@andreasio @LexPendragon @LJ filesystems definitely fall over when you have that many files in a single directory

MBox is easier as it is a single file. You can make pretty clean updates to it on a traditional filesystem; on a CoW filesystem each change is a more expensive operation of course.

There are tradeoffs. Having it in a real database is the best but then it's not really portable and in most situations will have a negative performance impact.

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