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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