Detailed Description

This explains how the MMAP V1 durability system is implemented.

Every server operation (OperationContext), must call Locker::lockGlobal as the first lock action (it is illegal to acquire any other locks without calling this first). This action acquires the global and flush locks in the appropriate modes (IS for read operations, IX for write operations). Having the flush lock in one of these modes indicates to the flush thread that there is an active reader or writer.

Whenever the flush thread(dur.cpp) activates, it goes through the following steps :

Acquire the flush lock in S mode using AutoAcquireFlushLockForMMAPV1Commit. This waits until all current write activity on the system completes and does not allow any new operations to start.

Once the S lock is granted, the flush thread writes the journal entries to disk (it is guaranteed that there will not be any modifications) and applies them to the shared view.

After that, it upgrades the S lock to X and remaps the private view.

NOTE: There should be only one usage of this class and this should be in dur.cpp

Public Member Functions
	AutoAcquireFlushLockForMMAPV1Commit (Locker *locker)

	~AutoAcquireFlushLockForMMAPV1Commit ()

void	upgradeFlushLockToExclusive ()
	We need the exclusive lock in order to do the shared view remap. More...

void	release ()
	Allows the acquired flush lock to be prematurely released. More...

Private Attributes
Locker *const	_locker

bool	_released

Constructor & Destructor Documentation

◆ AutoAcquireFlushLockForMMAPV1Commit()

mongo::AutoAcquireFlushLockForMMAPV1Commit::AutoAcquireFlushLockForMMAPV1Commit ( Locker * locker )

◆ ~AutoAcquireFlushLockForMMAPV1Commit()

mongo::AutoAcquireFlushLockForMMAPV1Commit::~AutoAcquireFlushLockForMMAPV1Commit ( )

Member Function Documentation

◆ release()

void mongo::AutoAcquireFlushLockForMMAPV1Commit::release ( )

Allows the acquired flush lock to be prematurely released.

This is helpful for the case where we know that we won't be doing a remap after gathering the write intents, so the rest can be done outside of flush lock.

◆ upgradeFlushLockToExclusive()

void mongo::AutoAcquireFlushLockForMMAPV1Commit::upgradeFlushLockToExclusive ( )

We need the exclusive lock in order to do the shared view remap.

Member Data Documentation

◆ _locker

Locker* const mongo::AutoAcquireFlushLockForMMAPV1Commit::_locker

private

◆ _released

bool mongo::AutoAcquireFlushLockForMMAPV1Commit::_released