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Confusing in boot from image (create new volume) and root disk size

I am a bit confused about the various boot options for instances disks in OpenStack. Currently I have a number of CentOS templates stored as images in Glance, and so far I have been creating new instances, and selecting 'Boot from image (create new volume)'. This lets me specify the volume size. My first boot scripts are able to resize the root partition with no problems. However, I am confused about the root disk size value in the instance flavor.

If I boot directly from the image, will this create a new image file at the root disk size? Will this use the existing image in Glance? I do not want to modify the Glance image, but keep it in its current state as a VM template. Maybe I am confused as to what the best-practices are for instance templates, and creating new VMs. If anyone can explain this to me I would appreciate it.

Confusing in Confused about boot from image (create new volume) and root disk size

I am a bit confused about the various boot options for instances disks in OpenStack. Currently I have a number of CentOS templates stored as images in Glance, and so far I have been creating new instances, and selecting 'Boot from image (create new volume)'. This lets me specify the volume size. My first boot scripts are able to resize the root partition with no problems. However, I am confused about the root disk size value in the instance flavor.

If I boot directly from the image, will this create a new image file at the root disk size? Will this use the existing image in Glance? I do not want to modify the Glance image, but keep it in its current state as a VM template. Maybe I am confused as to what the best-practices are for instance templates, and creating new VMs. If anyone can explain this to me I would appreciate it.

Confused about boot from image (create new volume) and root disk size

I am a bit confused about the various boot options for instances disks in OpenStack. Currently I have a number of CentOS templates stored as images in Glance, and so far I have been creating new instances, and selecting 'Boot from image (create new volume)'. This lets me specify the volume size. My first boot scripts are able to resize the root partition with no problems. However, I am confused about the root disk size value in the instance flavor.

If I boot directly from the image, will this create a new image file at the root disk size? Will this use the existing image in Glance? I do not want to modify the Glance image, but keep it in its current state as a VM template. Maybe I am confused as to what the best-practices are for instance templates, and creating new VMs. If anyone can explain this to me I would appreciate it.

UPDATE:

Thanks for the answer Bill. What you said makes sense. I guess my main concern is the best method for setting up images for consumption in a production cloud-provider environment (boot from image, vs boot from image (new volume), vs boot from volume snapshot (new volume)). If I boot from an image/or volume (without creating a new volume), that will make any changes to the image persistent (which I obviously don't want in the case of using a number of templates). However, if I boot from an image and create a new volume, it looks like the flavor disk size is completely ignored in favor of the disk size property which you can manually specify.

I haven't tried booting from a volume snapshot yet (maybe that is different, but I will try that next).