VMware Training

Thu Feb 20 09:50:22 UTC 2014

It means your VMs can run on any host and access the files it requires. If
this was not the case then you could not tolerate a hardware failure and
expect your VMs to survive. It also means you can do things like evacuate a
host and take it down for maintenance.

Of course you could build your application in a way that can tolerate the
failure of a host, and just use local storage for your guests. It really
depends on what you are trying to achieve though.

On 20 February 2014 03:46, Jay Ashworth <jra at baylink.com> wrote:

> Why bother with a clustering FS, then, if you cannot actually /use it/ as
> one?
> - jra
>
> On February 19, 2014 10:44:22 PM EST, Jimmy Hess <mysidia at gmail.com>
> wrote:
> >On Wed, Feb 19, 2014 at 2:06 PM, Jay Ashworth <jra at baylink.com> wrote:
> >
> >> ----- Original Message -----
> >> > From: "Eugeniu Patrascu" <eugen at imacandi.net>
> >> [snip]
> >> 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)?
> >>
> >
> >When one of the hosts has a virtual disk file open for write access on
> >a
> >VMFS cluster-aware filesystem,    it is locked to that particular host,
> > and  a process on a different host is denied the ability write to the
> >file,   or even open the file for read access.
> >
> >Another host cannot even read/write metadata about the file's directory
> >entry.
> >Attempts to do so,  get rejected with an error.
> >
> >So you don't really have to worry all that much about "as long you
> >don't
> >access the same files",  although: certainly you should not try to,
> >either.
> >
> >Only the software in ESXi can access the VMFS ---  there is no ability
> >to
> >run arbitrary applications.
> >
> >(Which is also, why I like NFS more than shared block storage; you can
> >conceptually use  the likes of  a storage array feature such as
> >FlexClone
> >to make    a copy-on-write clone of a file, take a storage level
> >snapshot,
> >and then do a granular restore of    a specific VM;  without  having to
> >restore the entire volume as a unit.
> >
> >You can't pull that off with a clustered filesystem on a block target!)
> >
> >
> >Also, the VMFS filesystem is cluster aware  by method of exclusion
> >(SCSI
> >Reservations) and separate journaling.
> >
> >Metadata locks are global  in the VMFS cluster-aware filesystem.  Only
> >one
> >host is allowed to write to
> >any of the metadata -on the entire volume a- time,   unless you have
> >VAAI
> >VMFS extensions, and your    storage vendor supports the ATS  (atomic
> >test
> >and set),
> >resulting in a performance bottleneck.
> >
> >For that reason,  while VMFS is cluster aware,  you cannot necessarily
> >have
> >a large number of cluster nodes,
> >or more than a few dozen open files,  before  performance degrades  due
> >to
> >the metadata bottleneck.
> >
> >
> >Another consideration is that;  in the event that you have a power
> >outage
> >which simultaneously  impacts your storage array and all your hosts:
> >you
> > may very well  be unable to  regain access to any of your files,
> >until the  specific host that had that file locked comes back up,   or
> >you
> >wait out a   ~30 to ~60 minute timeout period.
> >
> >
> >
> >
> >> Cheers,
> >> -- jra
> >>
> >--
> >-JH
>
> --
> Sent from my Android phone with K-9 Mail. Please excuse my brevity.
>