SHMVIRTSIZE

Hi I had a strange problem. I am running SAP on informix. I'd increased SHMVIRTSIZE parameter recently. After that, I'd shutdown and restarted both SAP and informix. No problem. Things were fine for a week. Last weekend we had to install a HP-UX patch. We installed a patch and rebooted. SAP refused to start. Investigation showed that there was not enough shared memory for SAP to start. Informix was OK. I changed SHMVIRTSIZE back to what it was and SAP starts. We even undid the HP-UX patch, still the same. My question is: Why did SAP start after I increased SHMVIRTSIZE and not after the machine was rebooted? Any ideas? Deepa