It looks like when matrix jobs try to discard old builds, it also flushes my perforce workspaces to 0.

Feb 04, 2016 3:34:40 AM hudson.plugins.perforce.PerforceSCM processWorkspaceBeforeDeletion
INFO: Workspace 'c:\jenkins\workspace\matrix_job' is being deleted; flushing workspace to revision 0.
Feb 04, 2016 3:34:40 AM hudson.plugins.perforce.PerforceSCM processWorkspaceBeforeDeletion
INFO: Using remote perforce client: buildmachine01
Feb 04, 2016 3:34:40 AM hudson.plugins.perforce.PerforceSCM processWorkspaceBeforeDeletion
INFO: [matrix_job] $ p4 workspace -o buildmachine01
Feb 04, 2016 3:34:40 AM hudson.plugins.perforce.PerforceSCM processWorkspaceBeforeDeletion
INFO: 'Don't update client' is set. Not saving the client changes.
Feb 04, 2016 3:34:40 AM hudson.plugins.perforce.PerforceSCM processWorkspaceBeforeDeletion
INFO: [matrix_job] $ p4 sync -k //buildmachine01/...#0
Feb 04, 2016 3:34:59 AM hudson.model.AsyncPeriodicWork$1 run
INFO: Finished Workspace clean-up. 26,634 ms

My perforce settings does not allow jenkins to create the workspace or manage it. Changelog, syncing, and workspace updating are disabled. This is quite bad since our p4 workspace is at least 300 GB to sync to again.