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

Michael O'Sullivan michael.osullivan at auckland.ac.nz
Mon May 19 21:15:16 UTC 2008

Thanks for your response Wendy. Please see a diagram of the system at 
http://www.ndsg.net.nz/ndsg_cluster.jpg/view (or 
http://www.ndsg.net.nz/ndsg_cluster.jpg/image_view_fullscreen for the 
fullscreen view) that (I hope) explains the setup. We are not using FC 
as we are building the SAN with commodity components (the total cost of 
the system was less than NZ $9000). The SAN is designed to hold files 
for staff and students in our department, I'm not sure exactly what 
applications will use the GFS. We are using iscsi-target software 
although we may upgrade to using firmware in the future. We have used 
CLVM on top of software RAID, I agree there are many levels to this 
system, but I couldn't find the necessary is hardware/software to 
implement this in a simpler way. I am hoping the list may be helpful here.

What I wanted to do was the following:

Build a SAN from commodity hardware that has no single point of failure 
and acts like a single file system. The ethernet fabric provide two 
paths from each server to each storage device (hence two NICs on all the 
boxes). Each device contains a single logical disk (striped here across 
two disks for better performance, there is along story behind why we 
have two disks in each box). These devices (2+) are presented using 
iSCSI to 2 (or more) servers, but are put together in a RAID-5 
configuration so a single failure of a device will not interrupt access 
to the data.

I used iSCSI as we use ethernet for cost reasons. I used mdadm for 
multipath as I could not find another way to get the servers to see two 
iSCSI portals as a single device. I then used mdadm and raided the two 
iSCSI disks together to get the RAID-5 configuration I wanted. Finally I 
had to create a logical volume for the GFS system so that servers could 
properly access the network RAID array. I am more than happy to change 
this to make it more effective as long as:

1) It doesn't cost very much;
2) The no single point of failure property is maintained;
3) The servers see the SAN as a single entity (that way devices can be 
added and removed with a minimum of fuss).

Thanks again for any help/advice/suggestions. I am very new to 
implementing storage networks, so any help is great.

Regards, Mike

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