On 02/26/2010 12:48 AM, romeotheriault wrote:
Hello, I'm trying to understand the benefits and/or use cases for
using ifaces with iscsi and am hoping someone can help me clarify
them.
Let's say I have a iscsi SAN with two nics, with different ip's, but
on the same subnet (10.10.2.1, 10.10.2.2). Both of the SAN's nics are
connected to different switches. Now I have a linux server with two
nics, with one of the nics connecting to one of the switches and the
other nic connecting to the other switch. Each of the servers nics
have different ip's (10.10.2.4, 10.10.2.5) but on the same subnet
again.
Now I setup iscsi connections to the SAN from the linux box to each of
the SAN's ip's/portal groups without using any iface config but I am
using multipathing.
What are the drawbacks or benefits to using ifaces or not using ifaces
in the above config?
If you do not use ifaces, then IO will be routed based on the route
table. So I think probably, IO would go through the same NIC on the
server. Is this what you are seeing? If you wanted to use dm-multipath
to round robin over both NICs on the linux server then you would use a
ifaces to bind each session to each NIC.
With your setup you could also accomplish this by just putting one
target and one server in one subnet and the other group in a different
one. The linux networking routing will take care of everything for you.
Are the two switches connected to each other? If they were and you are
using one subnet, you would have better redundancy. Above you have 2
paths to the target, but if the switches are connected you have 4 paths.
If the switches are not connected then if one switched dies you cannot
access that target portal, and then if you got really unlucky and the
other target portal died then you cannot access the target and you are
out of luck. If OTOH the switches were connected then you could still
access the other target portal with both server nics.
If not using ifaces and one of the switches or nics in the server go
down, will the multipathing know to switch to the other nic? even
though they are on the same subnet?
The network layer should figure out there is another NIC that can be
used and just use it. A problem might be while we are switching nics IO
could time out and both paths could be down if they both ended up using
the same nic due to the routing table. So you would want to setup
dm-multipath with a higher no_path_retry, because when you switch over
you might also have to relogin to the target through the new nic.
If you used ifaces then the failover should be smoother. The other path
would already be logged in, so dm-multipath could just restart the IO
right away.
Thank you for any help understanding this.
Romeo
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