From: | Simon Riggs <simon(at)2ndquadrant(dot)com> |
---|---|
To: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
Cc: | Alvaro Herrera <alvherre(at)alvh(dot)no-ip(dot)org>, Fujii Masao <masao(dot)fujii(at)gmail(dot)com>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org> |
Subject: | Re: reloption to prevent VACUUM from truncating empty pages at the end of relation |
Date: | 2018-04-18 15:44:53 |
Message-ID: | CANP8+jJKJDmkbh_DSwx-FMLEVBkPpW3suESQ4CioRVPNiOvvZw@mail.gmail.com |
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Lists: | pgsql-hackers |
On 17 April 2018 at 20:09, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> Alvaro Herrera <alvherre(at)alvh(dot)no-ip(dot)org> writes:
>> Andres was working on a radix tree structure to fix this problem, but
>> that seems to be abandoned now, and it seems a major undertaking. While
>> I agree that the proposed solution is a wart, it seems much better than
>> no solution at all. Can we consider Fujii's proposal as a temporary
>> measure until we fix shared buffers? I'm +1 on it myself.
>
> Once we've introduced a user-visible reloption it's going to be
> practically impossible to get rid of it, so I'm -1. I'd much rather
> see somebody put some effort into the radix-tree idea than introduce
> a kluge that we'll be stuck with, and that doesn't even provide a
> good user experience. Disabling vacuum truncation is *not* something
> that I think we should recommend.
The truncation at the end of VACUUM takes an AccessExclusiveLock,
which is already user visible. Using a radix tree won't alter that.
ISTM the user might be interested in having the *lock* NOT happen, so
I am +1 for the suggestion regardless of whether radix tree ever
happens.
The lock itself can be cancelled, so the user would also be interested
in explicitly requesting a retry with a separate command/function.
--
Simon Riggs http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
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