Great question—this is one of the most confusing parts of S3 because it doesn’t use actual folders, just a flat object store. Let’s break this down clearly:

Core S3 Truth: No Real Folders, Just Object Keys #

First, remember: S3 stores everything as objects with unique keys (like a/a1.txt or 2.txt). There’s no hierarchical folder structure under the hood—what you see as "folders" in the S3 console is just a visual trick to make navigation feel familiar.

What Exactly Is a Prefix? #

A prefix is a logical grouping term, not a physical object. It’s any leading part of an object’s key that you use to filter or organize objects. For example:

• For the object a/a1.txt, valid prefixes include a, a/, or even a/a (if you had another object like a/a2.txt).
• For b/b1.txt, prefixes could be b, b/, etc.
• For 2.txt, the only prefix is the empty string (or the key itself, since there’s no / to split on).

The key thing here: prefixes don’t need to correspond to any actual stored object. They’re just a way to reference all objects whose keys start with that specific string.

Are Console "Folders" (Objects Ending with /) Prefixes? #

No—they’re not the same thing. Here’s why:
When you manually create a "folder" in the S3 console, S3 automatically creates a zero-size object with a key ending in / (like a/). This is just a placeholder object to make the console display a folder icon.

But the prefix a/ exists regardless of whether that placeholder object exists. Even if you delete the a/ placeholder, all objects with keys starting with a/ (like a/a1.txt) will still be grouped under the logical prefix a/ when you list objects with that filter.

Your Example in Action #

Take your three objects: a/a1.txt, b/b1.txt, 2.txt

• The logical prefix a/ applies to a/a1.txt (no need for an a/ placeholder object)
• The logical prefix b/ applies to b/b1.txt
• 2.txt falls under the root prefix (empty string)

If you created a/ and b/ placeholder objects via the console, those are just extra objects—they don’t define the prefixes; the object keys do.

How This Relates to S3 Performance #

S3’s performance limits (3500 PUT/COPY/POST/DELETE per prefix per second, 5500 GET/HEAD per prefix per second) apply to these logical prefixes, not the placeholder folder objects. So if you split your objects across 10 distinct logical prefixes (e.g., prefix1/, prefix2/, ..., prefix10/), you can scale to 55,000 GET requests per second—regardless of whether you created placeholder objects for those prefixes.

Key Takeaways #

• Prefixes are logical, not physical—they’re based on object key patterns, not stored objects.
• Console "folders" are just zero-size placeholder objects with keys ending in /—they’re a visual aid, not the same as prefixes.
• You can use any leading substring of an object key as a prefix for filtering or scaling performance.

内容的提问来源于stack exchange，提问作者laxman