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

On Fri, Jan 12, 2018 at 8:43 PM, Claudio Freire <klaussfreire(at)gmail(dot)com>
wrote:

>
>
> 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
>
>
>
Sorry, missed the whole point. Of the *first* record's CRC