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From Teradata documentation, the definitions of these ResUsageSpma columns are:

FilePres         Total number of times a cylinder is loaded.

FilePreReads     Number of times a cylinder is loaded.

which look similar, but I observed on our lab's Teradata and found quite different values with an observation that FilePres = 2 x FilePreReads (approx.)

I also looked into KA but couldn't find any explanation. Please respond if you know the meaning of these fields.

The story doesn't end here. When I compared different versions of Teradata documents (V2R6, 12, 13) I found out quite a few differences among them.

The screenshots of the differences (from Excel) are attached. alt text alt text

Any idea on differences or this maybe a Teradata documentation problem?

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2 Answers

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In Chapter 13, pg 135 of the ResUsage Macros and Tables Manual (located in the Database Management section of the Teradata Manuals) the further describe the difference between these two columns on the ResUsageSvpr table. This table is the detail table of which ResUsageSpma summarizes.

File[seg]Pres - Total number of disk segments prefetched.

File[seg]PreReads - Total number of disk segment prefetches that caused a physical read

My interpretation is that the difference between these two columns would be which segments were able to be obtained from cache (logical read?) versus requiring a physical IO from the disk subsystem.

An interesting side note - This table also has what appears to be multi-temperature cylinder support in it.

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Right to the point. Much thanks. – Monis Iqbal Dec 23 at 6:27
Rob, please check the edited question. I found out some bizarre differences among the same fields in different Teradata versions. – Monis Iqbal Dec 24 at 10:56
@Rob, We asked Teradata about the differences and they said that Teradata 13 documentation is correct except for the Logical/Physical IO Counts, for which they provided the correct formulas. I've mentioned them in my answer. – Monis Iqbal Jan 26 at 18:18
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Below are the (more explanatory) definitions from Teradata 13 documentation:

FilePres - Total number of times a logical data prefetch was performed (either as a cylinder read or individual block reads).

FilePreReads - Number of times a data prefetch was physically performed either as a cylinder read or individual blocks read.

We asked Teradata about the major differences between the 'Physical IO Count' and 'Logical IO Count' formulas and they responded with these correct formulas:

Physical I/O Counts - FileAcqReads + FilePreReads + FileWrites

Logical I/O Counts - FileAcqs + FilePres + FileRels

From Teradata, the Teradata 13 documentation provides the correct definitions/formulas however for the physical/logical counts they have entered a DR to rectify it.

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