Database Reference
In-Depth Information
ASM Disks
The ASM disk is the basic element of the Oracle ASM storage structure. It forms the Oracle ASM diskgroup. An ASM
disk is usually based on a piece of a storage device such as a physical disk, or a storage volume like the LUN of a
storage array. Depending on how the storage is presented to the RAC node, an ASM disk can be built on a block device
or a partition of block device, network-attached files, or a pseudo-device.
As stated in the preceding “Multipath Device Configuration” section, in an Oracle RAC environment that has
multiple I/O paths to the external shared SAN storage, the same LUN in the SAN storage is presented as multiple
devices on a RAC node, for example /dev/sdc and /dev/sde. These devices point to the same storage volume and have
the same SCSI ID. We should not take these devices as different ASM disks; otherwise, Oracle ASM will find these ASM
disks for the same storage volume and report an error. We should also not use just one device name, such as /dev/sdc
as for an ASM disk, because the ASM disk based on this single I/O path device will not take advantage of the multiple
I/O paths to this storage volume. A highly recommended method is to use the multipath pseudo-device of this storage
volume for the ASM disk. Although Oracle ASM itself doesn't provide a way to implement multipathing, Oracle ASM
can work with the OS multipath utility by using the multipath pseudo-device for the ASM disk. This method ensures
that the Oracle ASM disk can benefit from load balancing as well as high availability against the single point of failure
on the I/O paths from the RAC node to the storage array.
In order for an ASM instance to use a storage volume for the ASM disk, the ASM instance needs to discover the
storage volume. To help an ASM discover a storage volume, a proper value may need to be set for the ASM instance
initialization parameter ASM_DISKSTRING with a pattern to match the device name that represents the storage
volume. For example, to discover a storage volume that is represented by a Linux multipath pseudo-device with
a name pattern like /dev/mapper/, the ASM instance initialization parameter can be set as
ASM_DISKSTRING =/dev/mapper/*.
Another requirement is ownership of the storage devices. For example, in the Linux environment, all the devices
are owned by the root user by default. In the previous section entitled “Set Ownership of the Devices,” I discussed the
method that uses the Linux udev rule utility to change the owner of the devices from root to the owner of the ASM
instance such as the grid user. The following will discuss how to use ASMLib to create ASM disks with the proper
ownership setting.
ASMLib: What and Why
For Linux platforms, especially Oracle Linux and Red Hat Enterprise Linux, Oracle introduced a support library called
Oracle ASMLib that provides an alternative interface to access disks and gives the Oracle Database more efficient
and capable access to the disk groups. Notice that since ASMLib is an optional tool, use of ASMLib is not required for
you to use ASM for your Oracle Database. ASMLib provides a simplified method to meet the two requirements for
preparing storage devices for ASM disks:
•
Ensure the name consistency of a storage volume across all the RAC nodes
•
Set the proper ownership and permissions on the disk devices that are used for ASM disks
ASMLib can achieve both requirements. If you use ASMLib in your RAC environment, you don't have to set the
udev rules. Table
5-2
. compares these three methods.
Table 5-2.
Comparison of Multipathing, udev Rules, and ASMLib
Multipathing Software
udev Rules
ASMLib
Device Name Consistency
Yes
No
Yes
Device Ownership Setting
No*
Yes
Yes
* As for Red Hat Enterprise Linux 6.x, the uid and gid parameters in multipath.conf file that
were previously used to set the ownership of devices have been deprecated. The ownership of
the device is set by means of udev rules.
