This article is Part of Becoming a VMware Storage Expert series

ALUA stands for “Asymmetric Logical Unit Access”, as per t10 SCSI-3 specifications (www.t10.org/cgi-bin/ac.pl?t=f&f=spc3r23.pdf). ALUA occurs when the access characteristics of one port may differ from those of another port. In English this means the LUN could be seen via both storage processors as an active but only one of them owns the Lun. Hence there will be optimized and unoptimized paths, the I/O is allowed to the LUN.

But the performance of the owner Storage Processor is better than that of the non-owner Storage Processor. To allow the initiators to identify which targets would provide the best I/O, the ports on each Storage Processor are grouped together into target port groups. Each target port group is given a distinctive “state” (asymmetric access state [AAS]), that denotes the optimization of ports on one SP compared to ports on the other SP (for example, active-optimized versus active-non-optimized).

Target Port Groups

A target port group is defined as a set of target ports that are in the same target port asymmetric access state at all times. A target port group asymmetric access state is defined as the target port asymmetric access state common to the set of target ports in a target port group. The grouping of target ports is vendor specific.

Target port Group Support

Target port Group Support provides a method for determining the access characteristics of a path to a LUN through a target port. Target Port Group Support is often used with arrays that handle load balancing and failover within the Storage Processors. Target Port Groups allows path grouping. Each port in the same Target Port Group has the same port state, which can be one of the following state:
Active/Optimized
Active/Non-optimized
Standby
Unavailable
In-Transition.

Some storage arrays might not support some of the latter three states.

Asymmetric Access State(AAS)

Ports in an ALUA TPG can be in the same AAS at all times with respect to a given LUN. TPG’s AAS are reported to the initiators in response to the REPORT TPGS command. The TPG descriptor is reported in byte 1 of that response.

Possible states are as follows:

• Active-optimized (AO)—Ports are on the owner SP and provided the best I/O to the LUN.
• Active-non-optimized (ANO)—Ports are on the non-owner SP. I/O to the LUN is less optimal compared to AO AAS.

ESXi 6 sends the I/O to TPGs that are in AO AAS, but if they are not available, I/O is sent to TPGs that are in ANO AAS. If the storage array receives sustained I/O on TPGs that are in ANO AAS, the array transitions the TPG’s state to AO AAS. Who makes that change depends on the ALUA management mode of the storage array

-Venkatesh Sekar

How to become a VMware Storage Expert. Let me make it clear this is not a new certification VMware has introduced. This is a series I am starting to help people understand the storage part in the ESXi and implement it better.

I have engaged in several support calls where customers complain that the storage isn’t performing well, very high latency, production environment is down after the LUN’s failed over between the storage processors.

Becoming a VMware Storage Expert means you have to understand and master how ESXi works with Storage. I am starting a series in this blog to explain the components involved when a Datastore is presented (Block or NFS) and mounted to an ESXi, how VVOL works, also will touch-base on vSan.

We will go over all the topics about Storage & availability and explain in detail.

What happens to ESXi when it encounters events like APD-PDL

Few other outage scenarios causing the storage to be unavailable

Here is the Series.

• ALUA- What is it and how it works

• Storage Plugins- PSA, NMP

• What are APD and PDL?

• How to Handle APD and PDL?