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



Alex Kompel wrote:
On Mon, May 19, 2008 at 2:15 PM, Michael O'Sullivan
<michael osullivan auckland ac nz> wrote:
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.


So what do you want to get out of this configuration? iSCSI SAN, GFS
cluster or both? I don't see any reason for 2 additional servers
running GFS on top of iSCSI SAN.
There are advantages (for 2 additional storage servers) because serving data traffic over IP network has its own overhead(s). They offload CPU as well as memory consumption(s) away from GFS nodes. If done right, the setup could emulate high end SAN box using commodity hardware to provide low cost solutions. The issue here is how to find the right set of software subcomponents to build this configuration. I personally never use Linux iscsi target or multi-path md devices - so can't comment on their features and/or performance characteristics. I was hoping folks well versed in these Linux modules (software raid, dm multi-path, clvm raid level etc) could provide their comments. Check out linux-lvm and/or dm-devel mailing lists .. you may be able to find good links and/or ideas there, or even start to generate interesting discussions from scratch.

So, if this configuration will be used as a research project, I'm certainly interested to read the final report. Let us know what works and which one sucks.

If it is for a production system to store critical data, better to do more searches to see what are available in the market (to replace the components grouped inside the "iscsi-raid" box in your diagram - it is too complicated to isolate issues if problems popped up). There should be plenty of them out there (e.g. Netapp has offered iscsi SAN boxes with additional feature set such as failover, data de-duplication, backup, performance monitoring, etc). At the same time, it would be nice to have support group to call if things go wrong.

From GFS side, I learned from previous GFS-GNBD experiences that serving data from IP networks have its overhead and it is not as cheap as people would expect. The issue is further complicated by the newer Red Hat cluster infra-structure that also places non-trivial amount of workloads on the TCP/IP stacks. So separating these IP traffic(s) (cluster HA, data, and/or GFS node access by applications) should be a priority to make the whole setup works.

-- Wendy







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