Optimizing MongoDB - mongodb

Question

I have around 105 milions records similar to this:
{
"post_id": 1314131221,
"date": 1309187001,
"upvotes": 2342
}
in MongoDB collection.
I also have an index on "post_id" and "date".
Then i need to do this:
db.fb_pages_fans.find({
post_id: 1314131221,
date: {"$gt": 1309117001, "$lta": 1309187001}
}).sort({date: 1});
If i set "date" on the specific date:
when it returns 30 records, it took ~130ms
when it returns 90 records, it took ~700ms
when it returns 180 records, it took ~1200ms
Of course i'm talking about first query, second and more queries are very fast, but i need to have first queries fast.
It's much more slower from 90 records than PostgreSQL, which I use now. Why is that so slow?
btw. creating index on mentioned two "cols" on 105mil records took around 24 hours.
It runs on one machine with 12GB RAM, here is a log from mongostats when i was executing the query:
insert query update delete getmore command flushes mapped vsize res faults locked % idx miss % qr|qw ar|aw netIn netOut conn time
0 0 0 0 0 1 0 23.9g 24.1g 8m 0 0 0 0|0 0|0 62b 1k 1 18:34:04
0 1 0 0 0 1 0 23.9g 24.1g 8m 21 0 0 0|0 0|0 215b 3k 1 18:34:05

Answer 1

If your first query is slow and all consequtive, similar queries fast then mongo is moving the queried data from disk to memory. That's relatively hard to avoid with data sets that size. Use mongostat and check faults statistic to see if you're getting pagefaults during your queries. Alternatively it might be that your index(es) do not fit into memory, in which case you can try and right balance them so that the relevant, high throughput parts of it are consistently in physical memory.
Also, are we talking a single physical database or a sharded setup?