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</o:shapelayout></xml><![endif]--></head><body lang=EN-GB link=blue vlink=purple><div class=WordSection1><p class=MsoNormal><span style='color:#1F497D'>Peter,<o:p></o:p></span></p><p class=MsoNormal><span style='color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='color:#1F497D'>I’m not sure I understand the situation you’re trying to describe.<o:p></o:p></span></p><p class=MsoNormal><span style='color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='color:#1F497D'>Consider 2 servers (S1, S2) connected to 2 routers (R1, R2) on 1 LNET (N1)<o:p></o:p></span></p><p class=MsoNormal><span style='color:#1F497D'>and clients connect to the routers via another LNET (N2). Normally both R1<o:p></o:p></span></p><p class=MsoNormal><span style='color:#1F497D'>and R2 carry traffic between any/all clients on N2 and either server. <o:p></o:p></span></p><p class=MsoNormal><span style='color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='color:#1F497D'>If (say) R1 fails, clients on N2 will see communications failures when they<o:p></o:p></span></p><p class=MsoNormal><span style='color:#1F497D'>attempt to send to either of the servers via R1 and stop using it. Similarly, both<o:p></o:p></span></p><p class=MsoNormal><span style='color:#1F497D'>servers will see communications failures when they attempt to send to any client<o:p></o:p></span></p><p class=MsoNormal><span style='color:#1F497D'>via R1 and they too will stop using it. <o:p></o:p></span></p><p class=MsoNormal><span style='color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='color:#1F497D'>Meanwhile, clients will time out RPCs that were affected by the failure of R1<o:p></o:p></span></p><p class=MsoNormal><span style='color:#1F497D'>and try to reconnect – first using the affected OST’s current NID, then trying<o:p></o:p></span></p><p class=MsoNormal><span style='color:#1F497D'>the failover NID. When they successfully reconnect, they will find that S1’s<o:p></o:p></span></p><p class=MsoNormal><span style='color:#1F497D'>OSTs are still the “same ones” as before and therefore just resend the failed RPCs.<o:p></o:p></span></p><p class=MsoNormal><span style='color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='color:#1F497D'>LNET running on both clients and servers will continue to avoid routing traffic<o:p></o:p></span></p><p class=MsoNormal><span style='color:#1F497D'>through R1, however they will try to ping R1 occasionally so that they notice<o:p></o:p></span></p><p class=MsoNormal><span style='color:#1F497D'>when it comes back and can start to reuse it.<o:p></o:p></span></p><p class=MsoNormal><span style='color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='color:#1F497D'>If (say) S1 fails concurrently with R1, clients reconnecting after RPCs have<o:p></o:p></span></p><p class=MsoNormal><span style='color:#1F497D'>timed out will only reconnect successfully to the failover OST NIDs and<o:p></o:p></span></p><p class=MsoNormal><span style='color:#1F497D'>discover that they need to participate in recovery.<o:p></o:p></span></p><p class=MsoNormal><span style='color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='color:#1F497D'>For all this to work smoothly, we require (a) multiple routers between N1 and N2<o:p></o:p></span></p><p class=MsoNormal><span style='color:#1F497D'>to ensure communications between clients and servers can continue in the face of<o:p></o:p></span></p><p class=MsoNormal><span style='color:#1F497D'>router failures. We also need router failure to be detected relatively promptly to<o:p></o:p></span></p><p class=MsoNormal><span style='color:#1F497D'>minimize the number of reconnection attempts the clients make.<o:p></o:p></span></p><p class=MsoNormal><span style='color:#1F497D'><o:p> </o:p></span></p><blockquote style='margin-top:5.0pt;margin-bottom:5.0pt'><p class=MsoNormal><span style='color:#1F497D'>Cheers,<br> Eric <o:p></o:p></span></p></blockquote><p class=MsoNormal><span style='color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='color:#1F497D'><o:p> </o:p></span></p><div style='border:none;border-left:solid blue 1.5pt;padding:0cm 0cm 0cm 4.0pt'><div><div style='border:none;border-top:solid #B5C4DF 1.0pt;padding:3.0pt 0cm 0cm 0cm'><p class=MsoNormal><b><span lang=EN-US style='font-size:10.0pt;font-family:"Tahoma","sans-serif"'>From:</span></b><span lang=EN-US style='font-size:10.0pt;font-family:"Tahoma","sans-serif"'> lustre-devel-bounces@lists.lustre.org [mailto:lustre-devel-bounces@lists.lustre.org] <b>On Behalf Of </b>Peter Braam<br><b>Sent:</b> 27 September 2011 1:47 PM<br><b>To:</b> lustre-devel@lists.lustre.org<br><b>Subject:</b> [Lustre-devel] question about failover<o:p></o:p></span></p></div></div><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>Greetings -<o:p></o:p></p><div><p class=MsoNormal><o:p> </o:p></p></div><div><p class=MsoNormal>The general question is how do router failures and server failover interact?<o:p></o:p></p></div><div><p class=MsoNormal><o:p> </o:p></p></div><div><p class=MsoNormal>My suspicion is that is it necessary for the routing topology and server topology to be such that server failures one wants to recover from always leave working servers connected to the router, so that at least some traffic makes it through that router, and it won't be declared failed also. Is that right?<o:p></o:p></p></div><div><p class=MsoNormal><o:p> </o:p></p></div><div><p class=MsoNormal>As an example, point to point connections between two routers and a singe failover pair are to be avoided, because it becomes impossible to distinguish server and router failures. Is that a rule that is generally followed?<o:p></o:p></p></div><div><p class=MsoNormal><o:p> </o:p></p></div><div><p class=MsoNormal>Thanks!<o:p></o:p></p></div><div><p class=MsoNormal><o:p> </o:p></p></div><div><p class=MsoNormal>Peter<o:p></o:p></p></div><pre>______________________________________________________________________<o:p></o:p></pre><pre>This email may contain privileged or confidential information, which should only be used for the purpose for which it was sent by Xyratex. No further rights or licenses are granted to use such information. If you are not the intended recipient of this message, please notify the sender by return and delete it. 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