CPUMiner - Do I also need to download Bitcoin client and 15gb chain? Just out of curiosity, I downloaded cpuminer, signed up at a mining pool site and started the miner.  It seems to be doing something.

[2014-01-22 22:51:09] 1 miner threads started, using 'sha256d' algorithm.
[2014-01-22 22:51:09] Starting Stratum on stratum+tcp://stratum.bitcoin.cz:3333 
[2014-01-22 22:51:10] Stratum detected new block
[2014-01-22 22:51:12] thread 0: 2097152 hashes, 1600 khash/s
[2014-01-22 22:51:42] Stratum detected new block
[2014-01-22 22:51:46] thread 0: 56289640 hashes, 1652 khash/s
[2014-01-22 22:52:47] thread 0: 99121136 hashes, 1624 khash/s
[2014-01-22 22:53:48] thread 0: 97418224 hashes, 1599 khash/s
[2014-01-22 22:54:40] thread 0: 95913336 hashes, 1821 khash/s


I was under the impression that mining could not be done without first downloading the whole chain (that takes about 3 days to download).  Is cpuminer really doing something or just showing me some fake data?
Mining by yourself requires that you have the whole blockchain. However, when you choose to mine as part of a pool, the pool operator maintains the full blockchain and just sends parcels of work out to the miners. (Of course, in this arrangement you have to trust the pool operator.)