Comment 0 for bug 1792710

When uploading a volume to glance as an image, the glance server don't know the image size, so the backend storage server(such as ceph) need to resize the image every time it received new chunk of data(by default 64K). So there will be huge times of resize operations that will terribly impact the performance. The patch aims to tell the image size to glance server at the initial phase of uploading, so that glance backend such as ceph can allocate an image with exact size, and don't need to resize the image again and again

This is an known issue which can be found in driver files of all kinds of backend storage system:
In file: glance_store/_drivers/rbd.py, function: add
In file: glance_store/_drivers/cinder.py, function: add
In file: glance_store/_drivers/sheepdog.py, function: add
In all these files, there're comments like below:
    # If the image size provided is zero we need to do
    # a resize for the amount we are writing. This will
    # be slower so setting a higher chunk size may
    # speed things up a bit.