"

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With four paths (for example) we have to sort the paths into an 
order in which they are chosen when the primary path fails. This 
order should depend on getting the best performance when a 
path segment fails. For example, if our performance is limited 
by the IOPS that can be handled by each RAID controller, then 
we want to balance the LUNs between the RAID controllers, 

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