Question

How 0.203125GB is calculated for MongoDB database size?

Is it same for all OS( 32 bit and 64 bit)?

Can we see the current usage of a particular database?

> show dbs
local   (empty)
tutorial        0.203125GB

Answer 1

MongoDB Documentation

Each datafile is preallocated to a particular size. (This is done to prevent file system fragmentation, among other reasons.) The first filename for a database is .0, then .1, etc. .0 will be 64MB, .1 128MB, et cetera, up to 2GB. Once the files reach 2GB in size, each successive file is also 2GB.

Thus, if the last datafile present is, say, 1GB, that file might be 90% empty if it was recently created.

Additionally, on Unix, mongod will preallocate an additional datafile in the background and do background initialization of this file. These files are prefilled with zero bytes. This initialization can take up to a minute (less on a fast disk subsystem) for larger datafiles. Pre-filling in the background prevents significant delays when a new database file is next allocated.

On Windows, additional datafiles are not preallocated. NTFS can allocate large files filled with zeroes relatively quickly, rendering preallocation unnecessary.

As soon as a datafile starts to be used, the next one will be preallocated.

You can disable preallocation with the –noprealloc command line parameter. This flag is nice for tests with small datasets where you drop the database after each test. It should not be used on production servers.

For large databases (hundreds of GB or more), this is of no significant consequence as the unallocated space is relatively small.