open_basedir performance issue, realpathcache vs. statcache

Hello, we're managing a shared hosting environment with about 600 virtual servers. All content for the websites is stored on a NFS share. We're suffering from a huge amount of lstat calls, and identified open_basedir as the reason for that. strace shows that each call needs 40µs in average, this is due to NFS. A lot of questions arises from that, but some answers are still missing. Maybe I can get them here. Did open_basedir rely on realpathcache? I haven't found a definitive answer to that. We have changed realpath cache size, without any impact. So I assume the answer is no. >From what I understand realpathcache is different from statcache. So using clearstatcache should not flush the realpathcache. Am I correct? Thanks for you help Roland