How to define sharding range for each shard in Mongo?

Question

let say, the document is

{
  x:Number
}

and I have 3 shards.

Instead of autosharding, can I define specifically shard1 only contains data x<0, shard2 only contains data 0 =< x =< 1000 , and shard 3 is 1000

Answer 1

You can. It's possible to pre-split chunks manually, it's described here: http://www.mongodb.org/display/DOCS/Splitting+Chunks

Think carefully about how you split your chunks. If you do it badly you can get lots of performance problems, but if you know enough about your keys you can gain a lot.

If you do it you probably want to turn off the balancer:

> use config
> db.settings.update({_id: "balancer"}, {$set: {stopped: true}}, true);

(this is described here: http://www.mongodb.org/display/DOCS/Sharding+Administration)

This is an example of how you might do it. Depending on exactly what you want to do you will have to modify it (I assume your shard key is not named x, for example, and your range isn't -1000 to 2000).

> use admin
> db.runCommand({split: "my_db.my_coll", middle: {x: 0}})
> db.runCommand({split: "my_db.my_coll", middle: {x: 1000}})
> db.runCommand({movechunk: "my_db.my_coll", find: {x:   -1}, to: "shard_1_name"})
> db.runCommand({movechunk: "my_db.my_coll", find: {x:    0}, to: "shard_2_name"})
> db.runCommand({movechunk: "my_db.my_coll", find: {x: 1000}, to: "shard_3_name"})

The split commands create the chunks. Each command splits the chunk containing the middle value into two, so the first splits the chunk containing min_value -> max_value into min_value -> 0 and 0 -> max_value. Then the second command splits the chunk containing 1000, the second chunk created by the previous command, into two new chunks. After that command you have three chunks:

• min_value -> 0
• 0 -> 1000
• 1000 -> max_value

The three following commands moves these chunks to separate shards. The docs say that the command will move the chunk containing the value in find, so I picked three values I know are in different chunks and used these (there is a symbol in BSON for min_key and max_key, but I'm not sure how to use it properly in this context).

Read this page too http://www.mongodb.org/display/DOCS/Moving+Chunks