EVA Max Transfer Size

Another HP EVA Post from the vaults:

Recently I saw an issue where an EVA was delivering poor performance when under load from a full backup. I used EVAPerf to investigate the issue and found that the disks were not under pressure, however the CPU was running up to 100% busy. During these spikes of high CPU usage the latency on the LUNS was inevitably very high.


I found the following advisory from HP, which describes performance issues which are created by the MAX IO limitations of the EVA. THE EVA was designed to deal with a max IO size of 128KB, when receiving a transfer size larger than this it has to buffer it and then break it down, putting an increased load on the system. The suggestion from HP is to limit the max IO size from the host system. The hosts connected to the EVA in question were Windows 2008 R2, the method to limit the IO size was via the HBA.


The hosts I was working on were using Qlogic HBA’s. To make the changes to a Qlogic HBA you need to be on a driver version of or later and then follow these steps from the Windows command line:

1 View current MAX IO size qlfc -tsize /fc

2 Set MAX IO size to 128KB qlfc -tsize /fc /set 128

3 Set as default qlfc -tsize /fc /set default


Since implementing the reduced block size the performance has returned to a good level.