AutoCAD file save process

Given initial Files on disk: Model.DWG and Model.BAK: 

Opening a drawing:

1. OPEN the Model.DWG.
2. Model.DWG is loaded from disk into RAM.
3. Model.DWL - WhoHas file created.
4. Model.DWL2 - XML version of WhoHas file created.
5. No .tmp is created on OPEN.

Full save:

1. Full SAVE (occurs if ISAVEPERCENT=0 and may also occur if ISAVEPERCENT>0).
2. SAVE the Model.DWG if a full-save occurs and SAVEBAK sysvar is enabled.
3. Model.DWG in RAM is written out to a .tmp.
4. Model.BAK deleted.
5. Model.DWG renamed to Model.BAK.
6. The .tmp renamed to Model.DWG.
7. Model.DWG remains in RAM for current session.

Model.DWL & Model.DWL2 remain as-is (until user closes Model.DWG, at which time they are deleted).

Incremental save:

1. Incremental SAVE (which may occur if ISAVEPERCENT>0).
2. SAVE the Model.DWG when SAVEBAK sysvar is enabled.
3. Model.DWG copied to a .tmp.
4. Model.DWG edits (changes currently in RAM since last save) are written incrementally the .tmp.
5. Model.BAK deleted.
6. Model.DWG renamed to Model.BAK.
7. The .tmp is renamed to Model.DWG.
8. Model.DWG remains in RAM for current session.

Diagram of the file process: 
 

Note: Save performance is improved in AutoCAD 2018. Objects that will experience the greatest improvement include blocks with annotation scaling, mtext with columns and other newer formatting, as well as attributes and attribute definitions with multilines. In addition, automatic save functionality is updated so that saves are most often performed incrementally rather than performing full saves, which are slower.