Comments on: IOUG Day 2 http://jonathanlewis.wordpress.com/2009/05/06/ioug-day-2/ Just another Oracle weblog Mon, 20 May 2013 17:10:14 +0000 hourly 1 http://wordpress.com/ By: Jonathan Lewis http://jonathanlewis.wordpress.com/2009/05/06/ioug-day-2/#comment-33181 Thu, 28 May 2009 22:25:18 +0000 http://jonathanlewis.wordpress.com/?p=1130#comment-33181 Cristian,

Sorry I missed this note when it was first posted. It would be worth knowing what sort of speeds you are talking about.

One of the “problems” with increasing numbers of nodes is that you have an increased chance that you have to split a large “read” request into multiple 3-way transfers.

When you compare this with the extreme case that you can get from a SAN (where SAN readahead pre-caches the next big multiblock read you’re going to ask for) it’s not too surprising that big tablescans can slow down.

You have to remember that the important comparison should be between “real disk” speed and “shared cache” speed – but SANs often “cheat” on tablescans.

]]> By: Cristian http://jonathanlewis.wordpress.com/2009/05/06/ioug-day-2/#comment-32917 Thu, 07 May 2009 08:04:56 +0000 http://jonathanlewis.wordpress.com/?p=1130#comment-32917 Hi Jonathan,
there is a problem with the link in “the oracle RAC SIG”. But i want take this chance to say that with a four nodes RAC 10.2.0.4 we have encountered performance problems due to the fact that in full table scans getting blocks by global cache from remote nodes is sometimes a lot slower than getting them from disks. (a lot of “gc cr multiblock request” wait events) a we have found no hardware o interconnect communication probles.

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