[Linux-cluster] RAW Devices performance issue

[Stefan Lesicnik]

----- Original Message -----
> From: "Shariq Siddiqui" <shariq.siddiqui at yahoo.com>
> To: linux4oracle at yahoogroups.com, linux-cluster at redhat.com
> Sent: Wednesday, 16 February, 2011 12:04:00 PM
> Subject: [Linux-cluster] RAW Devices performance issue
> 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.

Hi,

I dont know anything about using raw devices, but I do know the write speed through the multipath device for the SAN is slow.

Try fix that performance first - check SAN cache write is enabled, check your raid levels and over how many disks.
I cant say what you should get, but i've seen local non raided disks write much faster

Stefan