Expert Oracle Exadata - Parent Directory

C H A P T E R 5Exadata Smart Flash CacheThe marketing guys at Oracle must like the term “smart.” They have applied it to a dozen or so differentfeatures on the Exadata platform. They also seem to like the term “flash,” which is associated with atleast a half dozen features as well. To add to the confusion, there are two features in Oracle Database 11gRelease 2 that have almost exactly the same names, Database Smart Flash Cache (DBFC) and ExadataSmart Flash Cache (ESFC). While both features make use of flash-based memory devices, they are verydifferent. This chapter is focused on ESFC so we’ll only mention DBFC in passing.Cleary, one of the goals with Exadata V2 (and now X2) was to expand Exadata capabilities toimprove its performance with OLTP workloads. ESFC was the key component that was added to the V2configuration to accomplish this goal. The addition provides over 5TB of cache in a full rack. It’simportant to understand that this cache is managed by Oracle software that is aware of how the data isbeing used by the databases that the storage is supporting. Oracle has been working on software foreffectively managing database caches for over 30 years. Since the storage software knows what thedatabase is asking for, it has a much better idea of what should and shouldn’t be cached than aconventional storage array.DBFC VS. ESFCDBFC and ESFC are two completely different things. DBFC is an extension of the buffer cache on thedatabase server. It is a standard part of 11g and is implemented as a tier 2 buffer cache for an instance. Itis only supported on Solaris and Oracle Enterprise Linux. It is enabled by adding a flash card to a databaseserver and telling a single instance to use it. If a user needs a block that is not in the buffer cache, it willlook in the DBFC to see if it is there before requesting an I/O. When blocks are aged out of the buffer pool,they are moved to the DBFC instead of being simply flushed. ESFC is, of course, the disk cache on theExadata storage servers. It caches data for all instances that access the storage cell.HardwareEach Exadata storage server has 4 Sun Flash Accelerator F20 PCIe cards. Each card holds 96G for a totalof 384G on each storage server. These are the numbers that you normally see quoted in thespecifications although the amount that’s available for use is slightly less. The cards are actually madeup of 4 solid-state flash modules (FMods), sometimes called disk on modules (DOMs). Each of themodules contain eight 4GB SLC NAND components. So each of the 4 FMods has 32GB of storage, ofwhich 24GB is addressable. With 4 FMods on each of the 4 cards you have 16 of these modules in eachstorage server. The modules are presented to the O/S separately. Each module reserves an additional125