Scope

Write a detail spec explaining this future and make it ready for review in upstream.

Acceptance criteria

1. Spec should be ready for review in upstream

As the python-glanceclient uploads image data without requiring a prior specification of the image size, it functions as intended. However, this can lead to complications when Glance utilizes Cinder, RBD, or S3 as its backend. Since Glance does not have the data size beforehand, it conducts an iterative "resize-before-write" process, gradually expanding the Cinder volume by 1 GB at a time until all image data is received. This approach poses a challenge because the Cinder volume needs to be detached and reattached during each
iteration, which can significantly slow down the process.

The same issue arises when using Ceph (RBD) as the backend, where a similar "resize-before-write" operation occurs, resulting in time-consuming operations.

If Glance is configured to use S3 as a backend, the multipart upload feature cannot be utilized because the image size is unknown in advance.