No valid host was found

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This issue is really bugging me. I installed Juno on 2 baremetal servers (not on VM), one for compute/network and the other for compute. Everything went well during installation but I had to try many times to launch an instance even with m1.tiny. Most of the failures got an error like "No valid host was found" and I am sure the HW resource is good enough.

In nova-schedule.log, I see below error.

2015-05-06 22:18:34.902 1380 TRACE nova.servicegroup.drivers.db OperationalError: (OperationalError) database is locked u'UPDATE services SET updated_at=?, report_count=? WHERE services.id = ?' ('2015-05-06 14:18:34.901704', 319559, 4)

Has anybody ever see this kind of issue?

Really appreciated.