On Wed, 2008-09-10 at 19:42 -0400, Maurice Volaski wrote: > >On Wed, 2008-09-10 at 19:10 -0400, Maurice Volaski wrote: > >> >On Wed, 2008-09-10 at 18:37 -0400, Maurice Volaski wrote: > >> >> >On Wed, 2008-09-10 at 15:00 -0400, Maurice Volaski wrote: > >> >> >> >On Wed, 2008-09-10 at 14:36 -0400, Maurice Volaski wrote: > >> >> >> >> A disadvantage, however, is that Sun StorageTek > >>Availability Suite > >> >> >> >> (AVS), the DRBD equivalent in OpenSolaris, is much less > >> >>flexible than > >> >> >> >> DRBD. For example, AVS is intended to replicate in one > >>direction, > >> >> >> >> from a primary to a secondary, whereas DRBD can switch > >>on the fly. > >> >> >> >> See > >> >> >> >> > >>http://www.opensolaris.org/jive/thread.jspa?threadID=68881&tstart=30 > >> >> >> >> for details on this. > >> >> >> > > >> >> >> >I would be curious to see production environments > >>"switching" direction > >> >> >> >on the fly at that low level... Usually some top-level > >>brain does that > >> >> >> >in context of HA fail-over and so on. > >> >> >> > >> >> >> By switching on the fly, I mean if the primary services are taken > >> >> >> down and then brought up on the secondary, the direction of > >> >> >> synchronization gets reversed. That's not possible with > >>AVS because... > >> >> >> > >> >> >> >well, AVS actually does reverse synchronization and does > >>it very good. > >> >> >> > >> >> >> It's a one-time operation that "re-reverses" once it completes. > >> >> > > >> >> >When primary is repaired you want to have it on-line and retain the > >> >> >changes made on the secondary. > >> >> > >> >> Not necessarily. Even when the primary is ready to go back into > >> >> service, I may not want to revert to it for one reason or another. > >> >> That means I am without a live mirror because AVS' realtime mirroring > >> >> is only one direction, primary to secondary. > >> > > >> >This why I tried to state that this is not realistic environment for > >> >non-shared storage HA deployments. > >> > >> What's not realistic? DRBD's highly flexible ability to switch roles > >> on the fly is a huge advantage over AVS. But this is not to say AVS > >> is not realistic. It's just a limitation. > >> > >> >DRBD trying to emulate shared-storage > >> >behavior at a wrong level where in fact usage of FC/iSCSI-connected > >> >storage needs to be considered. > >> > >> This makes no sense to me. We're talking about mirroring the storage > >> of two physical and independent systems. How did the concept of > >> "shared storage" get in here? > > > >This is really outside of ZFS discussion now... But your point taken. If > >you want mirror-like behavior of your 2-node cluster, you'll get some > >benefits of DRBD but my point is that such solution trying to solve two > >problems at the same time: replication and availability, which is in my > >opinion plain wrong. > > Uh, no, DRBD addresses only replication. Linux-HA (aka Heartbeat) > address availability. They can be an integrated solution and are to > some degree intended that way, so I have no idea where your opinion > is coming from.
Because in my opinion DRBD takes some responsibility of management layer if you will. Classic, predominant replication in HA clusters schema is primary-backup (or master-slave) and backup by definition is not necessary primary-identical system. Having said that, it is noble for DRBD to implement role switching and not a bad idea for many small deployments. > For replication, OpenSolaris is largely limited to using AVS, whose > functionality is limited, at least relative to DRBD. But there seems > to be a few options to implement availability, which should include > Linux-HA itself as it should run on OpenSolaris! Everything is implementable and I believe AVS designers thought about dynamic switching of roles, but they end up with what we have today, they likely discarded this idea. AVS not switching roles and forces IT admins to use it as primary-backup data protection service only. > But relevant to the poster's initial question, ZFS is so far and away > more advanced than any Linux filesystem can even dream about that it > handily nullifies any disadvantage in having to run AVS. Right. _______________________________________________ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
