add key comparisons

Author: alimanfoo

slides/v3-update-20190619.md

@@ -486,7 +486,7 @@ document. Store.*
 * Metadata documents are defined using the JSON type system (objects,
   arrays, strings, numbers).
 
-* Metadata documents are encoded for storage.
+* Metadata documents are serialised (encoded) for storage.
 
   * Default encoding is JSON, but protocol extensions can define other
     encodings for group and array metadata, see below.
@@ -870,18 +870,67 @@ Operation | Implementation
 
 ### Making hierarchy discovery more efficient
 
-* Current v3 resolves this by two means:
+* Current v3 resolves this by three means:
 
   1. Split the [storage key
      space](https://zarr-specs.readthedocs.io/en/core-protocol-v3.0-dev/protocol/core/v3.0.html#storage-keys),
      so all metadata keys have prefix "meta/" and all chunk keys have
      prefix "data/".
+ 
+  2. Change the format of metadata keys slightly (see below).
 
-  2. Leverage the fact that many stores, including cloud object
+  3. Leverage the fact that many stores, including cloud object
      stores, can support `list_prefix(prefix)` and/or
      `list_dir(prefix)` operations, which natively list all keys with
      a given prefix.
 
+===
+
+### v2/3 comparison - storage keys
+
+Bootstrap metadata ...
+
+Version | Storage key
+--- | ---
+v2 | N/A
+v3 | `zarr.json`
+
+===
+
+### v2/3 comparison - storage keys
+
+Array metadata, e.g., for array at path "/foo/bar" ...
+
+Version | Storage key
+--- | ---
+v2 | `foo/bar/.zarray`
+v3 | `meta/root/foo/bar.array`
+
+===
+
+### v2/3 comparison - storage keys
+
+Group metadata, e.g., for group at path "/foo/baz" ...
+
+Version | Storage key
+--- | ---
+v2 | `foo/baz/.zgroup`
+v3 | `meta/root/foo/baz.group`
+
+===
+
+### v2/3 comparison - storage keys
+
+Chunk data, e.g., for chunk at grid index (0, 0) in 2D array at path "/foo/bar" ...
+
+Version | Storage key
+--- | ---
+v2 | `foo/bar/0.0`
+v3 | `data/foo/bar/0.0`
+v3* | `data/foo/bar/0/0`
+
+<small>* chunk key format can be configured in array metadata</small>
+
 ====
 
 ### [Core protocol](https://zarr-specs.readthedocs.io/en/core-protocol-v3.0-dev/protocol/core/v3.0.html#protocol-operations) - examples