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What is Alua? How it works?

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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.


Reference from Mostafa Khalil
Alua Storage Array

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

Storage Plugins – PSA, NMP

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This article is Part of Becoming a VMware Storage Expert series 

Storage Plugins – When you use ESXi with a block device presented using iSCSi or FC, you might have seen or heard terms like PSP, SATP, PSA, NMP and ALUA

While we have already discussed a few basics of ALUA, This post will be discussing what are the Plugins involved how they work. This post is a part of Becoming a VMware Storage Expert series

Pluggable Storage Architecture (PSA)

Starting ESXi 3.5 VMware uses Pluggable Storage Architecture (PSA). PSA enables Storage Vendors to build their own Multipathing Plugin(MPP) using API’s under PSA. This will allow 3rd party vendors to develop their Load balancing, failover mechanisms and plug it into the Architecture.

Availability is critical and redundant paths to storage are essential. One of the key functions of the storage component in vSphere is to provide multipathing (that is, which path a given I/O should use if there are multiple paths) and failover (that is, I/O switching to using another path when a path goes down).

VMware, by default, provides a generic multipathing plug-in (MPP) called the Native Multipathing Plug-in (NMP).

Native Multipathing Plug-in (NMP)

Native Multipathing Plug-in (NMP)NMP works with SATP and PSP to handle Failover and Multipathing and it takes care of

  • Registering logical devices with the PSA framework
  • Receiving input/output (I/O) requests for logical devices it registered with the PSA framework
  • Completes the I/Os and posts completion of the SCSI command block with the PSA framework, which includes the following operations:
  • Selecting the physical path to which it sends the I/O requests
  • Handling failure conditions encountered by the I/O requests
  • Handles task management operations, such as aborts/reset

PSA communicates with NMP for the following operations:

  • Opening/closing logical devices
  • Starting I/O to logical devices
  • Aborting an I/O to logical devices
  • Getting the name of the physical paths to logical devices
  • Getting the SCSI inquiry information for logical devices”

The below image explains how they fit together

PSA NMP Storage Plugins

SATP, PSP and MMP are explained in this article.

Becoming a VMware Storage Expert

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VMware Storage Expert VMware vSphere APD-PDL VVOL VSAN

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.