On 5/8/13 7:34 PM, Jeff Davis wrote:
On Wed, 2013-05-08 at 17:56 -0500, Jim Nasby wrote:
Apologies if this is a stupid question, but is this mostly an issue
due to torn pages? IOW, if we had a way to ensure we never see torn
pages, would that mean an invalid CRC on a WAL page indicated there
really was corruption on that page?

Maybe it's worth putting (yet more) thought into the torn page
issue... :/
Sort of. For data, a page is the logically-atomic unit that is expected
to be intact. For WAL, a record is the logically-atomic unit that is
expected to be intact.

So it might be better to say that the issue for the WAL is "torn
records". A record might be larger than a page (it can hold up to three
full-page images in one record), but is often much smaller.

We use a CRC to validate that the WAL record is fully intact. The
concern is that, if it fails the CRC check, we *assume* that it's
because it wasn't completely flushed yet (i.e. a "torn record"). Based
on that assumption, neither that record nor any later record contains
committed transactions, so we can safely consider that the end of the
WAL (as of the crash) and bring the system up.

The problem is that the assumption is not always true: a CRC failure
could also indicate real corruption of WAL records that have been
previously flushed successfully, and may contain committed transactions.
That can mean we bring the system up way too early, corrupting the
database.

Unfortunately, it seems that doing any kind of validation to determine
that we have a valid end-of-the-WAL inherently requires some kind of
separate durable write somewhere. It would be a tiny amount of data (an
LSN and maybe some extra crosscheck information), so I could imagine
that would be just fine given the right hardware; but if we just write
to disk that would be pretty bad. Ideas welcome.
What about moving some critical data from the beginning of the WAL record to the end? That would make it easier to detect that we don't have a complete record. It wouldn't necessarily replace the CRC though, so maybe that's not good enough.

Actually, what if we actually *duplicated* some of the same WAL header info at the end of the record? Given a reasonable amount of data that would damn-near ensure that a torn record was detected, because the odds of having the exact same sequence of random bytes would be so low. Potentially even just duplicating the LSN would suffice.

On the separate write idea, if that could be controlled by a GUC I think it'd be worth doing. Anyone that needs to worry about this corner case probably has hardware that would support that.
--
Jim C. Nasby, Data Architect jim@nasby.net
512.569.9461 (cell) http://jim.nasby.net

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