/var/log/lastlog gets written to when you log in. The size of this file is based off of the largest UID in the system. The larger the maximum UID, the larger this file is. Thankfully it's a sparse file so the size on disk is much smaller than the size ls reports (ls -s reports the size on disk).

On our system we're authenticating against an Active Directory server, and the UIDs users are assigned end up being really, really large. Like, say, UID 900,000,000 for the first AD user, 900,000,001 for the second, etc.

That's strange but should be okay. It results in /var/log/lastlog being huuuuuge, though--once an AD user logs in lastlog shows up as 280GB. Its real size is still small, thankfully.

This works fine when /var/log/lastlog is stored on the hard drive on an ext3 filesystem. It breaks, however, if lastlog is stored in a tmpfs filesystem. Then it appears that the max file size for any file on the tmpfs is 256GB, so the sessreg program errors out trying to write to lastlog.

Where is this 256GB limit coming from, and how can I increase it?

As a simple test for creating large sparse files I've been doing:

dd if=/dev/zero of=sparse-file bs=1 count=1 seek=300GB

I've tried Googling for "tmpfs max file size", "256GB filesystem limit", "linux max file size", things like that. I haven't been able to find much. The only mention of 256GB I can find is that ext3 filesystems with 2KB blocks are limited to 256GB files. But our hard drives are formatted with 4K blocks so that doesn't seem to be it--not to mention this is happening in a tmpfs mounted ON TOP of the hard drive so the ext3 partition shouldn't be a factor.

This is all happening on a 64-bit Red Hat Enterprise Linux 5.4 system. Interestingly, on my personal development machine, which is a 32-bit Fedora Core 6 box, I can create 300GB+ files in tmpfs filesystems no problem. On the RHEL5.4 systems it is no go.