When to use MongoDB

I'm writing an application that doesn't necessarily need scaling abilities as it won't be collecting large amounts data at the beginning. (However, if I'm lucky, I could down the road potentially.)

I will be running my web server and database on the same box (for now).

That being said, I am looking for performance and efficiency.

The main part of my application will be loading blog articles. Using an RDBMS (MySQL) I will make 6 queries (2 of the queries being joins), just to load a single blog article page.

select blog
select blog_album
select blog_tags
select blog_notes
select blog_comments (join with users)
select blog_author_participants (join with users)

However, with MongoDB I can de-normalize and flatten 6 tables into just 2 tables/collections and minimizes my queries to potentially just one 1 query,

users
blogs
    ->blog_album
    ->blog_tags        
    ->blog_notes
    ->blog_comments
    ->blog_author_participants

Now, going with the MongoDB schema, there will be some data redundancy. However, hard drive space is cheaper than CPU/servers.

1.) Would this be a good scenario to use MongoDB?

2.) Do you only benefit in performance using MongoDB when scaling beyond a single server?

3.) Are there any durability risks using MongoDB? I hear that there is potential for loss of data while performing inserts - as insert are written to memory first, then to the database.

4.) Should this stop me from using MongoDB in production?

However, with MongoDB I can de-normalize and flatten 6 tables into just 2 tables/collections and minimizes my queries to potentially just one 1 query

But you can easily query MySQL for 6 tables worth of information related to a single blog post with a single properly crafted SQL statement.

however hard drive space is cheaper than CPU/servers.

If performance and scaling is a priority then you are going to be concerned with having enough RAM to fit everything into main memory and enough CPU cores to run queries. An enterprise grade RAID 10 array is a requirement, don't get me wrong, but as soon as your database software (MongoDB or MySQL) needs to scan an index that can't fit into main memory you'll be in for a world of pain assuming a large active database. :)

I like MongoDB, but it's big strength in my mind is map/reduce and its document-orientation. You require neither of those features. MySQL is time-tested in large scale deployments and supports partitioning (but I would argue that your database would have to be in the order of 50-100 GB before you can realize substantial gain from partitioning vs a single (plus passive backup) server with tons (64 GB+) of RAM. I would also argue that if performance is truly a concern then MySQL would be preferable as you would have supreme control over your indexes.

That's not to say that MongoDB isn't high performance, but its place probably isn't serving blogs. Your concern with inserts is valid as well. MongoDB is not an ACID system. Google transactions in both systems and compare.