so I wonder, will skiplist be a better choice? obviously running quota(8) will be a very cheap operation, but I'm worried about contention on the quota database during delivery etc. (these users are for the most part not actively using the system -- they get less than one message per day each(!) -- so I probably worry too much.)
The problem I saw was that accessing a skiplist db involves locking the entire file I believe. This felt like a huge point of contention, especially since every email delivered would lock it. I never did end up trying it out to test that though, so I wouldn't quote me on that, Ken could confirm.
What filesystem are you using? Have you considered trying reiserfs with tails enabled. It's quite impressive how little disk space it actually uses (don't believe du output). We used to use NVRAM cards, and even with 100,000 users, it was only about 30 megabytes for the quota files if memory serves me right (300 bytes each for directory entry, stat data, and file data), and I imagine the entire thing ends up in the OS buffer cache in pretty short order...
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