From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mx1.redhat.com (mx1.redhat.com [172.16.48.31]) by int-mx1.corp.redhat.com (8.11.6/8.11.6) with ESMTP id i7CEAHa03334 for ; Thu, 12 Aug 2004 10:10:17 -0400 Received: from sbbmail1.sbb.ch (sbbmail1.sbb.ch [193.192.251.19]) by mx1.redhat.com (8.12.10/8.12.10) with ESMTP id i7CE9te1018097 for ; Thu, 12 Aug 2004 10:09:55 -0400 Received: from intmail1.sbb.ch (intmail1.sbb.ch [10.104.27.189]) by sbbmail1.sbb.ch (Switch-3.1.6/Switch-3.1.0) with ESMTP id i7CE9agK012796 (version=TLSv1/SSLv3 cipher=EDH-RSA-DES-CBC3-SHA bits=168 verify=FAIL) for ; Thu, 12 Aug 2004 16:09:36 +0200 Received: from gmx.net (i34390.sbb.ch [147.78.17.245]) by intmail1.sbb.ch (Switch-3.1.6/Switch-3.1.6) with ESMTP id i7CE9S6U002816 for ; Thu, 12 Aug 2004 16:09:36 +0200 Message-ID: <411B79CF.905E0C25@gmx.net> Date: Thu, 12 Aug 2004 16:08:15 +0200 From: Thomas Meller MIME-Version: 1.0 Subject: Re: [linux-lvm] SAN Storage/mirror/lvm References: <1092071992.3369.48.camel@localhost> Content-Transfer-Encoding: 7bit Reply-To: thomas.meller@gmx.net, LVM general discussion and development List-Id: LVM general discussion and development List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , List-Id: Content-Type: text/plain; charset="us-ascii" To: LVM general discussion and development Cott Lang wrote: > On the host, I see 4 SCSI devices, two valid, two invalid - I'm > guessing this is caused because only one storage processor exports > a particular LUN, so half the paths are invalid until a storage > processor fails over. > > sdd - SP A Path 1 > sde - SP B Path 1 > sdf - SP A Path 2 > sdg - SP B Path 2 > > I have a LUN currently on SP B, so I can use md multipath configured > with /dev/sde and /dev/sdg, and that seems to work. > > However, two things: > > 1) How do I set this up to handle a failover to SP A? > 2) Multipath only seems to work in failover mode, is there a way to > load balance across the two paths? > > Anyone have any hints? :) Hello Cott, your questions and the phenomena you see are likely to be specific to your hardware. I tried Emulex and QLogic HBA and found very different behaviour. Multipathing is handled by the storage systems differently from dumb disks. Load balancing over several physical paths is therefore a complicated task. Most vendors do not really support it. Some systems suffer from a Host switching paths. Unless you do not use a supported (read: expensive) Software you should keep your hands off. Even failover has a 'mechanical' quality: you can hear the door's angles squiek when it happens. I found my clusters diving after teardown of a storage's HBA. Failover while switching the physical path, i.e. switching the HBA takes at least 45 second on a (tuned) qlogic. Meanwhile the whole computer freezes. The driver seems to switch off interrupts or something. Have fun finding your solution. Thomas