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IDS Forum

Re: To 16K My Indices ... Or Not to 16K Them, That

Posted By: ANDREW FORD
Date: Thursday, 12 November 2009, at 11:48 a.m.

In Response To: To 16K My Indices ... Or Not to 16K Them, That is (Clifton Bean)

I think the answer to this question is...it depends.

We looked at 16K page size (also looked at 4K, 6K, 8K, 12K and 14K) for some of our busiest OLTP indexes with the thought that a larger page size = more index entries per page/btree node = fewer index levels = better performance.

It turns out that even though we reduced the number of index levels from 6 with a 2K page size to 4 with a 16K page size the smaller page size with more levels outperformed a larger page size with fewer levels.

I think this was result of the cost of an I/O for a buffer cache miss when reading an index. With a 16K page size if you need to read an index page from disk because it is not in the buffers you need to read 8x as much data from disk than if you were using 2K pages, so your penalty for a cache miss with 16K page sizes is greater.

This problem is magnified when your working data set is small relative to the size of the table and your data access is evenly distributed across the table. You will be bringing index leaf data in 16K at a time and will have a lower chance of a cache hit on your next read than if you were bringing data in at a more selective 2K.

All of this assumes you don't have enough memory to keep the 16K index pages you need in the buffers. If you can prevent Informix from going to disk for index reads then the cache miss penalty doesn't exist and fewer index levels should give a little bit of a performance boost.

I say a little bit because even with more levels the root node (level 1) and a lot of intermediate nodes (levels 2 to n-1) will have good chance of being in cache because they are accessed frequently. This comes into play when you're wondering why reducing the number of levels from 6 to 4 didn't improve performance by as much as you thought it would.

At least that is what happened when I did my testing. YMMV, it depends and all that stuff.

Andrew

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