Cluster File System Definition

Cluster File System Definition

[UZAIR LAKHANI]

Hello All,

I am getting some problems understanding the actual
idea behind Cluster File System implementation. From
one website 
http://www.stalker.com/notes/SFS.html#CFS
I got the idea that a cluster file system is a wrapper
file system. Since this cluster file system is a
wrapper file system or in other words this wrapper fs
will direct the requests to an actual fs like ext3
(the stacking concept), so we don't need a format
utility for this cluster file system which is actually
a wrapper fs and not an actual fs.

On the other hand OpenGFS 
http://opengfs.sourceforge.net/
which is also a cluster file system has its own format
utility and disk layout.

Now what is exactly the definition of a cluster file
system. I need some help understading the differences.

Thanks,
Uzair Lakhani,
Karachi, Pakistan.




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Re: Cluster File System Definition

[Bryan Henderson]

>I am getting some problems understanding the actual
>idea behind Cluster File System implementation. From
>one website 
>http://www.stalker.com/notes/SFS.html#CFS
>I got the idea that a cluster file system is a wrapper
>file system.
>...
>On the other hand OpenGFS 
>http://opengfs.sourceforge.net/
>which is also a cluster file system has its own format
>utility and disk layout.
>
>Now what is exactly the definition of a cluster file
>system.

Maybe you missed the word "usually," where that web page says that a 
cluster file system is usually a wrapper around some other filesystem.  I 
can't comment on whether "usually" is a fair description, but at least it 
resolves your inconsistency.

The definition of a cluster file system is quite simple:  A file system 
appropriate for use in a cluster.  A cluster is a bunch of computers 
working on the same problem (usually in order to solve it faster than a 
single computer could).  The thing about a cluster that is incompatible 
with many file systems is that because the computers are working on the 
same problem, they probably need to read and write the same files, at the 
same time.  Something like ext3 doesn't allow multiple computers to have 
the same filesystem mounted at the same time (unless read-only).

A "shared" filesystem _can_ be mounted on multiple computers at the same 
time, but may not be a cluster file system because it doesn't perform 
acceptably when multiple computers are accessing (especially writing) to 
the same file at the same time.  NFS, for example, is a shared filesystem 
type that usually isn't fast enough to satisfy a cluster.

So sometimes, the cheapest way to make a cluster filesystem is to wrap 
something around a non-cluster filesystem, but sometimes you want to build 
a filesystem for clustering from the ground up.

While we're talking filesystem class definitions, I'll throw in another 
that you often hear along with cluster and shared:  distributed 
filesystem.  While I've seen it used to mean "shared," what it really 
means is that the filesystem, or at least the entity through which you 
access it, is spread across multiple computers.  For example, two 
computers might each own a disk and the two disks together form a 
filesystem.  Or maybe the disks don't belong to any particular computer, 
but the locks that coordinate access to them are spread across multiple 
computers.

It is in the nature of cluster computing for a cluster to use a 
distributed filesystem (i.e. the filesystem itself is clustered), but it 
isn't required.

--
Bryan Henderson                     IBM Almaden Research Center
San Jose CA                         Filesystems