HBase Random Read Performance

[Joost Ouwerkerk]

I'm working on a web application with primarily read-oriented 
performance requirements.  I've been running some benchmarking tests 
that include our application layer, to get a sense of what is possible 
with Hbase.  A variation on the Bigtable test that is reproduced by 
org.apache.hadoop.hbase.PerformanceEvaluation, I'm randomly reading 1 
column from a table with 1 million rows.  In our case, the contents of 
that column need to be deserialized by our application (which adds some 
overhead that I'm also trying to measure), the deserialized contents 
represent a little over 1K of data. 

Although a single thread can only achieve 125 reads per second, with 12 
client threads (from 3 different machines) I'm able to read as many as 
500 objects per second.  Now, I've replicated my test on a basic MySQL 
table and am able to get a throughput of 2,300 reads/sec; roughly 5 
times what I'm seeing with Hbase.  Besides the obvious code maturity 
thing, is the discrepancy related to random reads not actually being 
served from memcache, but rather from the disk, by Hbase?  The HBase 
performance page 
(http://wiki.apache.org/hadoop/Hbase/PerformanceEvaluation) shows random 
reads(mem) as "Not implemented."

Can anyone shed some light on the state of HBase's memcaching?
Cheers,
Joost.