Highway 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_Tool or a TRESPASS command to the array. As a result, the array transfers LUN 1 ownership to SPB. Continue reading “not, one another computers was in fact configured having VMW_PSP_Repaired rather than VMW_PSP_MRU”