Before you can increase memory in oracle database, you must first understand four oracle memory parameters which govern the instance:

  • SGA_TARGET and SGA_MAX_SIZE
  • MEMORY_TARGET and MEMORY_MAX_TARGET


SGA_TARGET and SGA_MAX_SIZE

Important: If you set MEMORY_TARGET, they oracle will perform AMM (automatic memory management) and both SGA + PGA are managed within the allocated memory. No need to touch SGA_TARGET / SGA_MAX_SIZE parameters.

Let us assume we have 10 GB physical RAM on a server. Which mean, at any given point we can max use 10 GB of RAM but not more than that. Also it does not mean that every-time we are using entire 10 GB of RAM. The same way SGA_TARGET defines the total size of SGA. SGA_MAX_SIZE define the total max value SGA_TARGET can take.

For example, server RAM is 10 GB, SGA_MAX_SIZE is 5 GB and SGA_TARGET is 3 GB. This means that I can max increase SGA_TARGET to 5 GB and not beyond that.

SGA_MAX_SIZE is not dynamic parameter but SGA_TARGET is.


MEMORY_TARGET and MEMORY_MAX_TARGET

From 11g onward, you do not need to manage SGA and PGA separately. You can allocate MEMORY_TARGET parameter and oracle will handle both SGA + PGA. You do not even need to set SGA_TARGET or SGA_MAX_SIZE.

Let us assume our previous example, we have 10 GB RAM on server. And we want to allocate 5 GB to Oracle. This can be simple done by setting MEMORY_TARGET to 5 GB. MEMORY_MAX_TARGET defines the maximum value MEMORY_TARGET can go.

Example, server RAM is 10 GB, MEMORY_MAX_TARGET is 7 GB and MEMORY_TARGET is 5 GB. This means I can max increase MEMORY_TARGET to 7 GB and not beyond that.

MEMORY_MAX_TARGET is not dynamic parameter but MEMORY_TARGET is

If MEMORY_TARGET is set to 5 GB, oracle will manage PGA + SGA withing 5 GB.


IF YOU SET MEMORY_TARGET

  • SGA_TARGET, SGA_MAX_SIZE and PGA_AGGREGATE_TARGET are set to 0, 60% of memory mentioned in MEMORY_TARGET is allocated to SGA and rest 40% is kept for PGA
  • SGA_TARGET and PGA_AGGREGATE_TARGET are set to non-zero values, these values will be considered minimum values.
  • SGA_TARGET is set to non zero value and PGA_AGGREGATE_TARGET is not set. Still these values will be autotuned and PGA_AGGREGATE_TARGET will be initialized with value of (MEMORY_TARGET-SGA_TARGET).
  • PGA_AGGREGATE_TARGET is set and SGA_TARGET is not set. Still both parameters will be autotunes. SGA_TARGET will be initialized to a value of (MEMORY_TARGET-PGA_AGGREGATE_TARGET).


HOW TO INCREASE MEMORY_TARGET IN ORACLE

SQL> show parameter target;
SQL> ALTER SYSTEM SET MEMORY_MAX_TARGET=3G SCOPE=SPFILE;
shut immediate;

Also make sure that /dev/shm is more than 3G otherwise you will get error at instance startup

df -h /dev/shm                  -> in my case, it was 2 GB. Increasing to 5 GB
mount -t tmpfs shmfs -o size=5G /dev/shm
df -h /dev/shm

Make /dev/shm value permanent in /etc/fstab file

vi /etc/fstab
change
tmpfs                   /dev/shm                tmpfs   defaults        0 0
to
tmpfs                   /dev/shm                tmpfs   size=5G        0 0

Start the database instance and increase MEMORY_TARGET

startup;
show parameter target;
ALTER SYSTEM SET MEMORY_TARGET=2560m;


In this activity, we have increased memory in oracle database by adding size to MEMORY_TARGET parameter.