From GridLAB-D Wiki
TECHNICAL MANUAL
The checkpoint_keepall global variable is used to preserve all the checkpoints saved. Normally, the checkpoint system keeps only the last checkpoint. However, all checkpoint images can be preserved setting the checkpoint_keepall variable to 1 or TRUE.
GLM
#set checkpoint_keepall=1
Command line
host% gridlabd -D checkpoint_keepall=1
Version
Checkpoints were introduced in Version 3.0.