From 0dd6ff0ac8482f753405c5bdb091d2a8adc58e75 Mon Sep 17 00:00:00 2001 From: Michael Paquier Date: Mon, 18 Feb 2019 09:52:02 +0900 Subject: [PATCH] Avoid some unnecessary block reads in WAL reader When reading a new page internally and depending on the way the WAL reader facility gets used by plugins, the current implementation of the WAL reader may finish by reading a block multiple times while it is not actually necessary as the requested data length may be equal to what has been already read. This can happen for any size, but is more likely to happen at the end of a page. This can cause performance penalties in plugins which rely on the block reads to be purely sequential, zlib not liking backward reads for example. The new behavior also shaves some cycles when doing recovery. Author: Arthur Zakirov Reviewed-by: Andrey Lepikhov, Michael Paquier, Grigory Smolkin Discussion: https://postgr.es/m/2ddf4a32-517e-d6f4-d992-4a63b6035bfd@postgrespro.ru --- src/backend/access/transam/xlogreader.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/src/backend/access/transam/xlogreader.c b/src/backend/access/transam/xlogreader.c index 69b6226f8f..cbc7e4e7ea 100644 --- a/src/backend/access/transam/xlogreader.c +++ b/src/backend/access/transam/xlogreader.c @@ -536,7 +536,7 @@ ReadPageInternal(XLogReaderState *state, XLogRecPtr pageptr, int reqLen) /* check whether we have all the requested data already */ if (targetSegNo == state->readSegNo && targetPageOff == state->readOff && - reqLen < state->readLen) + reqLen <= state->readLen) return state->readLen; /* -- 2.39.5