Last week there was a short discussion on Twitter about why PFS pages (damaged header, not individual PFS bytes) can’t be repaired (prompted by a #sqlhelp question about why they can’t be single-page restored, like other per-database allocation bitmaps). Just for the record, they can’t be fixed by automatic page repair in a mirror or AG either.
PFS pages occur every 8088 pages in every data file and store a byte of information about itself and the following 8087 pages. The most important piece of information it stores is whether a page is allocated (in use) or not. You can read more about PFS pages and the other per-database allocation bitmaps in this blog post.
So why can’t they be rebuilt by DBCC CHECKDB, when all the other per-database allocation bitmaps can?
The answer is that the is-this-page-allocated-or-not information is not duplicated anywhere else in the database, and it’s impossible to reconstruct it in all cases.
You might think that DBCC CHECKDB could work out which pages are allocated by inferring that state if a page is linked to in some way from another page that’s known to be allocated, and it could do that, except for the case of a heap with no nonclustered indexes.
If a heap has no nonclustered indexes, there is no other structure in the database that links to any of the data pages in the heap. Therefore, without the information in a PFS page, there’s no way to tell which of the pages contained in extents allocated to the heap are actually allocated or not. This is because SQL Server does not touch a page when it is deallocated, so there’s nothing on a page that indicates whether a page is currently allocated or has been deallocated.
So what if DBCC CHECKDB can tell that there are no such cases in the database?
The answer is that the algorithm to rebuild a PFS page given the links from other pages to pages covered by that PFS range is extremely complicated and would involve searching through the entire database, reading and processing all pages a second time *after* they’ve been repaired, looking for linkages to pages in the broken range. While it sounds technically possible, when scoping out writing such an algorithm back in 2001-2002, I quickly ran into run-time and complexity challenges that made the work entirely infeasible.
It’s also not possible to just mark all the pages allocated – because then allocation-order scans would come across potentially unformatted pages and fail. It would also break backups that use WITH CHECKSUM. Such an algorithm could be made to work (in the absence of heaps with no nonclustered indexes), but has the same problems as the algorithm above.
So – PFS pages can’t be repaired, and unless the database structure changes to mirror that allocation information in some way, I don’t see that changing at any point in the future.
Hope you found this interesting!