[Linux-cluster] GFS, iSCSI, multipaths and RAID

Wendy Cheng s.wendy.cheng at gmail.com
Tue May 13 19:05:33 UTC 2008

Michael O'Sullivan wrote:
> Hi everyone,
> I have set up a small experimental network with a linux cluster and 
> SAN that I want to have high data availability. There are 2 servers 
> that I have put into a cluster using conga (thank you luci and ricci). 
> There are 2 storage devices, each consisting of a basic server with 2 
> x 1TB disks. The cluster servers and the storage devices each have 2 
> NICs and are connected using 2 gigabit ethernet switches.

It is a little bit hard to figure out the exact configuration based on 
this description (a diagram would help if you can). In general, I don't 
think GFS tuned well with iscsi, particularly the latency could spike if 
DLM traffic gets mingled with file data traffic, regardless your network 
bandwidth. However, I don't have enough data to support the speculation. 
It is also very application dependent. One key question is what kind of 
GFS applications you plan to dispatch in this environment ?

I see you have a SAN here .. Any reason to choose iscsi over FC ?

> I have created a single striped logical volume on each storage device 
> using the 2 disks (to try and speed up I/O on the volume). These 
> volumes (one on each storage device) are presented to the cluster 
> servers using iSCSI (on the cluster servers) and iSCSI target (on the 
> storage devices). Since there are multiple NICs on the storage devices 
> I have set up two iSCSI portals to each logical volume. I have then 
> used mdadm to ensure the volumes are accessible via multipath.

The iscsi target function is carried out by the storage device 
(firmware) or you use Linux's iscsi target ?
> Finally, since I want the storage devices to present the data in a 
> highly available way I have used mdadm to create a software raid-5 
> across the two multipathed volumes (I realise this is essentially 
> mirroring on the 2 storage devices but I am trying to set this up to 
> be extensible to extra storage devices). My next step is to present 
> the raid array (of the two multipathed volumes - one on each storage 
> device) as a GFS to the cluster servers to ensure that locking of 
> access to the data is handled properly.

So you're going to have CLVM built on top of software RAID ? .. This 
looks cumbersome. Again, a diagram could help people understand more.

-- Wendy
> I have recently read that multipathing is possible within GFS, but 
> raid is not (yet). Since I want the two storage devices in a raid-5 
> array and I am using iSCSI I'm not sure if I should try and use GFS to 
> do the multipathing. Also, being a linux/storage/clustering newbie I'm 
> not sure if my approach is the best thing to do. I want to make sure 
> that my system has no single point of failure that will make any of 
> the data inaccessible. I'm pretty sure our network design supports 
> this. I assume (if I configure it right) the cluster will ensure 
> services will keep going if one of the cluster servers goes down. Thus 
> the only weak point was the storage devices which I hope I have now 
> strengthened by essentially implementing network raid across iSCSI and 
> then presented as a single GFS.
> I would really appreciate comments/advice/constructive criticism as I 
> have really been learning much of this as I go.

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