Difference between FilePres and FilePreReads

Database
Enthusiast

Difference between FilePres and FilePreReads

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.

I've also posted this question on the Teradata Questions forum: http://teradataquestions.com/questions/48/difference-between-filepres-and-fileprereads

Thanks,
6 REPLIES
Enthusiast

Re: Difference between FilePres and FilePreReads

Got the right answer on the TeradataQuestions forum. Thanks Rob.
Enthusiast

Re: Difference between FilePres and FilePreReads

I got the response but there are still additional questions regarding different/ambiguous definition of these and related columns in different Teradata versions. Please see the question here: http://teradataquestions.com/questions/48/difference-between-filepres-and-fileprereads
Enthusiast

Re: Difference between FilePres and FilePreReads

Monis,

My understanding of this is that FilePres is the logical read, which every read will do, then the FilePreReads is the number of physical reads to support the logical reads when the cylinder is not in Cache. on a cylinder basis.

Thus you could work out the Cylinder Cache hit ratio as

((FilePres-FilePeReads)/filePres) * 100

Random
Enthusiast

Re: Difference between FilePres and FilePreReads

From Teradata 13:

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.

Which aligns with your point of view.

Now the ambiguity in Teradata documentation for different versions: Should it be brought to their notice?
Enthusiast

Re: Difference between FilePres and FilePreReads

We are still on 12 here. It probably means that the 13 documentation is up to date, and corrected from 12.

If that is the case then the formula above is actually the overall Cache hit ratio? If it is, I think the oxide will fall off our disks soon as we seem to have a CHR of 14%!

It might be a good idea it Teradata could respond here!
Enthusiast

Re: Difference between FilePres and FilePreReads

We opened an incident with Teradata and they have responded. I've summarized their response at the teradataquestions forum: http://teradataquestions.com/questions/48/difference-between-filepres-and-fileprereads/67#67