Note: "permalinks" may not be as permanent as we would like,
direct links of old sources may well be a few messages off.
On Tue, Dec 13, 2011 at 04:25:18PM -0500, John Lauro wrote: > IMHO, the web site should make it a little clearer that protocol B is not > safe for active/active even when there is no failure, as from the As you also quoted already: > > > >>> "in dual primary mode, only protocol C is allowed. Does not get much clearer than that, does it. You can not use protocol b for dual primary. DRBD will not allow you to do so. If it does, tell me, as that would be a bug, and needs to be fixed. > description it could (wrongly) be assumed that in memory buffers are in > sync on both nodes: > > Protocol B. Memory synchronous (semi-synchronous) replication protocol. > Local write operations on the primary node are considered completed as > soon as the local disk write has occurred, and the replication packet has > reached the peer node. Normally, no writes are lost in case of forced > fail-over. However, in the event of simultaneous power failure on both > nodes and concurrent, irreversible destruction of the primary's data > store, the most recent writes completed on the primary may be lost. > > > > > -----Original Message----- > > From: drbd-user-bounces at lists.linbit.com [mailto:drbd-user- > > bounces at lists.linbit.com] On Behalf Of Lars Ellenberg > > Sent: Tuesday, December 13, 2011 4:14 PM > > To: drbd-user at lists.linbit.com > > Subject: Re: [DRBD-user] protocol B in dual-primary mode > > > > On Tue, Dec 13, 2011 at 09:11:25AM -0700, Marcus Sorensen wrote: > > > On Tue, Dec 13, 2011 at 8:52 AM, Marcus Sorensen <shadowsor at gmail.com> > > wrote: > > > > On Tue, Dec 13, 2011 at 4:40 AM, Lars Ellenberg > > > > <lars.ellenberg at linbit.com> wrote: > > > >> On Mon, Dec 12, 2011 at 09:35:01PM -0700, Marcus Sorensen wrote: > > > >>> "in dual primary mode, only protocol C is allowed. > > > >>> > > > >>> cluster file system deals with cache coherence, > > > >>> DRBD has to deal with storage coherence. > > > >>> you cannot do that asynchronously. > > > >>> > > > >>> to do that in protocol B, the receiving node would need to be able > to > > > >>> access data of in-flight requests, which is not implemented." > > > > ^^^ > > You even quoted the paragraph. > > Anything unclear about that? > > > > > > > >>> > > > >>> But what if you're not using a filesystem? > > > >> > > > >> Ok, so you are using "whatever", and you think you want > dual-primary > > DRBD, > > > >> but you want it to replicate asynchronously despite of that. > > > >> > > > >> What if "whatever" writes something to node A, > > > >> node A completes the write, > > > >> and "whatever" has all right to assume this actually hit stable > > storage. > > > >> > > > >> Then (the same or a different instance of) "whatever" wants to read > > that > > > >> change from node B, but the change is not yet on B, because it > > > >> replicated async. > > > >> > > > >> "whatever" would be slightly surprised, > > > >> and you would start screaming drbd ate my data. > > > >> > > > >> Well, no, it did not. You screwed up. > > > >> And this particular screwup DRBD does not allow. > > > > > > > > So you're saying that protocol B, in active-active, would not read > > > > from memory on the second node. So even if we wait until a write to > > > > block 213456 has replicated to memory on node B, if a read comes in > > > > for block 213456 on node B, it's going to go to disk for the read, > and > > > > ignore the outstanding write until it's on disk. This is good to > know, > > > > and not what I would have expected. > > > > > > Ah, for clarification, in protocol B we're not talking about the write > > > being 'in buffer' waiting to be flushed to disk, but in drbd memory, > > > correct? > > > _______________________________________________ > > > drbd-user mailing list > > > drbd-user at lists.linbit.com > > > http://lists.linbit.com/mailman/listinfo/drbd-user > > > > -- > > : Lars Ellenberg > > : LINBIT | Your Way to High Availability > > : DRBD/HA support and consulting http://www.linbit.com > > > > DRBDR and LINBITR are registered trademarks of LINBIT, Austria. > > __ > > please don't Cc me, but send to list -- I'm subscribed > > _______________________________________________ > > drbd-user mailing list > > drbd-user at lists.linbit.com > > http://lists.linbit.com/mailman/listinfo/drbd-user -- : 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. __ please don't Cc me, but send to list -- I'm subscribed