when to use oracle database smart flash cache

How To Size the Database Smart Flash Cache [ID 1317950.1]


How to determine an optimal DB_FLASH_CACHE_SIZE for a PCI Express flash card for a 11gR2 database.


The Database Smart Flash Cache is a new feature in Oracle Database 11g Release 2 (11.2). The Database Smart Flash Cache is a transparent extension of the database buffer cache using solid state device (SSD) technology. The SSD acts as a Level 2 cache to the (Level 1) SGA.

Database Smart Flash Cache can greatly improve the performance of Oracle databases by reducing the amount of disk I/O at a much lower cost than adding an equivalent amount of RAM.

When to Configure the Flash Cache 

You can consider adding the flash cache when all of the following are true:
  • Your database is running on the Solaris or Oracle Linux operating systems.
    The flash cache is supported on these operating systems only.
  • The Buffer Pool Advisory section of your Automatic Workload Repository (AWR) report or STATSPACK report indicates that doubling the size of the buffer cache would be beneficial.
  • db file sequential read is a top wait event.
  • You have spare CPU.
You cannot share the flash cache among multiple instances.

Sizing the Flash Cache

As a general rule, size the flash cache to be between 2 times and 10 times the size of the buffer cache. Any multiplier less than two would not provide any benefit. If you are using automatic shared memory management, make the flash cache between 2 times and 10 times the size of SGA_TARGET. Using 80% of the size of SGA_TARGET instead of the full size would also suffice for this calculation.

When using the Flash Cache, a small amount of metadata about each block is kept in the buffer cache.
You must therefore take this extra memory requirement into account when adding the flash cache.

Information can be found in the following standard documentation:

Oracle® Database New Features Guide
11g Release 2 (11.2)
Section: Database Smart Flash Cache

Oracle® Database Administrator's Guide
11g Release 2 (11.2)
Part Number E17120-11
Section:  Configuring Database Smart Flash Cache

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