src/backend/storage/page/README

Checksums

---------

Checksums on data pages are designed to detect corruption by the I/O system.

We do not protect buffers against uncorrectable memory errors, since these

have a very low measured incidence according to research on large server farms,

http://www.cs.toronto.edu/~bianca/papers/sigmetrics09.pdf, discussed

2010/12/22 on -hackers list.

Current implementation requires this be enabled system-wide at initdb time.

The checksum is not valid at all times on a data page!!

The checksum is valid when the page leaves the shared pool and is checked

when it later re-enters the shared pool as a result of I/O.

We set the checksum on a buffer in the shared pool immediately before we

flush the buffer. As a result we implicitly invalidate the page's checksum

when we modify the page for a data change or even a hint. This means that

many or even most pages in shared buffers have invalid page checksums,

so be careful how you interpret the pd_checksum field.

That means that WAL-logged changes to a page do NOT update the page checksum,

so full page images may not have a valid checksum. But those page images have

the WAL CRC covering them and so are verified separately from this

mechanism. WAL replay should not test the checksum of a full-page image.

The best way to understand this is that WAL CRCs protect records entering the

WAL stream, and data page verification protects blocks entering the shared

buffer pool. They are similar in purpose, yet completely separate. Together

they ensure we are able to detect errors in data re-entering

PostgreSQL-controlled memory. Note also that the WAL checksum is a 32-bit CRC,

whereas the page checksum is only 16-bits.

Any write of a data block can cause a torn page if the write is unsuccessful.

Full page writes protect us from that, which are stored in WAL. Setting hint

bits when a page is already dirty is OK because a full page write must already

have been written for it since the last checkpoint. Setting hint bits on an

otherwise clean page can allow torn pages; this doesn't normally matter since

they are just hints, but when the page has checksums, then losing a few bits

would cause the checksum to be invalid. So if we have full_page_writes = on

and checksums enabled then we must write a WAL record specifically so that we

record a full page image in WAL. Hint bits updates should be protected using

MarkBufferDirtyHint(), which is responsible for writing the full-page image

when necessary.

New WAL records cannot be written during recovery, so hint bits set during

recovery must not dirty the page if the buffer is not already dirty, when

checksums are enabled. Systems in Hot-Standby mode may benefit from hint bits

being set, but with checksums enabled, a page cannot be dirtied after setting a

hint bit (due to the torn page risk). So, it must wait for full-page images

containing the hint bit updates to arrive from the master.