[Linux-cluster] RAW Devices performance issue

Fajar A. Nugraha list at fajar.net
Wed Feb 16 10:34:07 UTC 2011

On Wed, Feb 16, 2011 at 5:04 PM, Shariq Siddiqui
<shariq.siddiqui at yahoo.com> wrote:
> 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.

Shouldn't you ask Oracle about that?

My GUESS is that in the first one the I/O is buffered, while in the
second /dev/raw/raw* is simply a block device opened with O_DIRECT
(thus bypassing buffer cache). You may want to retry dd with
"oflags=direct" and compare the results.

You might want to look at

Again, better ask Oracle if you want to be sure.


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