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Re: Updated version of pg_receivexlog

From: Magnus Hagander <magnus(at)hagander(dot)net>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: Heikki Linnakangas <heikki(dot)linnakangas(at)enterprisedb(dot)com>, Fujii Masao <masao(dot)fujii(at)gmail(dot)com>, Jaime Casanova <jaime(at)2ndquadrant(dot)com>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Updated version of pg_receivexlog
Date: 2011-10-27 14:57:22
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-hackers
On Thu, Oct 27, 2011 at 16:54, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> Magnus Hagander <magnus(at)hagander(dot)net> writes:
>> On Thu, Oct 27, 2011 at 13:19, Heikki Linnakangas
>> <heikki(dot)linnakangas(at)enterprisedb(dot)com> wrote:
>>> On 27.10.2011 14:09, Fujii Masao wrote:
>>>> Yes. But that sounds unuserfriendly. Padding the WAL file manually
>>>> is easy-to-do for a user?
>> I'd definitely want to avoid anything that requires pg_receivexlog to
>> actually *parse* the WAL. That'll make it way more complex than I'd
>> like.
> What parsing?  Just pad to 16MB with zeroes.  In fact, I think the

I'm just sayihng that *if* parsing is required, it would be bad.

> receiver should just create the file that size to start with, and then
> write received data into it, much like normal WAL creation does.

So when pg_receivexlog starts up, how would it know if the last file
represents a completed file, or a half-full file, without actually
parsing it? It could be a 16Mb file with 10 bytes of valid data, or a
complete file with 16Mb of valid data.

We could always ask for a retransmit of the whole file, but if that
file is gone on the master, we won't be able to do that, and will
error out in a situation that's not actually an error.

Though I guess if we leave the file as .partial up until this point
(per my other patch just posted), I guess this doesn't actually apply
- if the file is called .partial, we'll overwrite into it. If it's
not, then we assume it's a complete segment.

 Magnus Hagander

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