We're VMDK for everything else can excluding support issues from vendors.
Like you, I initially assumed there was substantial overhead using VMDKs vs
RDM/direct mapping but about a year ago I read that same white paper and it
changed my mind. Since then I have provisioned everything using VMDKs and
are phasing out the old RDMs. In my testing using IOmeter I could see
absolutely no performance difference between one or the other.
The whole abstraction layer operates basically in basically a pass-through
mode so long as you don't have alignment issues, but you can get in to a
situation of degraded performance if you don't clean up old snapshots.
And don't forget to put your non-boot volumes on db or high-load servers on
a paravirtual HBA instead of the emulated LSI/whatever the other one is. I
wouldn't bother doing it for your boot volume unless for some reason that is
heavily loaded, but that configuration is supported now for most guest OS's.
On Tue, 10 Jan 2012 13:15:16 -0500, Steve Bogdanski <[log in to unmask]>
wrote:
>Hello,
>I recently had a discussion on another email list about this topic and was
wondering if anyone in the MSU community was interested in chiming in.
>
>The general argument, from other list, was that with Vsphere 4.x and 5.x
that you should use VMDKs for data storage in your VMs in 99% of all
situations. I had always been under the assumption that VMDKs were slower
than directly connecting to the same back-end storage from the guest VM's
OS. This was even more true when the data was I/O heavy (as in DB and email).
>
>After hearing this, and being told I was crazy for not using (and taking
advantage of) VMDKs, I decided to do a little more research on the topic. I
did find this white paper from VMware,
http://www.vmware.com/files/pdf/vmfs_rdm_perf.pdf, which concludes that
there is no real performance benefit between VMDKs and RDMs. Since the
reasons for using RDMs now is very small (outside of applications that
require low-level disk access), I can see how it would make sense to choose
VMDKs. The next step in reasoning would be that if there is no benefit in
using RDMs, then you'd likely also not see any benefit in using direct
connection via the guest VM (using iSCSI initiator).
>
>So I am curious what the experience of others on campus has been and what
they usually go with. Is there any benefit, performance or otherwise, for
still connecting to back-end storage directly from a guest VM?
>
>
>-Steve Bogdanski
>
>
>
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