[Linux-cluster] iscsi doubt

[Graham Wood]

On Mon, Apr 20, 2009 at 06:56:23PM +0200, ESGLinux wrote:
> Yes I agree with you, but I thought with iscsi i can do the same as with
> NFS.
No. iSCSI is a way of remotely getting access to a block device.  NFS is a way of accessing 
a network filesystem.  They server completely different purposes, and run at different parts 
of the "stack" as well.

> any suggestion that makes me see the light ;.)
As has already been stated, you cannot do it.

You cannot mount ext3 on two machines at the same time, and write data.  If you want to 
write from one at a time, don't mount it on the other.  It's THAT simple.

There is NO other answer with ext3.  If you want to have it mounted twice your choices are:
- NFS
- GFS/OCFS2/etc.

The standard reason for wanting to avoid GFS and the like is the complexity - unfortunately 
the complexity is required because (as previously stated) of cache coherency.

As for your example of NFS, I can tell you from experience that it doesn't work as well as 
you seem to think - it is still quite possible for NFS to get things wrong, and give you the 
wrong data - however, the protocol has been designed with that in mind, and you get 
"sensible" errors when it detects things.

We've got NFS servers that are accessed by 1000+ clients - and creating a file on one can 
take a few minutes to appear on the others, however the way NFS works means that this is 
done "safely", and it will recover.  Ext3 has NO functionality to do this.


So, in summary, "No".

Graham