Auto Archiving for capped collection

[Inkvizitor]

Auto Archiving. If you know you want data to automatically "roll out"
over time as it ages, a capped collection can be an easier way to
support than writing manual archival cron scripts.

Please, explain, how to realize this feature.

[Eliot Horowitz]

Do you mean how to use capped collections?
See: http://www.mongodb.org/display/DOCS/Capped+Collections

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[Inkvizitor]

Yes. I want to archive old data. For instance, size of capped
collection reaches limit and new records will replace the oldest. I
want to archive the oldest records before replacing.

[Eliot Horowitz]

There isn't a way to do that.
But doesn't that kind of defeat the purpose?
Why not just have 1 collection if you want to archive forever anyway?

[Inkvizitor]

Most of all queries will touch recent data of huge collection. I am
worrying about performance in that case.

What limit of usual collection do i get?

[Michael Dirolf]

The OS will handle LRUing out data, so if all of your queries are
touching the same portion of data that should stay in memory
independently of the total size of the collection.