Re: corrupt pages detected by enabling checksums

Andres,

Thank you for diagnosing this problem!

On Thu, 2013-04-04 at 16:53 +0200, Andres Freund wrote:
> I think the route you quickly sketched is more realistic. That would
> remove all knowledge obout XLOG_HINT from generic code hich is a very
> good thing, I spent like 15minutes yesterday wondering whether the early
> return in there might be the cause of the bug...

I like this approach. It may have some performance impact though,
because there are a couple extra spinlocks taken, and an extra memcpy.
The code looks good to me except that we should be consistent about the
page hole -- XLogCheckBuffer is calculating it, but then we copy the
entire page. I don't think anything can change the size of the page hole
while we have a shared lock on the buffer, so it seems OK to skip the
page hole during the copy.

Another possible approach is to drop the lock on the buffer and
re-acquire it in exclusive mode after we find out we'll need to do
XLogInsert. It means that MarkBufferDirtyHint may end up blocking for
longer, but would avoid the memcpy. I haven't really thought through the
details.

Regards,
Jeff Davis