Introspection of Images

Several image formats include metadata inside the image data. This spec proposes that Glance expose that metadata through introspection of the image. For example, we can read the metadata from a vmdk formatted image to know that the disk type of the image is “streamOptimized”. Allowing Glance to perform the introspection removes the burden from the administrator. Exposing this metadata also helps the consumer of the image; Nova’s workflow is very different based on the disk type of the image.

Problem description

Several image formats include metadata inside the image data. A stream optimized vmdk image could be introspected like so:

$ head -20 so-disk.vmdk

# Disk DescriptorFile

# Extent description
RDONLY 209714 SPARSE "generated-stream.vmdk"

# The Disk Data Base

ddb.adapterType = "buslogic"
ddb.geometry.cylinders = "102"
ddb.geometry.heads = "64"
ddb.geometry.sectors = "32"
ddb.virtualHWVersion = "4"

When a user of the image looks at the metadata they will see important information like the required disk type, “streamOptimized”. Extracting the metadata in Glance and exposing it through Glance’s API, means that administrators and image users alike do not need to perform the introspection themselves. Consumers like Nova will have very different workflows based on disk type alone and could also optimize the rest of its workflow based on other image metadata.

Proposed change

Using Glance’s Asynchronous Workers, we can extract the image metadata without requiring a separate node and without suffering significant performance degradation.


A separate worker node could ostensibly be controlled to prevent degradation on the Glance API nodes but is an unnecessary addition to the architecture.

Data model impact


REST API impact


Security impact


Notifications impact


Other end user impact


Performance Impact

Based on the maximum number of workers and the size that the administrator has configured for the introspection workspace, performance could be negatively affected if the operator misconfigures their maximum number of workers and allocated workspace.

Other deployer impact

When configuring this feature, operators will need to know the average size of the images that they are managing and appropriately configure the max_workers setting and an appropriate amount of space for the workers to use.

Developer impact




Primary assignee:


Other contributors:



Core reviewer(s):

nikhil-komawar kragniz icordasc

Work Items

  • Create specific workers for different introspection tasks

  • Create workflows for tasks

  • Update configuration files with additions

  • Update configuration documentation


This depends on Glance’s Asynchronous Workers.


Unit testing will be needed for the introspection tasks and for the new task flows.

Documentation Impact

This may have an impact on the upgrade and installation parts of the documentation. For operators upgrading, they’ll need to understand how to properly configure a system for image introspection. For new users, they’ll need to be warned about appropriately allocating space for the workers to use and possibly choosing a more conservative maximum worker number than is default until they can determine the appropriate number.