Re: block size

From: Greg Rahn <>
Date: Mon, 26 Oct 2009 13:31:58 -0700
Message-ID: <>

On Mon, Oct 26, 2009 at 11:32 AM, Niall Litchfield <> wrote:
> so, discussion with DW architect today
> "but this isn't OLTP,this is DW, in a datawarehouse you want the biggest block size you can have".
> my question to you (and him)
> "why? What's the theory and evidence for this?"
> In other words *why* do people think this?

Lets start with the last question first: People think this probably because of: 1) they think better is better for an unknown, unquantified reason 2) someone somewhere on the Internet wrote it, so it must be true

The reality (facts) of it is this:
- a larger block *may* give a slightly better compression ratio as more values are packed within a block and Oracle uses block level compression
- in a DW where physical table/partition scans are prevalent, the IO size matters most, not the block size (Oracle can do 1MB reads no matter what the block size)
- 8k blocks are the default and more tested value and block size can influence execution plan and most people know that a suboptimal execution plan will easily offset any *potential* gains from a non-8k block
- with RAC an 8k block fits within a jumbo frame (MTU 9000) and that helps reduce system time for interconnect traffic (bigger blocks will get split)

When people make claims on something, I generally put my engineering hat on and ask, "What data did you look at to draw those conclusions (and can I see it too)?". That question alone is enough to end many (silly) discussions.

Greg Rahn
Received on Mon Oct 26 2009 - 15:31:58 CDT

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