From: Lars E. <lar...@li...> - 2011-05-27 19:00:15
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On Fri, May 27, 2011 at 10:40:34PM +0400, George Shuklin wrote: > First, do not do two iSCSI targets on top of dual primary DRBD. It may > > look like a good idea at first, and unfortunately many people out there > > seem to think it is, but it does not work. > > > > The iscsi targets would need to be cluster aware, which they are not. > > Really: it won't work as you would like it to. > > > > Go for a failover solution with moving the target IP around. > > > > The rest, e.g. doing software raid5 over multiple iSCSI links, > > looks very fragile to me as well. > > > Good day. > > Can you say why iscsi (blockio) over two primary/primary DRBD with > multipath on initiator shall not work? I do some tests they works > extremly perfectly - I actually was amazed how smoothly it starts and > operates. > > PS clustering with IP sharing will reduce speed of solution - two iscsi > over two drbd will do twice read compare to cluster with single head. Think of what happens when you only lose the replication link. Did you take precautions for that case? Use fencing and stonith? Also, since the targets don't know of each other, they would happily give out concurrent reservations each. Same for any other "advanced" iSCSI command. Multipathing, whether MC/S or multi session, expects to have two paths to the same target, not one path each to independent targets that don't know of each other but happen to sit on top of data which looks identical - most of the time, until something bad happens. DRBD cannot magically make a non-cluster aware service cluster aware. Not even an iSCSI target. And if you still want to use it that way, because "it works for you", at least tell DRBD to do "fencing resource-and-stonith;", and get a working and tested fencing handler as well as stonith in place. If any part of the fencing depends on timeouts, make sure to chose them carefully ;-) Anyone with in-depth knowledge of iSCSI terminology, please add your take on why using independent targets for multipathing is a bad idea. -- : Lars Ellenberg : LINBIT | Your Way to High Availability : DRBD/HA support and consulting http://www.linbit.com DRBD® and LINBIT® are registered trademarks of LINBIT, Austria. |