As part of WiredTiger's durable history, the time window is written to the disk to support MVCC with better performance. The stored time window can become obsolete and no longer need storage. This obsolete time window is removed automatically when the page is reconciled again, otherwise, this obsolete time window information is saved on disk until the page is modified.

The time information isn't as noticeable when the table has bigger values. Suppose the time information is noticeable for indexes where the values are smaller. To reduce the unnecessary use of the disk space by the obsolete time window information, we want the checkpoint cleanup to look for the pages that have the obsolete time window information and mark the page dirty to let the reconciliation remove the obsolete time window information.

As part of the checkpoint cleanup that is going through the in-memory obsolete time window pages, mark them dirty to let the reconciliation to remove the obsolete time window from the pages.