WiredTiger has a variety of statistics that track the flow of data between the cache and storage. WIth the addition of page deltas some of these statistics appear to be incorrect (or misleading). See: WT-14469, WT-14470, WT-14546.

The goal of this ticket is to review the code paths where data moves between the cache and storage and make sure that the statistics that count bytes or pages read/written are correct and consistent.

At a minimum, this should include:

• Reading a page into the cache
• Evicting a dirty page, whether it generates a full page image or a delta
• Writes from checkpoint – again covering both full page and delta cases

There will be a few different sizes of interest. In some ways the point of this ticket is to make sure we are using the right size in the right place.

• Full page size – as in classic WT – used when reading or writing a full page image
• Delta size – used when reconciliation writes just a delta rather than a full page
• Multi-part read size – A read from storage returns a full page image plus zero or more deltas. The block manager "bytes read" should use the sum of these sizes, since they are all read into the system.

In addition to checking that we are using the right values when changing these stats, it would also be useful to compare the standard block manager implementation in src/block/ to that for the disagg block manager in src/block_disagg to make sure we are actually updating the same stats in equivalent ways.