VMware Training

Thu Feb 20 03:46:22 UTC 2014

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

-- 
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