Flash on Flash

Why exactly did Oracle create the Flash Cache concept? Well, generally speaking flash is going to be faster than disk for access of data, so, putting a bit of flash into your server and then using it as an L2 cache for the database cache makes sense. In this test we aren’t dealing with the Exadata Cell based flash cache, but the DB, server based flash cache.

But what happens when disk isn’t the storage media? Let’s look at a flash-on-flash test case using a RamSan630 flash solid state storage appliance and a RamSan 70 PCIe server mounted flash card.



Figure 1: Test Configuration

The flash cache was sized at the lower end of the suggested 2X to 10X the database cache size (90 gb) and then a run with the flash cache set to zero was run. Note that for the first run appropriate tables and indexes were assigned to be kept in the flash cache, other tables where set to default. Figure 2 shows the results from use of the Smart Flash Cache with Flash as storage.



Figure 2: TPS verses GB in the Flash cache

At least for our testing with the database on a RamSan630 SSD and the flash cache being placed on a RamSan70 PCIe card, the results are not encouraging towards the use of the flash cache with a flash based SAN. Review of the AWR results showed that the flash cache was indeed being used but, due to the small difference in overall latency between the RS630 with IB interfaces and the RS70 in the PCIe slot, the overall effect of the flash cache was negligible. According to AWR results when the flash cache was set to zero the predominate wait event was the db file sequential read, when the flash cache was set to 90 gb, the db flash cache single block physical read event dominated the report, thus showing that the cache was in fact being used.

These results demonstrate that for a database system that is based on high-speed flash storage, the DB flash cache will not be needed.