WiredTiger tracks the size of each checkpoint in its metadata. This size is the sum of the storage sizes of all of the blocks in the checkpoint BTree.

We need to make sure this statistic is accurate when using page deltas in disaggregated storage.

• When writing a delta, we increase checkpoint size by the size of the delta
• Freeing a page we decrease the checkpoint size by the size of the corresponding page plus any deltas

Note that "freeing" a page here simply means that a new checkpoint doesn't include a page that was part of the previous checkpoint. We don't need to track here whether the block is completely free (i.e., not used by any checkpoint).