GFS multiple server access

[nickl_1]

Hi folks,

all the GFS docs I've found state that the GFS filesystem can be written to by more than one server at the same time - this seems brain dead to me, how do I only let one server at a time activate a VG using RHCluster and GFS ?

Many thanks,
Nick .

(ps I know this is a sglinux forum but I could not find a rhcluster forum here)

[Matti_Kurkela · Accepted solution]

Brain dead???

The entire point of a cluster-aware filesystem like GFS *is* that it can safely be accessed by multiple co-operating hosts at the same time. Making it possible has required quite a lot of brainwork.

See this whitepaper: it describes the setup and the advantages of using a cluster filesystem with an Oracle database. A similar reasoning is likely to apply to (some) other clusters, too.
http://www.redhat.com/whitepapers/rha/gfs/Oracle9i_RAC_GFS.pdf

If you need the VG active at only one host at a time, you don't need GFS.

Apparently the RHCluster is written to assume that with a GFS disk resource you won't need to make sure only one server at a time is using it - so it doesn't even try to enforce that.

I'm not very familiar with RHCluster, but a quick glance through the manual would indicate two possible approaches:

- enable the "Force Umount" checkbox in the configuration of your GFS resource. This would seem to unmount the GFS resource when a service goes down, but it would probably not enforce the one-host-at-a-time restriction. So the VG could be activated and mounted manually, if the server gets an OK for that from the DLM system.

- configure the resource as a "file system" resource instead of a "GFS" resource. This would seem to use DLM to implement the locking scheme you expect. However, I'm not sure if the tools will allow you to specify the file system type as GFS in this case. You may have to use ext3 or some other "traditional" file system instead.

MK

[nickl_1]

Aha I see - so it appears that GFS is only required if you want to allow several servers to access a filesystem at the same time ?

Having worked with hp HP SG I've never come across a situation where I'd want 2 or more servers having write access to a filesystem at the same time.

Thanks for your help ;)

Nick .

[nickl_1]

Thread closed.