Subject Re: Database Page size = WAS RE: [firebird-support] What UDF - Lib to use ?
Author Johannes Pretorius
thanks alot everybody

At 16:53 20/07/2004, you wrote:

>At 09:45 AM 7/20/2004, Jonathan Neve wrote:
>>Is there ever a case for using a higher value [than 1024 for page
>>size]? What would be the effect?
>The most important effect of a larger page size is reduced index
>depth. Firebird indexes have a single page at the top and fan out from
>there. The larger the page size, the wider the fan - i.e. one 4096 byte
>page points to nearly four times as many pages at the next level down than
>a 1024 byte page. The deeper the index, the slower retrievals become. To
>a record through an index, the engine must read the top page of the index
>and one page at each level until it gets to the bottom. Then it reads the
>page with the record. The fewer levels, the fewer reads.
>This "fan out" benefit is also seen in large blobs. A smallish blob fits on
>a single page. Obviously, the larger the page size, the larger a simple,
>one part blob can be. A blob that is larger than a page has two parts, an
>array of page numbers and the pages that contain the data. The larger the
>page, the fewer pages are required to hold the blob, therefor the shorter
>the array, or the more data that can be stored in a blob before the array
>itself exceeds a page. A huge blob has three parts, an array of page
>numbers that identify pages of arrays of page numbers that identify pages
>containing data.
>A larger page size also reduces the number of pages used for internal
>structures - pointer pages, transaction inventory pages, and page inventory
>pages. The same number of bits are used, of course, but fewer pages, thus
>fewer page reads.
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