Further adventures with base images in OpenStack

    I was bored over the New Years weekend, so I figured I'd have a go at implementing image cache management as discussed previously. I actually have an implementation of about 75% of that blueprint now, but its not ready for prime time yet. The point of this post is more to document some stuff I learnt about VM startup along the way so I don't forget it later.

    So, you want to start a VM on a compute node. Once the scheduler has selected a node to run the VM on, the next step is the compute instance on that machine starting the VM up. First the specified disk image is fetched from your image service (in my case glance), and placed in a temporary location on disk. If the image is already a raw image, it is then renamed to the correct name in the instances/_base directory. If it isn't a raw image then it is converted to raw format, and that converted file is put in the right place. Optionally, the image can be extended to a specified size as part of this process.

    Then, depending on if you have copy on write (COW) images turned on or not, either a COW version of the file is created inside the instances/$instance/ directory, or the file from _base is copied to instances/$instance.

    This has a side effect that had me confused for a bunch of time yesterday -- the checksums, and even file sizes, stored in glance are not reliable indicators of base image corruption. Most of my confusion was because image files in glance are immutable, so how come they differed from what's on disk? The other problem was that the images I was using on my development machine were raw images, and checksums did work. It was only when I moved to a slightly more complicated environment that I had enough data to work out what was happening.

    We therefore have a problem for that blueprint. We can't use the checksums from glance as a reliable indicator of if something has gone wrong with the base image. I need to come up with something nicer. What this probably means for the first cut of the code is that checksums will only be verified for raw images which weren't extended, but I haven't written that code yet.

    So, there we go.

    Tags for this post: openstack cloud computing nova glance qemu image management canonical sre image_cache_management
    Related posts: Reflecting on Essex; Folsom Dev Summit sessions; Wow, qemu-img is fast; Juno nova mid-cycle meetup summary: slots; A first pass at glance replication; One week of Nova Kilo specifications

posted at: 23:13 | path: /openstack | permanent link to this entry