What Is an Overlay Filesystem?
An overlay filesystem merges several directory layers into one logical view, with lower layers read-only and an upper layer capturing all writes. Reads fall through to whichever layer holds the file, while writes and deletions are recorded only in the upper layer. This is the mechanism behind container image layering.
Why it matters
Container images are built from stacked overlay layers, which is why shared base layers can be cached and reused across CI jobs. Understanding the writable top layer explains why container changes vanish unless committed or mounted out.
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