Expert Oracle Exadata - Parent Directory

CHAPTER 2 OFFLOADING / SMART SCANA GENERIC VERSION OF EXADATA?The topic of building a generic version of Exadata comes up frequently. The idea is to build a hardwareplatform that in some way mimics Exadata, presumably at a lower cost than what Oracle charges forExadata. Of course, the focus of these proposals is to replicate the hardware part of Exadata, because thesoftware component cannot be replicated. (This realization alone should make you stop and questionwhether this approach is even feasible.) Nevertheless, the idea of building your own Exadata soundsattractive because the individual hardware components can be purchased for less than the package priceOracle charges. There are a few flaws with this thinking, however:1. The hardware component that tends to get the most attention is the flash cache.You can buy a SAN or NAS with a large cache. The middle-size Exadata package(1/2 rack) supplies around 2.5 Terabytes of flash cache across the storageservers. That’s a pretty big number, but what’s cached is as important as the sizeof the cache itself. Exadata is smart enough not to cache data that is unlikely tobenefit from caching. For example, it is not helpful to cache mirror copies ofblocks, since Oracle only reads primary copies (unless a corruption is detected).Oracle has a long history of writing software to manage caches. So it should comeas no surprise that it does a very good job of not flushing everything out when alarge table scan is processed so that frequently accessed blocks would tend toremain in the cache. The result of this database-aware caching is that a normalSAN or NAS would need a much larger cache to compete with Exadata’s flashcache. Keep in mind also that the volume of data you will need to store will bemuch larger on non-Exadata storage because you won’t be able to use HybridColumnar Compression.2. The more important aspect of the hardware, which oddly enough is occasionallyoverlooked by the DIY proposals, is the throughput between the storage anddatabase tiers. The Exadata hardware stack provides a more balanced pathwaybetween storage and database servers than most current implementations. So thesecond area of focus is generally the bandwidth between the tiers. Increasing theeffective throughput between the tiers is not as simple as it sounds, though.Exadata provides the increased throughput via InfiniBand and the ReliableDatagram Sockets (RDS) protocol. Oracle developed the iDB protocol to run acrossthe Infiniband network. The iDB protocol is not available to databases running onnon-Exadata hardware. Therefore, some other means for increasing bandwidthbetween the tiers is necessary. So you can use IPOB on a 10Ge network and useiSCSI or NFS, or you can use high-speed fiber-based connections. In any case youwill need multiple interface cards in the servers (which will need to be attached viaa fast bus). The storage device (or devices) will also have to be capable ofdelivering enough output to match the pipe and consumption capabilities (this iswhat Oracle means when they talk about a balanced configuration). You’ll alsohave to decide which hardware components to use and test the whole thing tomake sure that all the various parts you pick work well together without having amajor bottleneck at any point in the path from disk to database server.26