CODEWITHSUNDEEP
mongodboperating-system
user35662
user35662

Reputation: 37

What is memory map in mongodb?

I read about this topic at

http://docs.mongodb.org/manual/faq/storage/#faq-storage-memory-mapped-files

But didn't understand point .Does it is used to keep query data in physical memory ? How it is related with virtual memory ? Why it is important and how it effect at performance ?

Upvotes: 0

Views: 2752

Answers (1)

rsmorros
rsmorros

Reputation: 196

I'll try to explain in a simple way.

MongoDB (and other storage systems) stores data in files. Each database has its own files, created as they are needed. The first file weights 64 MB, the next 128 and so up to 2 GB. Then, new files created weigh 2 GB. Each of these files are logically divided into different blocks, that correspond with one virtual memory block.

When MongoDB needs to access a file or a part of it, loads all virtual blocks corresponding to that file or parts of the files into memory using mmap.On the other hand, mmap is a way for applications to leverage the system cache (linux).

So what really happens when you are doing a query is that MongoDB "tells" the OS to load the part it needs with the data requested, so the next time is requested will be faster. As you can imagine this is a very important feature to boost performance in databases like MongoDB, because accessing RAM is way faster than hard drive.

Another benefit of using mmap is that MongoDB memory will grow as it needs and the system memory is free.

Upvotes: 2

Related Questions