Durability Internals


The main durability page (not the internals page) is the Journaling page.


The data file format is unchanged.

Journal files are placed in /data/db/journal/.


Journaling is on by default. Run with --nojournal to disable journaling/durable storage. Both mongod and test support this option.

Declaring Write Intent

WHen writing mongod kernel code, one must now declare an intention to write. Declaration of the intent occurs before the actual write. See db/dur.h. The actual write must occur before releasing the write lock.

When you do your actual writing, use the pointer that dur::writing() returns, rather than the original pointer.

Foo *foo;
getDur().writing(thing)->bar = something;

int *x;
getDur().writingInt(x) += 3;

DiskLoc &loc;
loc.writing() = newLoc;

void *p;
unsigned len;
memcpy( getDur().writingPtr(p,len), src, len );

Try to declare intent on as small a region as possible. That way less information is journalled. For example:

BigStruct *b;

dur::writing(b)->x = 3; // less efficient

*dur::writing(&b->x) = 3; // more efficient

However, there is some overhead for each intent declaration, so if many members of a struct will be written, it is likely better to just declare intent on the whole struct.


jstests/dur/ contains tests for durability.

mongo --nodb jstests/dur/<testname>.js


# dump journal entries during any recover, and then start normally
mongod --journal --durOptions 1

# recover and terminate
mongod --journal --durOptions 4

# dump journal entries (doing nothing else) and then terminate
mongod --journal --durOptions 7

# extra checks that everything is correct (slow but good for qa)
mongod --journal --durOptions 8


Process Steps Diagram

Process Steps

Journal File Structure Diagram

Journal File Structure