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Chapter 17. Using Very Large Memory (VLM)
$ cat /etc/redhat-release
Red Hat Enterprise Linux AS release 4 (Nahant Update 2)
$ egrep "shm|tmpfs|ramfs" /proc/filesystems
nodev
tmpfs
nodev
ramfs
$
This means that if you try to mount a shmfs file system in Red Hat Enterprise Linux 4 or 5, you will get
the following error message:
mount: fs type shm not supported by kernel
The difference between shmfs and tmpfs is you do not need to specify the size of the file system if
you mount a tmpfs file system.
17.2. Configuring Very Large Memory (VLM)
The following example shows how to use the RAM disk ramfs to allocate 8 GB of shared memory
for the Oracle 10g database buffer cache on a 32 bit Red Hat Enterprise Linux 3, 4 or 5 systems
(hugemem kernel). If this setup is performed on a server that does not have enough RAM, then Linux
will appear to hang and the kernel will automatically start killing processes due to memory shortage
(ramfs is not swappable). Furthermore, ramfs is not backed by Huge Pages and therefore the Huge
Pages pool should not be increased for database buffers, see
are too many Huge Pages allocated, then there may not be enough memory for ramfs.
Since ramfs is not swappable, it is by default only usable by root. If you put too much on a ramfs file
system, you can easily hang the system. To mount the ramfs file system and to make it usable for the
Oracle account, execute:
# umount /dev/shm
# mount -t ramfs ramfs /dev/shm
# chown oracle:dba /dev/shm
When Oracle starts it will create a file in the /dev/shm directory that corresponds to the extended
buffer cache. Ensure to add the above lines to /etc/rc.local. If ointall is the primary group of
the Oracle account, use chown oracle:oinstall /dev/shm instead. For security reasons you
do not want to give anyone write access to the shared memory file system. Having write access to the
ramfs file system allows you to allocate and pin a large chunk of memory in RAM. In fact, you can kill
a machine by allocating too much memory in the ramfs file system.
To enable VLM, set the Oracle parameter use_indirect_data_buffers to true:
use_indirect_data_buffers=true
For 10g R1 and R2 databases it is important to convert DB_CACHE_SIZE and DB_xK_CACHE_SIZE
parameters to DB_BLOCK_BUFFERS, and to remove SGA_TARGET if set. Otherwise you will get errors
like these:
ORA-00385: cannot enable Very Large Memory with new buffer cache parameters
Here is an example for configuring a 8 GB buffer cache for a 10g R2 database with Red Hat
Enterprise Linux 3, 4 or 5 hugemem kernels:
60
Chapter 6, Swap
Space. In fact, if there
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