From: Michael Paquier Date: Mon, 18 Feb 2019 00:52:02 +0000 (+0900) Subject: Avoid some unnecessary block reads in WAL reader X-Git-Tag: REL_12_BETA1~716 X-Git-Url: https://granicus.if.org/sourcecode?a=commitdiff_plain;h=0dd6ff0ac8482f753405c5bdb091d2a8adc58e75;p=postgresql 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 --- 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; /*