Oracle RDBMS 11gR2 – Flash Cache

I noticed yesterday a new feature that could have the potential to be a small, by me unnoticed, gem called "Flash Cache". I hope someone on the internet, that has tested it, will post his or hers experiences on this…

While using DBMS_METADATA:

select dbms_metadata.get_ddl('TABLE','MYDATA', user) from dual;

DBMS_METADATA.GET_DDL(‘TABLE’,’MYDATA’,USER)
——————————————————————————–

  CREATE TABLE "XMLDB_USER"."MYDATA"
   (    "ID" NUMBER(9,0),
        "DOC" "SYS"."XMLTYPE"
   ) SEGMENT CREATION IMMEDIATE
  PCTFREE 10 PCTUSED 40 INITRANS 1 MAXTRANS 255 NOCOMPRESS NOLOGGING
  STORAGE(INITIAL 65536 NEXT 1048576 MINEXTENTS 1 MAXEXTENTS 2147483645
  PCTINCREASE 0 FREELISTS 1 FREELIST GROUPS 1 BUFFER_POOL DEFAULT
   FLASH_CACHE DEFAULT
   CELL_FLASH_CACHE DEFAULT)
  TABLESPACE "XML_DATA"
  XMLTYPE COLUMN "DOC" STORE AS SECUREFILE BINARY XML  (
  TABLESPACE "XML_DATA" ENABLE STORAGE IN ROW CHUNK 8192
  NOCACHE NOLOGGING  NOCOMPRESS  KEEP_DUPLICATES
  STORAGE(INITIAL 106496 NEXT 1048576 MINEXTENTS 1 MAXEXTENTS 2147483645
  PCTINCREASE 0 BUFFER_POOL DEFAULT
   FLASH_CACHE DEFAULT
   CELL_FLASH_CACHE DEFAULT
  ))
  XMLSCHEMA "http://www.myserver.com/MySchema.xsd" ELEMENT "ROOT"
  DISALLOW NONSCHEMA

So I looked it up in the 11gR2 manuals:

storage_clause

STORAGE
({ INITIAL size_clause
 | NEXT size_clause
 | MINEXTENTS integer
 | MAXEXTENTS { integer | UNLIMITED }
 | maxsize_clause
 | PCTINCREASE integer
 | FREELISTS integer
 | FREELIST GROUPS integer
 | OPTIMAL [ size_clause | NULL ]
 | BUFFER_POOL { KEEP | RECYCLE | DEFAULT }
 | FLASH_CACHE { KEEP | NONE| DEFAULT }
 
…and via a search on the docs it says:

FLASH_CACHE

The FLASH_CACHE clause lets you specify how buffers for a schema object are to be cached in flash memory. In order to use this clause, a database flash cache must be enabled on your system. A flash cache is an extension of the database buffer cache that lives on a flash disk, which is a solid state storage device that uses flash memory.

• Without flash cache, the database re-uses each clean buffer in main memory as needed, overwriting it. If the overwritten buffer is needed later, then the database must read it from magnetic disk. 
• With flash cache, the database can write the body of a clean buffer to the flash cache, enabling reuse of its main memory buffer. The database keeps the buffer header in an LRU list in main memory to track the state and location of the buffer body in the flash cache. If this buffer is needed later, then the database can read it from the flash cache instead of from magnetic disk.

  KEEP
  
  Specify KEEP if you want buffers to be written to the flash cache when they are aged out of main memory, and remain in the flash cache as long as the flash cache size is large enough.

  NONE
  
  Specify NONE if you want buffers to be discarded and re-used when they are aged out of main memory. These buffers will not be put on the flash cache. This is the behavior if flash cache is not enabled on your system.

  DEFAULT
  
  Specify DEFAULT if you want buffers to be written to the flash cache when they are aged out of main memory, and then be aged out of the flash cache with the standard LRU algorithm. This is the default if flash cache is enabled and you do not specify KEEP or NONE.

When to Configure Flash Cache (Database Administrators Guide 11gR2)

Consider adding flash cache when all of the following are true:

• 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.

…on the database parameter side:

SQL> show parameter flash

NAME                                 TYPE        VALUE
------------------------------------ ----------- ------------------------------
db_flash_cache_file                  string
db_flash_cache_size                  big integer 0


db_flash_cache_file

Specifies the path and file name for the file to contain flash cache, in either the operating system file system or an Oracle Automatic Storage Management disk group. If the file does not exist, the database creates it during startup. The file must reside on a flash disk. If you configure flash cache on a disk drive (spindle), performance may suffer.

The following is an example of a valid value for db_flash_cache_file:  /dev/fioa1

db_flash_cache_size

Specifies the size of the flash cache. Must be less than or equal to the physical memory size of the flash disk device. Expressed as nG, indicating the number of gigabytes (GB). For example, to specify a 16 GB flash cache, set db_flash_cache_size to 16G.

You can use ALTER SYSTEM to set db_flash_cache_size to zero to disable flash cache. You can also use ALTER SYSTEM to set flash cache back to its original size to reenable it. However, dynamically changing the size of the flash cache is not supported.
 

Interesting indeed. I only need a server that has this hardware so I could test its effect…

😎