Street Thrashing Condition 2
Figure 6.3 shows a variation on Scenario 1 in which the Fibre Channel fabric was designed according to VMware best practices. This by itself wouldn’t result in path thrashing 
. However, the designer decided to customize each host so that they have different preferred paths to LUN 1. These preferred path settings are represented by the interrupted lines (a path from Host A and another path from Host B). The expected behavior in this configuration is that as long as the defined preferred path to LUN 1 is available, the host insists on sending I/O via that path. As a result, Host A attempts to send its I/O to LUN 1 via SPA, and Host B sends it I/O via SPB. However, LUN 1 is owned by SPA and attempts to send I/O via SPB, resulting in a check condition with the sense key ILLEGAL_Demand (more on this in Chapter 7). Host B insists on sending the I/O via its preferred path. So, it sends a START_Equipment or a TRESPASS command to the array. As a result, the array transfers LUN 1 ownership to SPB. Now Host A gets really upset and tells the array to transfer the LUN back to SPA, using the START_Equipment or TRESPASS commands. The array complies, and the tug-of-war begins!
Stopping Roadway Thrashing
These instances encouraged VMware to make the fresh VMW_PSP_MRU plug-set for have fun with having effective/passive arrays. Inside the earlier launches, before ESX 4.0, this was once a policy form for each and every LUN. In the 4.0 and soon after, in addition to six.0 and 6.5, MRU try an excellent PSA connect-into the. (We the fresh PSP structure choices from inside the Chapter 7.) That have MRU, this new server directs the We/O with the lately made use of path. Whether your LUN movements to a different SP, this new I/O is sent into the new way to you to definitely SP rather to be sent to SP that has been the last holder. Note that MRU ignores the most popular path setting.
ALUA-capable arrays that provide AO AAS for TPGs on the owner SP and ANO AAS for TPGs on the non-owner SP allow I/O to the given LUN with high priority via the AO TPGs and, conversely, lower priority via the ANO TPGs. This means that the latter does not return a check condition with sense key ILLEGAL_Request if I/O to the LUN is sent through it. This means that using VMW_PSP_FIXED with these arrays can result in a lighter version of path thrashing. In this case, I/O does not fail to be sent to the ANO TPGs if that is the preferred path. However, the I/O performance is much lower compared to using the AO TPGs. If more hosts are using the AO TPGs as the preferred path, the LUN ownership stays on the original SP that owns it. As a result, the ANO TPGs are not transitioned to AO for the offending host.
To suit this example, VMware brought a unique function to be used having ALUA devices; but not, this is not outlined about ALUA specification. This feature is referred to as ALUA followover.
ALUA followover only means if servers finds an excellent TPG AAS alter so it didn’t cause alone, it will not you will need to revert the change regardless if it only has use of TPGs that will be ANO. Effectively, so it inhibits this new computers away from fighting having TPG AAS and you can, rather, they stick to the TPG AAS of one’s number. Data 6.cuatro and you will six.5 instruct ALUA followover correspondence that have TPG AAS.
Shape 6.4 reveals a medical sites drawing the spot where the button fabric was eliminated to help you explain the diagram . Here, TPG ID step one is the AO with the Health spa, and you may one another servers send the newest We/O to that TPG. TPG ID 2 try ANO, and that i/O is not delivered to it. These TPGs was set up having ALUA Explicit form.