To suit this case, VMware delivered a new feature to be used with ALUA gizmos; not, this is simply not discussed regarding ALUA specification.
Query
ALUA followover just implies that if the server finds a beneficial TPG AAS change it failed to end in by itself, it doesn’t just be sure to return the change although they has only entry to TPGs which can be ANO. Effectively, it prevents the fresh machines out-of fighting to own TPG AAS and you will, as an alternative, they stick to the TPG AAS of your number. Rates 6.cuatro and you may six.5 show ALUA followover communication having TPG AAS.
Contour six.4 reveals a systematic sites diagram where key fabric was indeed removed so you can simplify the brand new drawing . Right here, TPG ID 1 ‘s the AO into the Salon, and both hosts send the fresh I/O compared to that TPG. TPG ID 2 was ANO, and i/O isn’t delivered to they. These TPGs was designed having ALUA Explicit form.
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 actually a device means configured with the sites array. The latest default means may vary according to provider and you may model.
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 https://www.datingmentor.org/sugar-daddies-canada/ 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.
Identifying Unit 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.