Hey thezero,
As you mentioned, the datamodel_summary directory contains accelerated data from datamodels. While you can delete it, that may not solve the issue, as if Splunk sees that a retention policy is not being met, it is just going to kick off accelerations, and the reclaim the disk space. Your best bet would be to figure out which data model is causing the issue, and then adjusting it as needed.
Under "Settings" check out the "Data Models" page, and look for accelerated data models that may be contributing to the disk space usage. If you don't find any, then it is possible that there was an accelerated data model at one time, but it has since been removed, and Splunk hasn't reaped the accelerations, in which case you can delete that directory.
Hope this helps,
Dave
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