Created Async Filer Metadata Backup (markdown)

Async-Filer-Metadata-Backup.md

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+It is desirable to have a copy of the filer metadata. However, it is not always easy to setup replications for filer metadata store:
+* The store does support stream replication.
+* The store is embedded leveldb, RocksDB, etc.
+* It is just complicated.
+
+There are two ways to ensure a copy of the meta data.
+
+# Separate Filer instances connected with "-peers" option
+
+With `-peers` option, if the filers are not sharing the same filer metadata store, the metadata changes are asynchronously propagated to all peers.
+
+See https://github.com/chrislusf/seaweedfs/wiki/Filer-Store-Replication#file-store-replication
+
+# Async filer metadata backup to a remote store
+
+We can also continuously backup filer metadata to a remote store, without running a filer instance.
+```
+         Metadata Changes
+ Filer --------------------> `weed filer.meta.backup` ----> Remote Store(LevelDB/RocksDB/
+                                                                      Mysql/Postgres/
+                                                                      Redis/Cassanra/...)
+```
+Just need to configure the remote store the same way as `filer.toml`.
+
+`weed filer.meta.backup` can be stopped and resumed. The metadata backup progress is tracked in the remote store itself. So you can pause/resume any time, and even resume from a separate machine.