document usage and design of blockfile snapshotter

[deitch]

Based on this Slack discussion thread, I captured how to use the block file snapshotted, what its use case is, and how it works.

I am fairly certain that there are errors in here, but if we can capture this here and correct it, it will be known for future users and contributors, and might save people from having to answer the same questions over again on Slack 😄

Input to this is based on insights from @fuweid @samuelkarp @dcantah

There are 2 or 3 TODO comments here, where I either had open questions or was missing something. Help appreciate there.

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[dcantah]

Left some comments to start, thanks for this!!

[deitch]

Addressed all of your comments and raised some questions.

[deitch]

Filled in the TODO sections, removed the "Draft" status. I am sure there are errors, but it is ready to have them fixed.

[dmcgowan]

Looks good, in the last section it could be mentioned that if the underlying filesystem supports reflink (such as btrfs, xfs, apfs) then the layer copy is similarly as efficient storage wise as other snapshotters which support copy on write snapshotters. To consider as a follow up or if something else wants to play around with some examples there.

[dcantah]

Thanks for the quick fixups, LGTM!

[deitch]

I am not sure why the tests failed (and it blocked the merge). The entire PR is a document changes. 🤷‍♂️