VMware Training

Eugeniu Patrascu eugen at imacandi.net
Thu Feb 20 18:47:55 UTC 2014

On Thu, Feb 20, 2014 at 8:16 PM, Jay Ashworth <jra at baylink.com> wrote:

> ----- Original Message -----
> > From: "Eugeniu Patrascu" <eugen at imacandi.net>
> > On Wed, Feb 19, 2014 at 10:06 PM, Jay Ashworth <jra at baylink.com>
> > wrote:
> >
> > > ----- Original Message -----
> > > My understanding of "cluster-aware filesystem" was "can be mounted at
> the
> > > physical block level by multiple operating system instances with
> complete
> > > safety". That seems to conflict with what you suggest, Eugeniu; am I
> > > missing something (as I often do)?
> >
> > What you are saying is true and from VMware's point of view, an ISCSI
> > volume is a physical disk.
> > And you can mount the same ISCSI disk on many VMware hosts. Just write
> > into different directories on the disk.
> >
> > Am I missing something in your question ?
> I guess.  You and Jimmy seem to be asserting that, in fact, you *cannot*
> mount a given physical volume, with a clustering FS in its partition,
> onto multiple running OS images at the same time... at which point, why
> bother using a clustering FS?
OK, let me give it another try:

You have a machine that exports an ISCSI disk (such as a SAN or a plain
Linux box).

You have 2-3-5-X machines (hosts) that run ESXi.

You can mount that ISCSI disk on all ESXi hosts at the same time and use it
as a datastore for VMs and run the VMs from there.

What I said (and maybe this caused some confusion) is that you should not
access the same files from different hosts at the same time, but you can
run VM1 on host1, VM2 on host2 and so on without any issues from the same
ISCSI target.

> The point of clustering FSs (like Gluster, say), as I understood it,
> was that they could be mounted by multiple machines simultaneously: that
> there was no presumed state between the physical blocks and the FS driver
> inside each OS, which would cause things to Fail Spectacularly if more
> than one machine was simultaneously using them in realtime.
In the scenario described above and how VMware ESXi works in general, only
VMware accesses the filesystem (the filesystem is called VMFS). The hard
drives for the virtual machines are actually represented by files on the
VMFS and so the virtual machines does not touch the filesystem on the ESXi
hosts directly.

> You and Jimmy seem to be suggesting that multiple OSs need to be
> semaphored.
He says that multiple ESXi hosts need to be semaphored when they update the
metadata on the VMFS.
I don't have any opinion on this as no matter how much I abused VMware, the
filesystem stayed intact.

> One of three understandings here is wrong.  :-)
I hope I cleared up some of the confusion.


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