Re: Shared Devices with XFS

[Frank Hellmann <frank@xxxxxxxxxxxxx>]

Hi!

see below...

Steve Lord wrote:
> Frank Hellmann wrote:
>
> > Hi Guys!
> >
> > Sorry to be a bit Off-Topic, but unfortunatly I can't test this out by 
> > myself...
> >
> > I am checking out a way to speed up data-transfers between a server 
> > and a couple of clients. A fullblown SAN (CXFS etc) approach is 
> > currently not in our budget, but we already have a couple of directly 
> > attached fibre-channel diskarrays here. I will buy a FC-switch to get 
> > the arrays easily switched over to the clients to speed up things 
> > (instead of copying over network).
> >
> > Now, my question: What will happen if I configure the FC-switch to 
> > route  a single diskarray to all machines, mount that device 
> > read-write on the server and read-only on the clients? Will XFS allow 
> > me to do that? How propable is non-integrity data on the clients 
> > (considering that the server won't write to that device at that time 
> > anymore)? What will happen if all clients strom out to get data at the 
> > same time?
>
>
> The software will not stop you from doing this, but DON'T!

;-)))) I knew you would say that...

> There is a good chance what will happen here is your read-only nodes
> will crash at some point, or at least decide the fs is corrupted and
> needs to be shutdown. The read only nodes will have a lot of
> metadata in cache, and will not know that it is stale and should be
> refreshed from disk. Once they get a mix of inconsistent metadata
> all bets are off. You will tend to see stale data as well, and
> things which the read/write node has put into the filesystem may
> not show up for a while.

So, there is no way of striktly working read-only of disk without 
caching/relying on the meta-data locally.

> Sharing a SAN filesystem between machines involves a lot more than
> just cabling up the storage ;-).

Right. Most of it has to do with loads of money... ;-)

> Shared read only on all nodes should be fine, if you want to
> update content, you have to unmount all but one, make it rw,
> update content, make it ro again then allow the other nodes
> to remount. That mode of operation does not fly for many applications.

Actually, this is fine for our application. We aquire a lot of hires 
50MB sized images that are not changed after that. The clients work on 
proxy images that they generate locally of these hires images. So during 
aquisition I can unmount all clients and do the r/w operations and after 
that mount it on all clients as read-only.

That is still better then putting the arrays directly on the single 
clients...

> Finally, if two nodes both need to read the same data at the same time,
> they issue scsi commands to the device, and one gets to wait for the
> other to complete - but at least the second one gets to use cache on
> the device.

:-)
                Thanks a lot Steve. Cheers,

                                Frank...

> Steve

--
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Frank Hellmann          Optical Art GmbH           Waterloohain 7a
Digital Cinema          http://www.opticalart.de   22769 Hamburg
frank@xxxxxxxxxxxxx     Tel: ++49 40 5111051       Fax: ++49 40 43169199