On Mon, 2008-02-04 at 15:12 -0800, Nicholas A. Bellinger wrote: > On Mon, 2008-02-04 at 17:00 -0600, James Bottomley wrote: > > On Mon, 2008-02-04 at 22:43 +0000, Alan Cox wrote: > > > > better. So for example, I personally suspect that ATA-over-ethernet is way > > > > better than some crazy SCSI-over-TCP crap, but I'm biased for simple and > > > > low-level, and against those crazy SCSI people to begin with. > > > > > > Current ATAoE isn't. It can't support NCQ. A variant that did NCQ and IP > > > would probably trash iSCSI for latency if nothing else. > > > > Actually, there's also FCoE now ... which is essentially SCSI > > encapsulated in Fibre Channel Protocols (FCP) running over ethernet with > > Jumbo frames. It does the standard SCSI TCQ, so should answer all the > > latency pieces. Intel even has an implementation: > > > > http://www.open-fcoe.org/ > > > > I tend to prefer the low levels as well. The whole disadvantage for IP > > as regards iSCSI was the layers of protocols on top of it for > > addressing, authenticating, encrypting and finding any iSCSI device > > anywhere in the connected universe. > > Btw, while simple in-band discovery of iSCSI exists, the standards based > IP storage deployments (iSCSI and iFCP) use iSNS (RFC-4171) for > discovery and network fabric management, for things like sending state > change notifications when a particular network portal is going away so > that the initiator can bring up a different communication patch to a > different network portal, etc. > > > > > I tend to see loss of routing from operating at the MAC level to be a > > nicely justifiable tradeoff (most storage networks tend to be hubbed or > > switched anyway). Plus an ethernet MAC with jumbo frames is a large > > framed nearly lossless medium, which is practically what FCP is > > expecting. If you really have to connect large remote sites ... well > > that's what tunnelling bridges are for. > > > > Some of the points by Julo on the IPS TWG iSCSI vs. FCoE thread: > > * the network is limited in physical span and logical span (number > of switches) > * flow-control/congestion control is achieved with a mechanism > adequate for a limited span network (credits). The packet loss > rate is almost nil and that allows FCP to avoid using a > transport (end-to-end) layer > * FCP she switches are simple (addresses are local and the memory > requirements cam be limited through the credit mechanism) > * The credit mechanisms is highly unstable for large networks > (check switch vendors planning docs for the network diameter > limits) – the scaling argument > * Ethernet has no credit mechanism and any mechanism with a > similar effect increases the end point cost. Building a > transport layer in the protocol stack has always been the > preferred choice of the networking community – the community > argument > * The "performance penalty" of a complete protocol stack has > always been overstated (and overrated). Advances in protocol > stack implementation and finer tuning of the congestion control > mechanisms make conventional TCP/IP performing well even at 10 > Gb/s and over. Moreover the multicore processors that become > dominant on the computing scene have enough compute cycles > available to make any "offloading" possible as a mere code > restructuring exercise (see the stack reports from Intel, IBM > etc.) > * Building on a complete stack makes available a wealth of > operational and management mechanisms built over the years by > the networking community (routing, provisioning, security, > service location etc.) – the community argument > * Higher level storage access over an IP network is widely > available and having both block and file served over the same > connection with the same support and management structure is > compelling– the community argument > * Highly efficient networks are easy to build over IP with optimal > (shortest path) routing while Layer 2 networks use bridging and > are limited by the logical tree structure that bridges must > follow. The effort to combine routers and bridges (rbridges) is > promising to change that but it will take some time to finalize > (and we don't know exactly how it will operate). Untill then the > scale of Layer 2 network is going to seriously limited – the > scaling argument > Another data point from the "The "performance penalty of a complete protocol stack has always been overstated (and overrated)" bullet above: "As a side argument – a performance comparison made in 1998 showed SCSI over TCP (a predecessor of the later iSCSI) to perform better than FCP at 1Gbs for block sizes typical for OLTP (4-8KB). That was what convinced us to take the path that lead to iSCSI – and we used plain vanilla x86 servers with plain-vanilla NICs and Linux (with similar measurements conducted on Windows)." --nab - To unsubscribe from this list: send the line "unsubscribe linux-scsi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
