To accommodate this situation, VMware delivered yet another element for use that have ALUA gadgets; not, it is not discussed regarding the ALUA spec.
Inquiry
ALUA followover merely means that when the server finds an excellent TPG AAS changes that it didn’t cause itself, it does not try to revert the change regardless of if it only has use of TPGs which might be ANO. Efficiently, that it suppresses the servers from fighting getting TPG AAS and you can, rather, they follow the TPG AAS totally free farmers dating sites Australia of the variety. Numbers six.4 and you may 6.5 train ALUA followover interaction with TPG AAS.
Shape 6.cuatro shows a scientific stores drawing where the option materials were got rid of to clear up the newest drawing . Right here, TPG ID step one is the AO towards the Salon, and you will one another hosts posting the fresh new We/O to this TPG. TPG ID dos are ANO, and i/O is not provided for it. Such TPGs are set up with ALUA Explicit setting.
Figure 6.5 shows that Host A lost its path to the AO TPG (based on Figure 6.4). As a result, this host takes advantage of the ALUA Explicit mode on the array and sends a SET_TPGS command to the array so that TPG ID 2 is changed to AO and TPG ID 1 is changed to ANO. Host B recognizes that it did not make this change. But because ALUA followover is enabled, Host B just accepts this change and does not attempt to reverse it. Consequently, the I/O is sent to TPG ID 2 because it is now the AO TPG. (Notice that the array moved the LUN ownership to SPB because this is where the AO TPG is located.)
ALUA followover is a tool mode designed on shop array. The default mode may vary according to supplier and you may design.
Some storage arrays implement the PREF (preference) bit, which enables an array to specify which SP is the preferred owner of a given LUN. This allows the storage administrator to spread the LUNs over both SPs (for example, even LUNs on one SP and odd LUNs on the other SP). Whenever the need arises to shut down one of the SPs, the LUNs owned by that SP (say SPA) get transferred to the surviving nonpreferred SP (SPB). As a result, the AAS of the port group on SPB is changed to AO. ALUA followover honors this change and sends the next I/O intended for the transferred LUNs to the port group on SPB. When SPA is brought back online, the LUNs it used to own get transferred back to it. This reverses the changes done earlier, and the AAS of the port group on SPA is set to AO for the transferred LUNs. Conversely, the AAS of the port group on SPB, which no longer owns the LUNs, is changed to ANO. Again, ALUA followover honors this change and switches the I/O back to the port group on SPA. This is the default behavior of ALUA-capable HP EVA storage arrays.
Pinpointing Tool ALUA Configuration
ESXi 6 host configuration that enables use of ALUA devices is a PSA component in the form of a SATP (see Chapter 5, “vSphere Pluggable Storage Architecture [PSA]”). PSA claim rules determine which SATP to use, based on array information returned in response to an command. As mentioned earlier, part of the inquiry string is the TPGS field. The claim rules are configured such that if a field’s value is nonzero, the device is claimed by the defined ALUA SATP. In the following sections, I show how to list these claim rules and how to identify ALUA configurations from the device properties.