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The NoValidHost error is encountered when the scheduler filters eliminate all available hosts as potential targets for the instance. To confirm which filter(s) are rejecting hosts set debug=True in the /etc/nova/nova.conf on the node(s) running the scheduler and restart the scheduler service. Then re-attempt instance creation and watch the /var/log/nova/scheduler.log file for lines like:

[req-... None] Starting with 3 host(s)
[req-... None] Filter RetryFilter returned 3 host(s)
[req-... None] Filter AvailabilityZoneFilter returned 3 host(s) 
[req-... None] Filter RamFilter returned 2 host(s)
...
[req-... None] Filtered [(localhost.localdomain, localhost.localdomain) ram:3208 disk:7168 io_ops:0 instances:1] _schedule ...
[req-... None] Weighed [WeighedHost [host: (localhost.localdomain, localhost.localdomain) ram:3208 disk:7168 io_ops:0 instances:1, weight: 1.0]] ...

Note that these lines were redacted somewhat for brevity.

The NoValidHost error is encountered when the scheduler filters eliminate all available hosts as potential targets for the instance. To confirm which filter(s) are rejecting hosts set debug=True in the /etc/nova/nova.conf on the node(s) running the scheduler and restart the scheduler service. Then re-attempt instance creation and watch the /var/log/nova/scheduler.log file for lines like:

[req-... None] Starting with 3 host(s)
[req-... None] Filter RetryFilter returned 3 host(s)
[req-... None] Filter AvailabilityZoneFilter returned 3 host(s) 
[req-... None] Filter RamFilter returned 2 host(s)
...
[req-... None] Filtered [(localhost.localdomain, localhost.localdomain) ram:3208 disk:7168 io_ops:0 instances:1] _schedule ...
[req-... None] Weighed [WeighedHost [host: (localhost.localdomain, localhost.localdomain) ram:3208 disk:7168 io_ops:0 instances:1, weight: 1.0]] ...

Note that these lines were redacted somewhat for brevity.brevity. From this output you can determine which filter(s) eliminated hosts.