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Message Posted: Thu, 18 Mar 2005 @ 01:49:48 GMT


     
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Subj:   Re: Reconfiguration and Database Space
 
From:   Dennis Calkins

Reconfig doesn't change any database or User settings for MAX PERM, SPOOL or TEMP space, except to add all of the new disk space added by the new amps to user DBC to allow the DBA after the reconfig to Dole it out to who needs more space.

One thing to keep in mind is that MAX PERM is actually managed at the AMP level stat, not a system level.

If you have an 10 AMP system and you create a database with 1 GIG of space, each AMP will have a MAXPERM of 100 MEG for that Database.

If you have data skew and one amp has 90 MEG in use and the others only have 50 MEG ( 540 MEG ) then you might have issues after the reconfig.

Lets say you add 10 more amps. Now that 1 GIG of MAX PERM is divided across 20 AMPS ( 10 old ones + 10 new ones ) so each amp is allocated 50 MEG of space for this database.

Now each of the existing amps will give up 50% of their HASH codes to the new 10 AMP and the data will be redistributed based upon the location of those hash codes. ( we never move a hash code from an existing amp to another existing amp, it always to a new amp ).

In the unlikely event the SKEW on the HASH CODES is bad enough that one of the AMPS still has more than 50 MEG of space on it afte the reconfig, that AMP will have negative FREEPERM and you won't be able to insert any rows on that AMP. The other amps will be fine and the inserts will work (unless the FULL AMP is required to insert an USI orfallback row in association with the insert of the primrary row on an amp that isn't full. )

The DBA will have to ( after the reconfig ) allocate more space to this database to over come the NEGATIVE value.

     create database somespace as perm = 1e9;
     give somespace to negative_user;
     drop database somespace;

Now I guess one could run a query after the reconfig like....

     sel vproc, DatabaseName,
         ( MaxPerm - CurrentPerm )
            (NAMED freeperm, FORMAT '---,---,---,---,--9')
     from diskspace
     where  maxperm > 0
            and freeperm <= 0 ;

Maxperm > 0 elimnates all the users who have logons but no diskspace.



     
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