Created Filer Redis Setup (markdown)

Filer-Redis-Setup.md

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+Currently there are two Redis implementations, i.g., `redis2` and `redis3`. What is the difference?
+
+# Storing directory list
+
+Since Redis is a key-value store, the directory list could not be efficiently iterated by scanning the keys. So the directory list is stored as a [sorted set](https://redis.io/topics/data-types#sorted-sets) in `redis2`:
+```
+ <directory_path>: <sorted set of names>
+```
+
+The sorted set in Redis is fairly efficient. Storing 100K ~ 1 million entries is still fairly fast. In most of the cases, your use case should work very well with a directory this size.
+
+# Super Large Directories
+
+In some cases, the directory may have 10s of millions or billions of files or sub directories. The directory list would be too big to fit into one Redis entry. For `redis2`, you can specify which folder is super large, and avoid storing this fat list of names, but giving up the directory listing capability.
+
+This is where `redis3` can help. The internal data structure is:
+
+```
+ <directory_path>: <skip list>
+ <skip list item 1>: <sorted set>
+ <skip list item 2>: <sorted set>
+ ...
+```
+The directory list is stored as a skip list, and the child names are spread into the list items.
+
+This prevents each Redis entry from being too large and slower to access.