To match this case, VMware brought a different sort of ability to be used that have ALUA devices; but not, this is not laid out regarding the ALUA specification.
Query
ALUA followover simply means that in the event the machine finds an excellent TPG AAS transform which didn’t produce itself, it does not try to revert the change although it has only entry to TPGs that will be ANO. Effortlessly, that it suppresses the newest machines off attacking getting TPG AAS and you will, instead, it follow the TPG AAS of one’s range. Figures six.cuatro and six.5 train ALUA followover communication with TPG AAS.
Figure 6.4 reveals a medical storage diagram where in actuality the key fabrics was in fact got rid of so you’re able to describe the fresh diagram . Here, TPG ID step 1 ‘s the AO with the Spa, and both computers upload the brand new We/O to that particular TPG. TPG ID dos try ANO, and i/O isn’t provided for it. These TPGs try designed with ALUA Specific 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 try a device function configured into the shops range. The brand new default mode varies with regards to the seller and you will 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.
Determining 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.