6.6 Configuring Database Smart Flash Cache

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The Database Smart Flash Cache feature is a transparent extension of the database buffer cache using solid state device (SSD) technology. 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 Database Smart Flash Cache

  • You should consider configuring Database Smart Flash Cache when certain conditions are met.
  • Consider adding Database Smart Flash Cache when all of the following conditions are true:
  • Your database is running on the Solaris or Oracle Linux operating systems. Database Smart 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.
Note: You cannot share one flash file among multiple instances. However, you can share a single flash device among multiple instances if you use a logical volume manager or similar tool to statically partition the flash device.

◉ Sizing Database Smart Flash Cache

As a general rule, size Database Smart 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 Database Smart 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.

◉ Tuning Memory for Database Smart Flash Cache

For each database block moved from the buffer cache to Database Smart Flash Cache, a small amount of metadata about the block is kept in the buffer cache.

For a single instance database, the metadata consumes approximately 100 bytes. For an Oracle Real Application Clusters (Oracle RAC) database, it is closer to 200 bytes. You must therefore take this extra memory requirement into account when adding Database Smart Flash Cache.

To tune memory for the Database Smart Flash Cache, complete one of the following actions:
  • If you are managing memory manually, then increase the size of the buffer cache by an amount approximately equal to the number of database blocks that fit into the Database Smart Flash Cache as configured, multiplied by 100 (or 200 for Oracle RAC).
  • If you are using automatic memory management, then increase the size of MEMORY_TARGET using the algorithm described above. You may first have to increase the size of MEMORY_MAX_TARGET.
  • If you are using automatic shared memory management, then increase the size of SGA_TARGET.
Also, for an Oracle RAC database that uses the flash cache, additional memory must be allocated to the shared pool for Global Cache Service (GCS) resources. Each GCS resource requires approximately 208 bytes in the shared pool.

Note: You can choose to not increase the buffer cache size to account for Database Smart Flash Cache. In this case, the effective size of the buffer cache is reduced. In some cases, you can offset this loss by using a larger Database Smart Flash Cache.

◉ Database Smart Flash Cache Initialization Parameters

You can use a set of initialization parameters to configure Database Smart Flash Cache.

Table 6-4 Database Smart Flash Cache Initialization Parameters

Parameter Description
DB_FLASH_CACHE_FILE Specifies a list of paths and file names for the files to contain Database Smart Flash Cache, in either the operating system file system or an Oracle Automatic Storage Management disk group. If a specified file does not exist, then the database creates it during startup. Each file must reside on a flash device. If you configure Database Smart Flash Cache on a disk drive (spindle), then performance may suffer. A maximum of 16 files is supported.
DB_FLASH_CACHE_SIZE Specifies the size of each file in your Database Smart Flash Cache. Each size corresponds with a file specified in DB_FLASH_CACHE_FILE. The files and sizes correspond in the order that they are specified. An error is raised if the number of specified sizes does not match the number of specified files.
Each size specification must be less than or equal to the physical memory size of its flash device. The size is expressed as nG, indicating the number of gigabytes (GB). For example, to specify a 16 GB Database Smart Flash Cache, set DB_FLASH_CACHE_SIZE value to 16G.

For example, assume that your Database Smart Flash Cache uses following flash devices:

File Size
/dev/sda 32G
/dev/sdb 32G
/dev/sdc 64G

You can set the initialization parameters to the following values:

DB_FLASH_CACHE_FILE = /dev/sda, /dev/sdb, /dev/sdc

You can query the V$FLASHFILESTAT view to determine the cumulative latency and read counts of each file and compute the average latency.

You can use ALTER SYSTEM to set DB_FLASH_CACHE_SIZE to zero for each flash device you wish to disable. You can also use ALTER SYSTEM to set the size for any disabled flash device back to its original size to reenable it. However, dynamically changing the size of Database Smart Flash Cache is not supported.

◉ Database Smart Flash Cache in an Oracle Real Applications Clusters Environment

Oracle recommends that you configure a Database Smart Flash Cache on either all or none of the instances in an Oracle Real Application Clusters environment. Also, the total flash cache size configured on each instance should be approximately the same.

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