VSAM DB2 and number of extent

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Re: VSAM DB2 and number of extent

Postby willy jensen » Tue Nov 05, 2019 1:50 pm

Well, I did analyze the listcat and the result is 163 extents with a total of 87375 tracks.
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Re: VSAM DB2 and number of extent

Postby samb01 » Tue Nov 05, 2019 3:18 pm

Yes of course and that is the problem.
We would like to get less extent for DB2 perfomance.

For example this DB2 VSAM has the same size and only 22 extents :

DBDCL.CFS.DBRL.TKSL.J0001.A002             87375    ?    22
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Re: VSAM DB2 and number of extent

Postby enrico-sorichetti » Tue Nov 05, 2019 3:58 pm

vsam extent consolidation ???
When I tell somebody to RTFM or STFW I usually have the page open in another tab/window of my browser,
so that I am sure that the information requested can be reached with a very small effort
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Re: VSAM DB2 and number of extent

Postby steve-myers » Wed Nov 06, 2019 2:20 am

Back in the days before RAID type DASD, multiple extents had a potential performance impact because they often required unproductive movement of the the disk's access mechanism.

With RAID type DASD, in a technical sense, this is no longer true. The RAID mechanism scatters your data all over the physical media in ways you cannot control regardless of the extent definition. In any event, opening the data usually loads the data into the RAID controller's cache memory, so no additional I/O to the RAID device's physical media is required.

The primary advantage of reducing DASD extents is it forces the logical data into contiguous logical space, which looks better on reports. There is little, if any, performance impact from consolidating extents.

This appears to be something dreamed up by a more ignorant than usual PHB type.
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