Calculating space requirements for a user-owned index.

[Anon Anon]

How do you calculate the space requirements for a user-owned index?

We have a program that calculates the space needed for a system-owned index. Would that be close enough?

Kay Rozeboom
Information Technology Enterprise
Iowa Department of Administrative Services
Telephone: [phone] Fax: [phone]
Email: [e-mail]

[Dick.Weiland]

I would think that the existing calculations used for system-owned indexes should give you a reasonable number for the overall amount of space that you might use within the area. However with user-owned indexes you have to remember that each owner becomes the owner of an index structure. The SR8 structure is stored as if it is a VIA record of the owner so you are also increasing the cluster size of the owner records. I would at least determine the average length of the owner's set in terms of data records and use that number as input to the index formulas to get an idea of how much each owner's data cluster is going to grow. Then use the standard rules that you employ to determine page size given the current cluster sizes. Where you might have just allocated more pages to accomodate a new system-owned index you might want to consider increasing page sizes of existing pages so that the larger cluster sizes can be accomodated.

[Anon Anon]

Thanks, Dick. Can I ask a couple more questions?

1) What do you recommend for determining the best IBC size when there is a lot of variation in set length? If the IBC is too large, we will waste a lot of space on the small set occurrences. If it’s too small, we risk the overhead of splitting and spawning on the large set occurrences.

2) Is there any point in specifying a displacement value? It seems to me that we would be better off with a displacement of zero so that the SR8’s would be stored on the same page as the owner record.

Kay Rozeboom
Information Technology Enterprise
Iowa Department of Administrative Services
Telephone: [phone] Fax: [phone]
Email: [e-mail]

From: CA IDMS (IUA EIUA) Global User Community [mailto:[e-mail]]
Sent: Monday, June 03, 2013 4:28 PM
To: [e-mail]
Subject: [DBA Issues - Open Forum] RE: Calculating space requirements for a user-owned index.

I would think that the existing calculations used for system-owned indexes should give you a reasonable number for the overall amount of space that you might use within the area. However with user-owned indexes you have to remember that each owner becomes the owner of an index structure. The SR8 structure is stored as if it is a VIA record of the owner so you are also increasing the cluster size of the owner records. I would at least determine the average length of the owner's set in terms of data records and use that number as input to the index formulas to get an idea of how much each owner's data cluster is going to grow. Then use the standard rules that you employ to determine page size given the current cluster sizes. Where you might have just allocated more pages to accomodate a new system-owned index you might want to consider increasing page sizes of existing pages so that the larger cluster sizes can be accomodated.
Posted by:dick.weiland
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[Dick.Weiland]

1. I guess the first question really revolves around what is a small set and what is a large set. Remember that the Level 0 SR8 records are reduced in size to only hold the number of entries that exist within the SR8 occurrence. As a result an IBC of 100 with only one entry will have only enough space for the SR8 header and one entry so there is no great amount of space wasted. If a large percentage of the sets have a small number of members you may wish to try and optimize the larger set occurrences with the anticipation that the small sets will have a single Level 0 SR8 and not even need multiple levels of SR8 records.

2. In regards to the use of PAGE DISPLACEMENT for user-owned indexes I'd forget that the parameter exists. This parameter is designed to allow for separation of the intermediate SR8s from the Level 0 occurrences. However it is very possible that displacing one set occurence's Level 0 SR8s may place them on the page(s) used by another owner's intermediate SR8s generally defeating the preceived advantage of the parameter. Linda just published a paper on indexing page displacement about a week ago on this site's blogs and this topic should be within that material. You might get some advantage if the owner's area is sparsely populated but in most cases I believe that it buys no benefit.

[Anon Anon]

Thank you, Dick! I didn’t realize that the Level 0 SR8 records start out so small. That makes this much easier.

Kay Rozeboom
Information Technology Enterprise
Iowa Department of Administrative Services
Telephone: [phone] Fax: [phone]
Email: [e-mail]

From: CA IDMS (IUA EIUA) Global User Community [mailto:[e-mail]]
Sent: Tuesday, June 04, 2013 4:05 PM
To: [e-mail]
Subject: RE: [DBA Issues - Open Forum] RE: Calculating space requirements for a user

1. I guess the first question really revolves around what is a small set and what is a large set. Remember that the Level 0 SR8 records are reduced in size to only hold the number of entries that exist within the SR8 occurrence. As a result an IBC of 100 with only one entry will have only enough space for the SR8 header and one entry so there is no great amount of space wasted. If a large percentage of the sets have a small number of members you may wish to try and optimize the larger set occurrences with the anticipation that the small sets will have a single Level 0 SR8 and not even need multiple levels of SR8 records.

2. In regards to the use of PAGE DISPLACEMENT for user-owned indexes I'd forget that the parameter exists. This parameter is designed to allow for separation of the intermediate SR8s from the Level 0 occurrences. However it is very possible that displacing one set occurence's Level 0 SR8s may place them on the page(s) used by another owner's intermediate SR8s generally defeating the preceived advantage of the parameter. Linda just published a paper on indexing page displacement about a week ago on this site's blogs and this topic should be within that material. You might get some advantage if the owner's area is sparsely populated but in most cases I believe that it buys no benefit.
Posted by:dick.weiland
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[Dick.Weiland]

Kay, I guess I'm pretty bad at using these message boards. Check out the thread for Arch Consulting's anniversary. I put a response to your index sizing questions there by mistake. Oh well, congratulations to Arch Consulting!

[Anon Anon]

HI just as a general observation, do the rules from the 1970s still apply?

We live in a world of tera giga peta bytes etc. and increased processing power.
expoentially many times the availability of the 1970'S, and also the physical hardware technology is very different.

So being overly precise on calculations, will stilll make a significant difference in performance etc....

What are some bottlenecks which still exist today in terms of IDMS performance issues.., if they are not defined correctly.?

[Chris Hoelscher]

No – the rules from the 1970s do not apply anymore – nowadays LEISURE SUITS ARE NEVER IN STYLE!!!!!

Chris hoelscher
Technology Architect | Database Infrastructure Services
Technology Solution Services
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123 East Main Street |Louisville, KY 40202
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Humana.com
[phone] – office
[phone] - blackberry
Keeping CAS and Metavance safe for all HUMANAty

From: CA IDMS (IUA EIUA) Global User Community [mailto:[e-mail]]
Sent: Tuesday, June 04, 2013 9:24 AM
To: [e-mail]
Subject: [DBA Issues - Open Forum] RE: Calculating space requirements for a user-owned index.

HI just as a general observation, do the rules from the 1970s still apply?

We live in a world of tera giga peta bytes etc. and increased processing power.
expoentially many times the availability of the 1970'S, and also the physical hardware technology is very different.

So being overly precise on calculations, will stilll make a significant difference in performance etc....

What are some bottlenecks which still exist today in terms of IDMS performance issues.., if they are not defined correctly.?
Posted by:acp
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