Chapter 13. Configuring I/O for Raw Partitions

Raw partitions allow Oracle to bypass the OS cache. A raw device can be assigned or bound to block devices such as disk or disk partitions. When a raw device is bound to a disk, any read or write access to the raw device will perform a raw I/O with the disk. A raw I/O through the /dev/raw interface bypasses the kernel's block buffer cache entirely that is normally associated with block devices and goes right to the low level device itself. By bypassing the cache it accesses a physical device directly which allows applications such as Oracle databases to have more control over the I/O to the physical device. In fact, Oracle does it is own data caching and raw devices allow Oracle to ensure that data gets written to the disk immediately without OS caching.

Since Automatic Storage Management (ASM) is the recommended option for large amounts of storage in RAC environments, the focus of this article and section is on the usage of raw devices and block devices for ASM. ASM offers many advantages over conventional file systems. The ASM file system is not buffered and supports asynchronous I/O. It allows you to group sets of physical disks to logical entities as disk groups. You can add or remove disks without downtime. In fact, you could move a whole database from one SAN storage to another SAN without downtime. ASM spreads I/O over all the available disks automatically to avoid hot spots. ASM does also it is own striping and offers mirroring. ASM can be setup using the ASM library driver or raw devices. Starting with 10g R2, neither is necessarily required.

alias scsi_hostadapter1 aic7xxx

alias scsi_hostadapter1 aic7xxx

alias scsi_hostadapter2 lpfc

alias scsi_hostadapter2 lpfc