Nobody loses any user data until they do.
[1] But as Artur Bergman so eloquently points out, if your data can fit in ram, just use a native data-structure (http://youtu.be/oebqlzblfyo?t=13m35s)
We have done billions of ops with Mongo and have never lost any data.
I am sorry, to sound blunt, but that's an irrelevant data point. With a data set that fits comfortably into RAM (much less SSDs in RAID!), most any data store will work (including MySQL or Postgres).
> Operations per second: 450
Again, not a relevant data point. With a 10 ms seek time on a SATA disk, this is (again) well within the IOPS capacity of a single commodity machine (with RAID, a SAS drive, row cache, and operating system's elevator scheduling).
Context is the article which made is seem like even 220GB is an ok amount, still can fit in memory.
What's your breakdown between the operation types, and what kind of hardware are you on?
This type of load can easily be handled by a simple SQL box. We did these types of #s with a single SQL Server box 4 years ago, except that your "total documents" was our daily write load.
We don't want page faults on indexes, we design them to keep them in memory.
I don't get the safety issue, 20 months and we haven't lost any user data. shrug