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

[Simon Hobson <linux@xxxxxxxxxxxxxxxx>]

Jeff Sturm wrote:

> That said, the aoe protocol also supports an asynchronous write
> operation, which I suppose really is "fire and forget", unlike normal
> reads and writes.  I haven't used an aoe driver that implements
> asynchronous writes however, and I'm not sure I would if I had the
> option since you have no guarantee that the writes succeed.
Agreed, I can't see many uses. However there are some applications 
(such as video capture for instance) where it may be better to miss 
an occasional block than to suffer the overhead of error correction. 
I know someone who works for a surveying outfit where they drive 
around with vans (a bit like the Google camera cars) recording video 
of roads etc - the video is analysed later by the client to spot 
things like broken street lights, potholes, or whatever it is they 
are looking for. IN a situation like this, it's better to have a 
glitch in the video than potentially lose a big chunk because the 
system pauses to correct an error. As it is, they use SDLT tape (I 
think) because it's cheaper* than spinning disk and more suitable for 
the streaming data streams they are writing.
* Presumably at the time the decision was made, I suspect that may 
have changed now.
James Harper wrote:

> I use DRBD locally and used to regularly see messages about concurrent
> outstanding requests to the same sector. DRBD logs this because it can't
> guarantee the serialising of requests so two write requests to the same
> sector might be reordered at any layer different between the two
> servers. It sounds like AoE would make this even worse if the 'first'
> write was lost resulting in the 'second' write being performed first
> followed by the 'first' write.
Bear in mind that with modern disks it is normal for them to have 
command queuing and reordering built in. So unless you specifically 
turn it off, your carefully ordered writes may be re-ordered by teh 
drive itself.
Jeff Sturm wrote:

>  > 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.
Thanks, that's a useful insight.

--
Simon Hobson

Visit http://www.magpiesnestpublishing.co.uk/ for books by acclaimed
author Gladys Hobson. Novels - poetry - short stories - ideal as
Christmas stocking fillers. Some available as e-books.

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