Re: Changing WAL Header to reduce contention during ReserveXLogInsertLocation()

On Sat, Dec 30, 2017 at 5:32 AM, Simon Riggs <simon@2ndquadrant.com> wrote:
> So we can't completely remove xl_prev field, without giving up some
> functionality. But we don't really need to store the 8-byte previous
> WAL pointer in order to detect torn pages. Something else which can
> tell us that the WAL record does not belong to current WAL segno would
> be enough as well. I propose that we replace it with a much smaller
> 2-byte field (let's call it xl_walid). The "xl_walid" (or whatever we
> decide to call it) is the low order 16-bits of the WAL segno to which
> the WAL record belongs. While reading WAL, we always match that the
> "xl_walid" value stored in the WAL record matches with the current WAL
> segno's lower order 16-bits and if not, then consider that as the end
> of the stream.
>
> For this to work, we must ensure that WAL files are either recycled in
> such a way that the "xl_walid" of the previous (to be recycled) WAL
> differs from the new WAL or we zero-out the new WAL file. Seems quite
> easy to do with the existing infrastructure.

I have some reservations about whether this makes the mechanism less
reliable.  It seems that an 8-byte field would have almost no chance
of matching by accident even if the location was filled with random
bytes, but with a 2-byte field it's not that unlikely.  Of course, we
also have xl_crc, so I'm not sure whether there's any chance of real
harm...

-- 
Robert Haas
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