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Any time you run a virtual setup with many similar images, you probably ask the question, "Why do I have copies of everything?" This applies to virtual desktops and web servers, but when containers takeoff, you'll see the true scope of this problem.
The issues that this particular form of data sprawl creates are not just limited to using up extra space. High copy counts make updates time-consuming and error-prone -- especially if the copies are in multiple folders -- and loading images into servers and clients uses much more bandwidth. This can bring services to their knees, as the infamous boot storm exemplifies.
Image cloning could be the solution to the data sprawl problem, but not all NAS systems support it.
Using image cloning, the solution is to have a single copy of an image that can be shared by all its consumers. That's the essence of a containers approach, where removing redundant loading of the identical operating system stack allows virtual machine (VM) counts to treble. With fewer images, the cost and risk of updates goes way down.
Image cloning methods vary in implementation. Deduplication of objects achieves that single image goal by replacing all the extra images with pointers to a specific object. As long as the system allows it, changing that object occurs with a single file swap.
Adding complexity to image cloning
Things become more complex when the clone is a set of files -- a complete image. Now, deletion and addition of files at a folder level has to be accomplished, which typically falls outside the scope of deduplication code. Deleting one copy deletes a pointer, leaving the single copy and all the other pointers intact.
True image cloning requires a mechanism to create copies of a single set of folders and to then link them using the clone software to generate pointers for images for perhaps thousands of virtual desktops or other VMs. Updating these clones can be done by building the new image and then changing all the pointers to reference the new data. This is a very fast operation, so any period where some of the clones are out of date is very short.
A single copy on HDD would be easily overloaded, so a good image cloning process uses a fast SSD or PCI Express flash drive to hold the copy. Network performance is a crucial factor in such a configuration, especially in a boot storm scenario.
Since not all NAS software products support cloning, if it looks like it fits your needs, it is worth asking vendors if their NAS has image cloning capability or another service that simplifies image management.
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