Oracle Database Smart Flash Cache

In case you don’t have budget to buy Exadata you can still buy huge number of flash disks and put on them part of your database. But what should be stored on flash disks(very fast) and what on magnetic disks(very slow) ?

It’s not your businesses to know let decide database 🙂


DB Smart Flash Cache is new extension for buffer cache area. This extra area should be defined on solid state disks (SSD) and has following features:

  • make performance improvements at moderate cost(cheaper than DRAM)
  • low latency compared to magnetic disks
  • higher throughput compared to magnetic disks
  • easy to setup
  • easy to control
  • can be used for RAC cache fusion keeps consistency
  • direct I/O bypasses buffer cache so as well bypasses DB smart flash cache
  • can cache only clean blocks from buffer cache
  • flash cache is not auto-tuned
  • only blocks from standard buffer pool are cached in DB smart flash cache

Oracle recommends:

flash disks should have comparable read IOPs and IOPs write
this new layer should be at least 2-10 times bigger than buffer cache in DRAM
mainly for OLTP systems



if a oracle server process needs to read a block from database at first it must read it from magnetic disk(physical read). Then the block is stored in buffer cache memory and added to standard “LRU chain” list.

When “in memory buffers area” is getting full Oracle must decide which blocks needs to be removed from cache. If you have DB Smart Flash Cache enabled “clean” blocks are written to “Flash cache” by DBWR process so next time they can be read into memory from Flash Cache and improve your performance.

NOTE: “Dirty” blocks are never stored in Flash Cache
List of blocks cached in DB smart flash cache are stored in buffer cache area on two dedicated flash “LRU lists” depending on object attribute FLASH_CACHE:

DEFAULT – standard last recently used algorithm decides how long such blocks are cached in flash cache. It’s default value assigned to each object in database.
KEEP – such blocks are not removed from flash cache as long as the flash cache is large enough

alter|create table|index objectname
buffer_pool { keep | recycle | default }
flash_cache { keep | none | default }

NONE value for FLASH_CACHE is blocking flash caching for a given object.


All I/O operations from DB smart flash cache are counted as physical I/O however Oracle also collects such informations in new columns.


select name from v$statname where name like 'physical%optimized%';

physical read requests optimized
physical read total bytes optimized

You can see such stats in V$SESSTAT and V$SYSSTAT


Two parameters must be set on database level to turn on DB smart flash cache:

DB_FLASH_CACHE_FILE – defines (OS disk path or ASM disk group) and file name to store this data
DB_FLASH_CACHE_SIZE – defines size of the flash cache

DB_FLASH_CACHE_FILE='/os path/flash_cache_file.dbf'

After setting both parameters you need to restart database.

can’t be shared between many databases or instances


can’t be dynamically resized
can be set to 0 to disable DB smart flash cache
can be set to original size to re-enable DB smart flash cache

Performance improvements

Oracle conducted interesting test for a OLTP database 70GB size with 8GB SGA.
From below picture you can see improvements for Transactions versus size of DB smart cache size.

Following picture shows improvement in transaction response time versus DB smart cache size



I simulate SSD disk by creation ramdisk – disk based in memory using following steps:

1. create directory to mount ramdisk and change owner to oracle and group dba

[root@oel5 /]mkdir /ramdisk
[root@oel5 /]chown oracle:dba -R /ramdisk

2. mount ramdisk and check it

[root@oel5 /]# mount -t tmpfs none /ramdisk -o size=256m
[root@oel5 /]# mount | grep ramdisk
none on /ramdisk type tmpfs (rw,size=256m)

3. set parameters for database and restart it as user oracle

SQL> alter system set db_flash_cache_file='/ramdisk/ram.dbf'
SQL> scope=spfile;

System altered.

SQL> alter system set db_flash_cache_size=200M scope=spfile;

System altered.

SQL> startup force
ORACLE instance started.

Total System Global Area 835104768 bytes
Fixed Size 2232960 bytes
Variable Size 507514240 bytes
Database Buffers 322961408 bytes
Redo Buffers 2396160 bytes
Database mounted.
Database opened.
SQL> show parameter flash_cache

———————– ———– ——————————
db_flash_cache_file string /ramdisk/ram.dbf
db_flash_cache_size big integer 200M

4. Check new file exists in /ramdisk

[root@oel5 ramdisk]# ll
total 8
-rw-r—– 1 oracle asmadmin 209715200 Feb 24 22:54 ram.dbf

5. Let’s create tables with flash_cache keep reference in storage clause so Oracle will try to keep the blocks in DB smart cache as long as possible.

create table test_tbl1
(id number,
id1 number,
id2 number)
storage(flash_cache keep);

for i in 1..1000000
insert into test_tbl1 values(i, i, i);
end loop;

6. Eventually after some time you can see some data in flash cache – v$bh view.

select status, count(*) from v$bh
group by status;

———- ———-
xcur 36915
flashcur 25583
cr 13

7. If you clean buffer cache as well db smart flash cache is purged

alter system flush buffer_cache;
system FLUSH altered.

———- ———-
xcur 520
free 36411

Have a fun 🙂

Oracle Database 11g Release 2 introduced a new database feature:
Database Smart Flash Cache. This feature is available on Solaris and
Oracle Enterprise Linux and allows customers to increase the effective size
of the Oracle database buffer cache without adding more main memory to the
system. For transaction-based workloads, Oracle database blocks are normally
loaded into a dedicated shared memory area in main memory called the System
Global Area (SGA). Database Smart Flash Cache allows the database buffer
cache to be expanded beyond the SGA in main memory to a second
level cache on flash memory. The Sun Storage F5100 Flash Array and the
Sun Flash Accelerator F20 PCIe Card provide a natural fit for Oracle Database
Smart Flash Cache and offer an excellent opportunity for end users to take
advantage of this new functionality. :)



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