how openstack cinder distribute volume without special type while configure two ceph backend pools

Question

I found that cinder-volume will distribute the volume to the pool which dependent on their virtual or actual free capacity while request to create a new volume without special volume type and there are two or more backend pools without setting the default_volume_type configuration option.

Actually, in my case, there is a ceph_common pool which remain 30 TiB for MAX AVAIL, the other is a ceph_specs pool which remain 10 TiB for MAX AVAIL, it will create a new volume in ceph_specs pool while creating without volume type as openstack volume create --size 10 test_vol_without_type.

I had check from these links and can't get any clue:

• https://docs.openstack.org/cinder/latest/admin/default-volume-types.html
• https://docs.openstack.org/cinder/latest/configuration/block-storage/samples/cinder.conf.html

Could anyone give any advice? What's the principle of this situation? THX.

Answer 1

According to the default cinder.conf, there are two steps(filter and weigh) to determine which host for distributing the volume.

# Which filter class names to use for filtering hosts when not specified in the
# request. (list value)
#scheduler_default_filters = AvailabilityZoneFilter,CapacityFilter,CapabilitiesFilter

# Which weigher class names to use for weighing hosts. (list value)
#scheduler_default_weighers = CapacityWeigher

• 1, We use default availability zone, and don't define the special volume type to create the new volume, so the AvailabilityZoneFilter and CapabilitiesFilter return True.
• 2, In CapacityFilter, it will return True for the host if there is sufficient capacity for creating the size of volume.
• • available capacity calculate by:

total_physical_capacity * max_over_subscription_ratio - backend_provisioned

total_physical_capacity:     the pool TOTAL size get by `ceph df` command
max_over_subscription_ratio: Default ratio is 20.0, meaning provisioned capacity can be 20 times of the total physical capacity
backend_provisioned:         the pool PROVISIONED TOTAL size get by `rbd du -p pool_name`

• 3, In CapacityWeigher, it will select the most available capacity host to deploy the volume.

So it seems like randomly to distribute the new volume to different backend pool in some situation. Actually, it because of the CapacityWeigher.

How to solve my problem:

• 1, create a volume type by define the backend: openstack volume type create --property volume_backend_name='ceph_common' vol_type_common
• 2, configure default_volume_type as vol_type_common in all cinder components such as cinder-api, cinder-scheduler, cinder-volume.
• 3, reference: Default Volume Types, Untyped volumes to default volume type