Question
Kevin McGinn · May 31

System impact of compacting to reclaim large space

I have a database with almost 1T of free space. I want to run the compaction process to reclaim and subsequently free that space. I am aware that this process is a background process but will the level of processing required to move this volume of space to the end of the file to allow for reclamation impact system usage while this process is running. Essentially, does this process run at a low priority and use  processing as available. I am trying to determine if I should run this over a weekend or can this be run at any time.

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Kevin, 

You should run it over the weekend, if the system is busy, the operation may take longer or not fully complete. 

See notes from the documentation: 

Compacting a Database

The compact database operation is designed to run concurrently with normal database activity. The operation does consume some system resources, however, and may not complete if the system is under extremely high load. For these reasons, InterSystems recommends running this and other database reorganization operations (including compacting and defragmenting globals) during off-peak hours, and running only one such operation on a system at a time.

One thing you can do is run it in steps for both the compaction and the free so you have performance effect data on your actual system. Do a run to compact and free of maybe 1 or 10 Gb.

Be ready for the WIJ to expand.

Try it on a non production copy if you can first and run an Integchk on the dataset after.

Make sure you are on a current version of Cache or IRIS since some versions had issues.

Know when your system really has it low utilization period. You might have midnight/end of day processing or someone might do the weekly ETL on the weekend.