| From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
|---|---|
| To: | Andres Freund <andres(at)anarazel(dot)de> |
| Cc: | Robert Haas <robertmhaas(at)gmail(dot)com>, Simon Riggs <simon(at)2ndquadrant(dot)com>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org> |
| Subject: | Re: Changing WAL Header to reduce contention during ReserveXLogInsertLocation() |
| Date: | 2018-01-12 22:24:54 |
| Message-ID: | 22118.1515795894@sss.pgh.pa.us |
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| Lists: | pgsql-hackers |
Andres Freund <andres(at)anarazel(dot)de> writes:
> On 2018-01-12 10:45:54 -0500, Tom Lane wrote:
>> The xl_prev field is our only way of detecting that we're looking at
>> old WAL data when we cross a sector boundary.
> Right. I wonder if it be reasonable to move that to a page's header
> instead of individual records? To avoid torn page issues we'd have to
> reduce the page size to a sector size, but I'm not sure that's that bad?
Giving up a dozen or two bytes out of every 512 sounds like quite an
overhead. Also, this'd mean that a much larger fraction of WAL records
need to be split across page boundaries, which I'd expect to produce a
performance hit in itself --- a page crossing has to complicate figuring
out how much space we need for the record.
regards, tom lane
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