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Configure and use driver filter and weighing for scheduler
OpenStack Block Storage enables you to choose a volume back end based on back-end specific properties by using the DriverFilter and GoodnessWeigher for the scheduler. The driver filter and weigher scheduling can help ensure that the scheduler chooses the best back end based on requested volume properties as well as various back-end specific properties.
What is driver filter and weigher and when to use it
The driver filter and weigher gives you the ability to more finely
control how the OpenStack Block Storage scheduler chooses the best back
end to use when handling a volume request. One example scenario where
using the driver filter and weigher can be if a back end that utilizes
thin-provisioning is used. The default filters use the
free capacity
property to determine the best back end, but
that is not always perfect. If a back end has the ability to provide a
more accurate back-end specific value you can use that as part of the
weighing. Another example of when the driver filter and weigher can
prove useful is if a back end exists where there is a hard limit of 1000
volumes. The maximum volume size is 500 GB. Once 75% of the total space
is occupied the performance of the back end degrades. The driver filter
and weigher can provide a way for these limits to be checked for.
Enable driver filter and weighing
To enable the driver filter, set the
scheduler_default_filters
option in the
cinder.conf
file to DriverFilter
or add it to
the list if other filters are already present.
To enable the goodness filter as a weigher, set the
scheduler_default_weighers
option in the
cinder.conf
file to GoodnessWeigher
or add it
to the list if other weighers are already present.
You can choose to use the DriverFilter
without the
GoodnessWeigher
or vice-versa. The filter and weigher
working together, however, create the most benefits when helping the
scheduler choose an ideal back end.
Important
The support for the DriverFilter
and
GoodnessWeigher
is optional for back ends. If you are using
a back end that does not support the filter and weigher functionality
you may not get the full benefit.
Example cinder.conf
configuration file:
scheduler_default_filters = DriverFilter
scheduler_default_weighers = GoodnessWeigher
Note
It is useful to use the other filters and weighers available in
OpenStack in combination with these custom ones. For example, the
CapacityFilter
and CapacityWeigher
can be
combined with these.
Defining your own filter and goodness functions
You can define your own filter and goodness functions through the use
of various properties that OpenStack Block Storage has exposed.
Properties exposed include information about the volume request being
made, volume_type
settings, and back-end specific
information about drivers. All of these allow for a lot of control over
how the ideal back end for a volume request will be decided.
The filter_function
option is a string defining an
equation that will determine whether a back end should be considered as
a potential candidate in the scheduler.
The goodness_function
option is a string defining an
equation that will rate the quality of the potential host (0 to 100, 0
lowest, 100 highest).
Important
The drive filter and weigher will use default values for filter and
goodness functions for each back end if you do not define them yourself.
If complete control is desired then a filter and goodness function
should be defined for each of the back ends in the
cinder.conf
file.
Supported operations in filter and goodness functions
Below is a table of all the operations currently usable in custom filter and goodness functions created by you:
Operations | Type |
---|---|
+, -, *, /, ^ | standard math |
not, and, or, &, |, ! | logic |
>, >=, <, <=, ==, <>, != | equality |
+, - | sign |
x ? a : b | ternary |
abs(x), max(x, y), min(x, y) | math helper functions |
Caution
Syntax errors you define in filter or goodness strings are thrown at a volume request time.
Available properties when creating custom functions
There are various properties that can be used in either the
filter_function
or the goodness_function
strings. The properties allow access to volume info, qos settings, extra
specs, and so on.
The following properties and their sub-properties are currently available for use:
Host stats for a back end
- host
-
The host's name
- volume_backend_name
-
The volume back end name
- vendor_name
-
The vendor name
- driver_version
-
The driver version
- storage_protocol
-
The storage protocol
- QoS_support
-
Boolean signifying whether QoS is supported
- total_capacity_gb
-
The total capacity in GB
- allocated_capacity_gb
-
The allocated capacity in GB
- reserved_percentage
-
The reserved storage percentage
Capabilities specific to a back end
These properties are determined by the specific back end you are creating filter and goodness functions for. Some back ends may not have any properties available here.
Requested volume properties
- status
-
Status for the requested volume
- volume_type_id
-
The volume type ID
- display_name
-
The display name of the volume
- volume_metadata
-
Any metadata the volume has
- reservations
-
Any reservations the volume has
- user_id
-
The volume's user ID
- attach_status
-
The attach status for the volume
- display_description
-
The volume's display description
- id
-
The volume's ID
- replication_status
-
The volume's replication status
- snapshot_id
-
The volume's snapshot ID
- encryption_key_id
-
The volume's encryption key ID
- source_volid
-
The source volume ID
- volume_admin_metadata
-
Any admin metadata for this volume
- source_replicaid
-
The source replication ID
- consistencygroup_id
-
The consistency group ID
- size
-
The size of the volume in GB
- metadata
-
General metadata
The property most used from here will most likely be the
size
sub-property.
Extra specs for the requested volume type
View the available properties for volume types by running:
$ cinder extra-specs-list
Current QoS specs for the requested volume type
View the available properties for volume types by running:
$ cinder qos-list
In order to access these properties in a custom string use the following format:
<property>.<sub_property>
Driver filter and weigher usage examples
Below are examples for using the filter and weigher separately, together, and using driver-specific properties.
Example cinder.conf
file configuration for customizing
the filter function:
[default]
scheduler_default_filters = DriverFilter
enabled_backends = lvm-1, lvm-2
[lvm-1]
volume_driver = cinder.volume.drivers.lvm.LVMVolumeDriver
volume_backend_name = sample_LVM
filter_function = "volume.size < 10"
[lvm-2]
volume_driver = cinder.volume.drivers.lvm.LVMVolumeDriver
volume_backend_name = sample_LVM
filter_function = "volume.size >= 10"
The above example will filter volumes to different back ends depending on the size of the requested volume. Default OpenStack Block Storage scheduler weighing is done. Volumes with a size less than 10 GB are sent to lvm-1 and volumes with a size greater than or equal to 10 GB are sent to lvm-2.
Example cinder.conf
file configuration for customizing
the goodness function:
[default]
scheduler_default_weighers = GoodnessWeigher
enabled_backends = lvm-1, lvm-2
[lvm-1]
volume_driver = cinder.volume.drivers.lvm.LVMVolumeDriver
volume_backend_name = sample_LVM
goodness_function = "(volume.size < 5) ? 100 : 50"
[lvm-2]
volume_driver = cinder.volume.drivers.lvm.LVMVolumeDriver
volume_backend_name = sample_LVM
goodness_function = "(volume.size >= 5) ? 100 : 25"
The above example will determine the goodness rating of a back end based off of the requested volume's size. Default OpenStack Block Storage scheduler filtering is done. The example shows how the ternary if statement can be used in a filter or goodness function. If a requested volume is of size 10 GB then lvm-1 is rated as 50 and lvm-2 is rated as 100. In this case lvm-2 wins. If a requested volume is of size 3 GB then lvm-1 is rated 100 and lvm-2 is rated 25. In this case lvm-1 would win.
Example cinder.conf
file configuration for customizing
both the filter and goodness functions:
[default]
scheduler_default_filters = DriverFilter
scheduler_default_weighers = GoodnessWeigher
enabled_backends = lvm-1, lvm-2
[lvm-1]
volume_driver = cinder.volume.drivers.lvm.LVMVolumeDriver
volume_backend_name = sample_LVM
filter_function = "stats.total_capacity_gb < 500"
goodness_function = "(volume.size < 25) ? 100 : 50"
[lvm-2]
volume_driver = cinder.volume.drivers.lvm.LVMVolumeDriver
volume_backend_name = sample_LVM
filter_function = "stats.total_capacity_gb >= 500"
goodness_function = "(volume.size >= 25) ? 100 : 75"
The above example combines the techniques from the first two examples. The best back end is now decided based off of the total capacity of the back end and the requested volume's size.
Example cinder.conf
file configuration for accessing
driver specific properties:
[default]
scheduler_default_filters = DriverFilter
scheduler_default_weighers = GoodnessWeigher
enabled_backends = lvm-1,lvm-2,lvm-3
[lvm-1]
volume_group = stack-volumes-lvmdriver-1
volume_driver = cinder.volume.drivers.lvm.LVMVolumeDriver
volume_backend_name = lvmdriver-1
filter_function = "volume.size < 5"
goodness_function = "(capabilities.total_volumes < 3) ? 100 : 50"
[lvm-2]
volume_group = stack-volumes-lvmdriver-2
volume_driver = cinder.volume.drivers.lvm.LVMVolumeDriver
volume_backend_name = lvmdriver-2
filter_function = "volumes.size < 5"
goodness_function = "(capabilities.total_volumes < 8) ? 100 : 50"
[lvm-3]
volume_group = stack-volumes-lvmdriver-3
volume_driver = cinder.volume.drivers.LVMVolumeDriver
volume_backend_name = lvmdriver-3
goodness_function = "55"
The above is an example of how back-end specific properties can be
used in the filter and goodness functions. In this example the LVM
driver's total_volumes
capability is being used to
determine which host gets used during a volume request. In the above
example, lvm-1 and lvm-2 will handle volume requests for all volumes
with a size less than 5 GB. The lvm-1 host will have priority until it
contains three or more volumes. After than lvm-2 will have priority
until it contains eight or more volumes. The lvm-3 will collect all
volumes greater or equal to 5 GB as well as all volumes once lvm-1 and
lvm-2 lose priority.