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RE: [Xen-users] AoE (Was: iscsi vs nfs for xen VMs)

  • To: Simon Hobson <linux@xxxxxxxxxxxxxxxx>, <xen-users@xxxxxxxxxxxxxxxxxxx>
  • From: Jeff Sturm <jeff.sturm@xxxxxxxxxx>
  • Date: Thu, 27 Jan 2011 19:24:21 -0500
  • Cc:
  • Delivery-date: Thu, 27 Jan 2011 16:25:16 -0800
  • List-id: Xen user discussion <xen-users.lists.xensource.com>
  • Thread-index: Acu+WMr2tQZQbonIR7yDiDxjOkHxWwAJKPYA
  • Thread-topic: [Xen-users] AoE (Was: iscsi vs nfs for xen VMs)

> -----Original Message-----
> From: xen-users-bounces@xxxxxxxxxxxxxxxxxxx [mailto:xen-users-
> bounces@xxxxxxxxxxxxxxxxxxx] On Behalf Of Simon Hobson
> I must admit, AoE does seem to have it's upsides - in past threads
(here and
> elsewhere) I've only ever seen it being criticised.

Many of those threads seem to delve into performance claims, which isn't
very helpful in order to objectively compare the protocols.   I don't
frankly care that either of iSCSI or AoE is more efficient than the
other by a few percent on the wire--if your storage implementation
depends on such a small margin to determine success or failure, think
very carefully about your tolerances.  You'd better give yourself more
headroom than that.

Although the reality is complex, the basic truth is that networks are
fast and (non-SSD) disks are slow.  On sequential performance, a good
disk will have more bandwidth than a single GigE link, but under any
sort of random I/O the disk latency dominates all others and network
performance is marginalized.  And you can forget about relying on the
performance of sequential I/O in any large application cluster with e.g.
tens of nodes and central storage.

The real benefit of AoE that seems to get lost on its detractors is its
simplicity.  The protocol specification is brief and the drivers are
easy to install and manage.  The protocol supports self-discovery (via
broadcast) so that once you connect your initiator to your targets and
bring your Ethernet interface up, device nodes just appear and you can
immediately use them exactly as you would local devices.  Multipath over
AoE can be as easy as connecting two or more Ethernet interfaces rather
than one--the new transports will be discovered and utilized with zero
incremental configuration provided your targets and initiators support
it, as the commercial ones I use do.

The supposed benefits of iSCSI, which include security and routeability,
and meaningless to me.  Whether I use iSCSI or not I would never let my
storage network touch any of our general networks.  I want my storage
connected to my hosts over the shortest path possible, if not with
crossover cables, then with a dedicated switch.  AoE is not inherently
more or less secure than a SAS cable, and shouldn't be, since you need
to physically secure your storage regardless of interconnects.  For me
the security features of iSCSI only add to the complexity and overhead
inherent in the protocol.


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