Re: Clear empty space in a page. - Mailing list pgsql-hackers

From Andres Freund
Subject Re: Clear empty space in a page.
Date
Msg-id 20210530210712.5mklwl4egotev3ir@alap3.anarazel.de
Whole thread Raw
In response to Clear empty space in a page.  (Yura Sokolov <y.sokolov@postgrespro.ru>)
Responses Re: Clear empty space in a page.
List pgsql-hackers
Hi,

On 2021-05-30 03:10:26 +0300, Yura Sokolov wrote:
> While this result is not directly applied to stock PostgreSQL, I believe
> page compression is important for full_page_writes with wal_compression
> enabled. And probably when PostgreSQL is used on filesystem with
> compression enabled (ZFS?).

I don't think the former is relevant, because the hole is skipped in wal page
compression (at some cost).


> Therefore I propose clearing page's empty space with zero in
> PageRepairFragmentation, PageIndexMultiDelete, PageIndexTupleDelete and
> PageIndexTupleDeleteNoCompact.
> 
> Sorry, didn't measure impact on raw performance yet.

I'm worried that this might cause O(n^2) behaviour in some cases, by
repeatedly memset'ing the same mostly already zeroed space to 0. Why do we
ever need to do memset_hole() instead of accurately just zeroing out the space
that was just vacated?

Greetings,

Andres Freund



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