[PATCH] xlogreader: do not read a file block twice

From: Arthur Zakirov <a(dot)zakirov(at)postgrespro(dot)ru>
To: pgsql-hackers(at)lists(dot)postgresql(dot)org
Cc: Grigory Smolkin <g(dot)smolkin(at)postgrespro(dot)ru>
Subject: [PATCH] xlogreader: do not read a file block twice
Date: 2019-02-11 16:25:39
Message-ID: 2ddf4a32-517e-d6f4-d992-4a63b6035bfd@postgrespro.ru
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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?

--
Arthur Zakirov
Postgres Professional: http://www.postgrespro.com
Russian Postgres Company

Attachment Content-Type Size
xlogreader-reqlen.patch text/x-patch 545 bytes

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