External storage
From Wikitech
The external storage database cluster are MySQL servers storing page revision text. The main database only stores a link of the form DB://clusterx/blob_id, where x is the number of the cluster and blob_id is the key to the blob table.
External storage clusters are replicated, there's a master and several slaves. For clusters that are not written to, a failure of the master is not critical.
There are currently 5 external storage clusters in Tampa:
| Cluster | Master | Slaves | Cage/Rack |
|---|---|---|---|
| cluster1 | srv30 | srv28,srv29 | Core2,Core4,Core2 |
| cluster2 | srv27 | srv25,srv24 | all Core2 |
| cluster3 | srv34 | srv33,srv32,srv36 | Core5,Core5,Core5,Sec2 |
| cluster4 | srv73 | srv72,srv71 | all Sec2 |
| cluster5 | srv76 | srv75,srv74 | Sec2,unknown,Sec2 |
Currently, clusters 4 and 5 are used for storage of new revisions' text blobs, using a round robin load balancing.
Cluster3 has 4 nodes because it stores the text of many current revisions and has a high read load.
