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Too many maps?

I am testing my Hadoop-based FreeEed <http://freeeed.org/>, an open source
tool for eDiscovery, and I am using the Enron data
that. In my processing, each email with its attachments becomes a map,
and it is later collected by a reducer and written to the output. With the
(PST) mailboxes of around 2-5 Gigs, I begin to the see the numbers of emails
of about 50,000. I remember in Yahoo best practices that the number of maps
should not exceed 75,000, and I can see that I can break this barrier soon.

I could, potentially, combine a few emails into one map, but I would be
doing it only to circumvent the size problem, not because my processing
requires it. Besides, my keys are the MD5 hashes of the files, and I use
them to find duplicates. If I combine a few emails into a map, I cannot use
the hashes as keys in a meaningful way anymore.

So my question is, can't I have millions of maps, if that's how many
artifacts I need to process, and why not?

Thank you. Sincerely,