ALUA stands for Asymmetric Logical Unit Access. ALUA is also called as SCSI Target Port groups, TPGS(Target Port Group Support).
The ALUA pulled itself into effect into the SCSI standards to standardize the ways host multipathing software can know about the attributes of paths available to an asymmetric storage system having multi-port access.
Every asymmetric array vendor ( asymmetric arrays - multiported arrays which have differing performance characteristics when accessed through different ports) had a unique/ vendor specific method of identifying the characteristics of the path. They used a lot of vendor specific mechanisms to provide access to ( LUs - logical units) in case of path failures to trespass LUs.
This caused a lot of interoperability issues for the host multipathing vendors, they need to have a vendor specific mechanism for each and every asymmetric model out there and need to take of them in each of their support matrix. ALUA defines a set of SCSI command through which hosts can determine the path attributes by querying the target.
Say, for example if an Array 1 (Vendor X) has two paths P1 and S1 and Array 2 (Vendor Y) has two paths P1" and S1". Rather using 2 different vendor command sets to detect the primary and secondary path, the standard SCSI commands can be used to determine the same. This made the job of the multipathing software to handle things in a very clean fashion and handle the right multipathing failover polices/ load balancing policies in a universal fashion.
Access characteristics of a path to a logical unit can be determined using the Report Target Port Group (RTPG)scsi command. Access characteristics can be set using Set Target Port Group (STPG).
The ALUA pulled itself into effect into the SCSI standards to standardize the ways host multipathing software can know about the attributes of paths available to an asymmetric storage system having multi-port access.
Every asymmetric array vendor ( asymmetric arrays - multiported arrays which have differing performance characteristics when accessed through different ports) had a unique/ vendor specific method of identifying the characteristics of the path. They used a lot of vendor specific mechanisms to provide access to ( LUs - logical units) in case of path failures to trespass LUs.
This caused a lot of interoperability issues for the host multipathing vendors, they need to have a vendor specific mechanism for each and every asymmetric model out there and need to take of them in each of their support matrix. ALUA defines a set of SCSI command through which hosts can determine the path attributes by querying the target.
Say, for example if an Array 1 (Vendor X) has two paths P1 and S1 and Array 2 (Vendor Y) has two paths P1" and S1". Rather using 2 different vendor command sets to detect the primary and secondary path, the standard SCSI commands can be used to determine the same. This made the job of the multipathing software to handle things in a very clean fashion and handle the right multipathing failover polices/ load balancing policies in a universal fashion.
Access characteristics of a path to a logical unit can be determined using the Report Target Port Group (RTPG)scsi command. Access characteristics can be set using Set Target Port Group (STPG).
http://frankdenneman.nl/2010/03/esx4-alua-and-hp-continuous-access/
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