Subject: Re: [PATCH] xlogreader: do not read a file block twice
To: pgsql-hackers@lists.postgresql.org
References: <2ddf4a32-517e-d6f4-d992-4a63b6035bfd@postgrespro.ru>
From: Grigory Smolkin <g.smolkin@postgrespro.ru>
Message-ID: <87e30af6-f85a-dc95-4574-95cded028786@postgrespro.ru>
Date: Mon, 11 Feb 2019 19:32:59 +0300
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Hm, looks like it could speed up PostgreSQL recovery, but is it safe?


On 02/11/2019 07:25 PM, Arthur Zakirov wrote:
> Hello hackers,
>
> Grigory noticed that one of our utilities has very slow performance 
> when xlogreader reads zlib archives. We found out that xlogreader 
> sometimes reads a WAL file block twice.
>
> zlib has slow performance when you read an archive not in sequential 
> order. I think reading a block twice in same position isn't 
> sequential, because gzread() moves current position forward and next 
> call gzseek() to the same position moves it back.
>
> It seems that the attached patch solves the issue. I think when reqLen 
> == state->readLen the requested block already is in the xlogreader's 
> buffer.
>
> What do you think?
>

-- 
Grigory Smolkin
Postgres Professional: http://www.postgrespro.com
The Russian Postgres Company