[CentOS-virt] KVM virtual machine and SAN storage with FC

Fri Jul 5 14:56:09 UTC 2013
Gene <gh5046 at gmail.com>

On Thu, Jul 4, 2013 at 10:25 PM, denis bahati <djbahati at yahoo.co.uk> wrote:
> Hi Team,
> Thanks for the good explanation.
> If that is not workable for the database, can anyone recommend me for the
> setup of the database clients and data files in order to achieve HA and load
> balancing? How should I set up my VMs and stations (Two machines with two
> VMs each)? I will appreciate for a workable approach and that is practical
> for the HA/Load balancing.

It depends on what database product you're using.  If it's Oracle
Database Server it's designed to work with shared devices/file
systems.  You shouldn't have a problem running it in an active/active
(load balanced) configuration.  If it's MySQL/MariaDB the best you can
hope for, as far as I know, is an active/passive configuration with

I'm guessing you're using MySQL.  Make your database highly available
in an active/passive configuration with replication and use some sort
of failover (heartbeat, carp, etc) or a network load balancer.
Depending on your application you can still run it in a active/active
(load balanced) configuration.

> Regards
> ________________________________
> From: Nico Kadel-Garcia <nkadel at gmail.com>
> To: denis bahati <djbahati at yahoo.co.uk>; Discussion about the virtualization
> on CentOS <centos-virt at centos.org>
> Cc: "brett at worth.id.au" <brett at worth.id.au>
> Sent: Thursday, 4 July 2013, 18:32
> Subject: Re: [CentOS-virt] KVM virtual machine and SAN storage with FC
> On Thu, Jul 4, 2013 at 12:44 AM, denis bahati <djbahati at yahoo.co.uk> wrote:
>> Hi Brett,
>> On my plan is as follows:
>> I have two machine (Server) that will host two VM each. One for database
>> and
>> one for application. Then the two machine will provide (Load Balance and
>> High availability). My intention is that all application files and data
>> file
>> for the database should reside on the SAN storage for easy access and
>> update.
> Don't... do this. Two database clients writing to the same database
> filesystem back ends, simultaneously, is an enormous source of excited
> sounding flow charts and proposals which simply do not work and are
> very, very likely to corrupt your database beyond recover. These
> problems have been examined, for *decades* with shared home
> directories and saved email and for high performance or clustered
> databases that need to not have "split brain" skew, It Does Not Work.
> Set up a proper database *cluster* with distinct back ends.
>> Therefore the storage should be accessible to both VMs through mounting
>> the
>> SAN storage to the VMs. The connection between SAN storage and the servers
>> is through Fiber Channel.
> Survey says *bzzzt*. See above for databases. For shared storage, you
> should really be using some sort of network based access to a
> filesystem back end. NetApp and EMC spend *billions* in research
> building high availability shared storage, and even they don't pull
> stunts like this the last I looked. I can vaguely imagine one of the
> hosts doing write access and the other having read-only access. But
> really, most databases today support good clustering configurations
> that avoid precisely these issues.
>> I have seen somewhere talking about DM-Multipath but i dont know if this
>> can
>> help or the use of VT-d if can help. I will also appreciate if you provide
>> some links to give me insight of how to do this.
> Multipath does not mean "multiple clients of the same hardware
> storage". That's effectively like letting two kernels write to the
> same actual disk at the same time, and it's quite dangerous.
> Now, if you want each client to access their own fiber channel disk
> resource, that should be workable. Even if you have to mount the fiber
> channel resources on the KVM host, and make disk images for the KVM
> guest, that should at least get you a testable resource. But the
> normal approach is have a fiber channel storage server that makes disk
> images available via NFS, so that the guest VM's can be migrated from
> one server to another with the shared storage more safely.
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