MMAPv1

Storage Engine determines data file format & format of indexes and are key to performance for the mongo database.It is the middletier between the database and the underlying hardware.

 
A storage engine effects:
* How data is written to disk
*How data is deleted/removed from disk
*How data is read from disk
* The data structure used to store data

From mongo 3.0 we can choose from two storage engines:

MMAPv1 and WiredTiger(opensource & is used in other DBs)

 MMAPv1

• Grew from original Storage Engine & Uses unix MMAP system call
• Maps data files directly into virtual memory
• mongod –storageEngine mmapv1 (we can check the storage engine using db.serverStatus())
• Collection level locking with Mongo 3.0

• Multiple reader and single writer lock on shared resources like data,metadata,index , journals etc in mongo db

• Journal – write ahead log (for  consistency of data)

MMAPv1:Documents & Data Files

Within data files, different extents would be present to hold the documents.

 

If D2 grows, it has to move after D4 leaving space in D2, we also have to update indexes( to update address of D2)

Power of 2 sized allocation(padding factor is true by default):document is first given 32bytes, then 64,128..2 MB to grow

Advantage of MMAPv1 padding and power of 2 sized allocation is:

• Documents will not have to move as soon as they grow in size

• Record spaces are likely to get re-used

• Documents that grow at a constant rate will move less often as time goes on. 

Wired tiger storage engine will be discussed in next blog