From mboxrd@z Thu Jan 1 00:00:00 1970 From: James Bottomley Subject: Re: Is there a grand plan for FC failover? Date: 28 Jan 2004 10:57:29 -0600 Sender: linux-scsi-owner@vger.kernel.org Message-ID: <1075309052.2254.6.camel@mulgrave> References: <401521A7.5030808@thekelleys.org.uk> <1075131446.2290.29.camel@mulgrave> <20040128100236.D11527@vienna.EGENERA.COM> Mime-Version: 1.0 Content-Type: text/plain Content-Transfer-Encoding: 7bit Return-path: Received: from stat1.steeleye.com ([65.114.3.130]:63630 "EHLO hancock.sc.steeleye.com") by vger.kernel.org with ESMTP id S266090AbUA1Q5j (ORCPT ); Wed, 28 Jan 2004 11:57:39 -0500 In-Reply-To: <20040128100236.D11527@vienna.EGENERA.COM> List-Id: linux-scsi@vger.kernel.org To: "Philip R. Auld" Cc: Simon Kelley , SCSI Mailing List On Wed, 2004-01-28 at 09:02, Philip R. Auld wrote: > 1) load balancing when possible: it's not enough to be just a > failover mechanism. For first out, a failover target supplies most of the needs. Nothing prevents an aggregation target being added later, but failover is essential. > 2) requiring a userspace program to execute for failover is > problematic when it could be the root disk that needs > failing over. Userspace is required for *configuration* not failover. The path targets failover automatically as they see I/O down particular paths timing out or failing. That being said, nothing prevents async notifications via hotplug also triggering a failover (rather than having to wait for timeout etc) but it's not required. > 3) Handling partitions is a problem. It is? How? The block approach can sit either above or below partitions (assuming a slightly more flexible handling of partitions that dm provides). > I see multipath and md/RAID as two different animals. Multipathing is > multiple ways to reach the same physical block. That is it's under > the logical layer. While RAID is multiple ways to reach the same > logical block. It's basically many-to-one vs one-to-many. Having > multiple physical paths to a logical partition is a little > counter-intuitive. Nothing in the discussion assumed them to be similar. The notes were that md already had a multi-path target, and that translated error indications would be useful to software raid. Thus designing the fastfail to cater to both looks like a good idea, but that's just good design. Robust multi-pathing and raid will be built on top of scsi/block fastfail. James