Re: VMware Training

[Jay Ashworth <jra () baylink com>]

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 () gmail com> wrote:
> On Wed, Feb 19, 2014 at 2:06 PM, Jay Ashworth <jra () baylink com> wrote:
>
> > ----- Original Message -----
> > > From: "Eugeniu Patrascu" <eugen () 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.