Linux -- Problems writing session temp files

On a fairly high-traffic server, we are getting the following error several times a minute: PHP Warning: Failed to write session data (files). Please verify that the current setting of session.save_path is correct (/var/session) in Unknown on line 0 (I am logging errors, instead of displaying in browser) What sense should I make of this? System details: - Linux Redhat, kernel 2.4.17 - Dual processor Athlon MP - SCSI RAID 0+1 (four disks total), running ReiserFS for the /var and www/data partitions, but ext2 for / and /usr - Apache 1.3.22, PHP 4.0.6, MySQL 3.23.49 On a previous system I was having trouble with sessions because the system was running out of filehandles, but that was kernel 2.2. On this system, I have set /proc/sys/fs/file-max to 32768. At any one time, there might be 300 or 400 httpd processes running. I can't see how I could be anywhere near the file-max.