Re: [Xen-users] moving LV's devices to a SAN server.

[Javier Guerra <javier@xxxxxxxxxxx>]

On Mon, Nov 2, 2009 at 11:39 AM, Israel Garcia <igalvarez@xxxxxxxxx> wrote:
> Can you help me?

not much, unfortunately.  even if there are some standards, compliance
is spotty at best, so you'll have to test if your devices collaborate.

in any case, this was my reasoning for mentioning port aggregation (or
more precise, Link aggregation):

- the 'usual' topology for all things ethernet (including iSCSI), is
to simply put the switches at the middle and pull one cable to each
host.

- in a SAN, this creates a bottleneck since it's common to have just
one or two storage boxes for several hosts (specially when just
starting!).  The single Ethernet port going to the storage box limits
the total access bandwidth to just 1Gb for all hosts.

- most iSCSI devices currently include several (4-6) GbE ports.

- the naÃve way to use all these ports would be to ditch the Ethernet
switch, and just connect one host on each port.  This gives you 1Gb
dedicated for each host, and the total data bandwidth is limited to
the platter and internal backbone speeds.

- unfortunately, this strategy is too limiting for later growth.  Not
only you have a limited number of ports, but it also makes nearly
impossible to add a second storage box.

- so, what you can do is to keep the central switch, plug each host on
a single port of the switch; but for the storage box, use several
ports connected to several ports in the switch.  if the link
aggregation features of both the storage box and the switch match, now
you have a single very fat link between the box and the switch.  From
the point of view of the hosts, it's exactly the same as the 'usual'
topology (one device on each switch port); but a single host won't be
able to saturate the storage bandwith.

- expandability also isn't impaired, you can add extra hosts without
any change, and also extra storage just by creating extra link
aggregation groups.

hope it helps, at least in clarifying the general concepts.  for
details you'll have to consult the docs of both your storage box and
switches, and experiment a lot!

-- 
Javier

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