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 <mysi...@gmail.com> wrote: >On Wed, Feb 19, 2014 at 2:06 PM, Jay Ashworth <j...@baylink.com> wrote: > >> ----- Original Message ----- >> > From: "Eugeniu Patrascu" <eu...@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.
