From: | Claudio Freire <klaussfreire(at)gmail(dot)com> |
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To: | Simon Riggs <simon(at)2ndquadrant(dot)com> |
Cc: | PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org> |
Subject: | Re: Changing WAL Header to reduce contention during ReserveXLogInsertLocation() |
Date: | 2018-01-12 23:43:51 |
Message-ID: | CAGTBQpYZ8BvmGhQq2KN4jGMV_ARc90geXdHOteJZW_r1BiUouQ@mail.gmail.com |
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Lists: | pgsql-hackers |
On Sat, Dec 30, 2017 at 7:32 AM, Simon Riggs <simon(at)2ndquadrant(dot)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.
>
Or, you can use the lower 16-bits of the previous record's CRC
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