What is MemcacheDB?
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MemcacheDB is a distributed key-value storage system designed for persistent. It is NOT a cache solution, but a persistent storage engine for fast and reliable key-value based object storage and retrieval. It conforms to memcache protocol(not completed, see below), so any memcached client can have connectivity with it. MemcacheDB uses Berkeley DB as a storing backend, so lots of features including transaction and replication are supported. 

Features
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* High performance read/write for a key-value based object
* High reliable persistent storage with transaction
* High availability data storage with replication
* Memcache protocol compatibility

Supported memcache commands
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get(also mutiple get) 
set, add, replace
append/prepend
incr, decr
delete
stats

Private commands
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rget
db_checkpoint
db_archive
rep_set_priority
stats rep
stats repmgr
stats repms

Some Warning
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Expire time has been discarded in MemcacheDB(we are for persistent:p), so you should not use any corresponding features of clients. The daemon does nothing while you give a expire time to an item.

For more info, see: http://memcachedb.org