Dear All,

I am going to install Oracle RAC on two Servers, With shared SAN storage (Servers and Storage is IBM)
OS = RHEL 5u5 x64 bit

And we used multipathing mechanism and created multipathing devices.
i.e. /dev/mapper/mpath1.

Then I created raw device /dev/raw/raw1 of this /dev/mapper/mpath1 Block device as per pre-reqs for Oracle Cluster.

Every thing looks good, But we faced the performance issue as under...

when we run command :
#dd if=/dev/zero of=/dev/mapper/mpath1 bs=1024 count=1000
the writing rate is approx. 34 MB/s

But If we run command
#dd if=/dev/zero of=/dev/raw/raw1 bs=1024 count=1000
the writing rate is very slow like 253 KB/s

Please advice how to tune the performance.

raw by definition does not buffer - it goes straight to the device. The original block device however buffers so is actually writing to memory then to disk.

Oracle DB (including RAC) doesn't use disk buffering but instead creates an SGA which is its own memory space so will give better performance than you're seeing doing a dd as it works similar to the way disk buffering does.

If this is a new installation you might consider using OCFS2 instead of raw devices. Back when I set up Oracle RAC I went through much of Oracle's documentation and saw it mostly recommended raw/ASM but then after I was all done I found a recommendation at one Oracle link to use OCFS2 instead as raw devices are deprecated in Linux (meaning they may go away in future). We have been running on raw since 2007 but if I'd found that note first I'd have gone with OCFS2. In an earlier RAC environment we had OCFS which was the predecessor to OCFS2 - my reading suggests OCFS2 is much better than OCFS was. You can get OCFS2 from Oracle (OCFS = Oracle Cluster File System).