VMware Training

Thu Feb 20 18:48:50 UTC 2014

On Wed, Feb 19, 2014 at 9:46 PM, Jay Ashworth <jra at baylink.com> wrote:

> Why bother with a clustering FS, then, if you cannot actually /use it/ as
> one?
>

It is used as one.    It is also a lot more convenient to have a shared
filesystem,  than a distributed volume manager.
You could think of  VMDK files on a VMFS volume as  their alternative to a
Clustered Linux LVM.
Just because you have some sort of clustered volume manager,  doesn't make
your  guest operating systems cluster-aware.

With VMFS... if two guest operating systems  try to open the same disk,
 hypothetically, the most likely reason would be
that there is a split brain in your HA cluster,   and two hosts are trying
to startup the same VM.

The locking restrictions are for your own protection. If the filesystem
inside your virtual disks is not a clustered filesystem;
two instances of a VM  simultaneously  mounting the same NTFS volume and
writing some things, is an absolute disaster.

Under normal circumstances,  two applications should never be writing to
the same file.  This is true on clustered filesystems.
This is true when running multiple applications on a single computer.

There is such a thing as 'Shared disk mode':  if your block target is Fibre
Channel,  and  you are using Microsoft Cluster Services in VMs,
but  an extra virtual SCSI adapter on a VM in shared mode,  is something
that  you have to explicitly configure  (It's not the default).

There are many good things to be said about having a single-purpose
filesystem, which can be placed on a shared storage device --- which
multiple hosts can mount simultaneously, and view the same file/folder
structure, and touch resources corresponding
to which applications are running on that node...

That  does not require cluster votes and majority nodeset Quorums to keep
the cluster from totally going out,
and it does not need  questionable techniques such as STONITH ("Shoot the
other node in the head")  as a fencing strategy,
for providing exclusive access to resources.

Different hosts can access files corresponding to resources running on that
host,  and HA is able to fail virtual disks over,
so   the   Highly-specialized filesystem achieves  all the objectives,
 that it needs to be a clustered filesystem to solve.

> - jra
>
--
-JH