On Thu, Sep 15, 2016 at 04:38:07AM -0700, Christoph Hellwig wrote: > On Thu, Sep 15, 2016 at 12:49:35PM +0200, Wouter Verhelst wrote: > > A while back, we spent quite some time defining the semantics of the > > various commands in the face of the NBD_CMD_FLUSH and NBD_CMD_FLAG_FUA > > write barriers. At the time, we decided that it would be unreasonable > > to expect servers to make these write barriers effective across > > different connections. > > Do you have a nbd protocol specification? https://github.com/yoe/nbd/blob/master/doc/proto.md > treating a flush or fua as any sort of barrier is incredibly stupid. Maybe I'm not using the correct terminology here. The point is that after a FLUSH, the server asserts that all write commands *for which a reply has already been sent to the client* will also have reached permanent storage. Nothing is asserted about commands for which the reply has not yet been sent, regardless of whether they were sent to the server before or after the FLUSH; they may reach permanent storage as a result of the FLUSH, or they may not, we don't say. With FUA, we only assert that the FUA-flagged command reaches permanent storage before its reply is sent, nothing else. If that's not a write barrier, then I was using the wrong terminology (and offer my apologies for the confusion). The point still remains that "X was sent before Y" is difficult to determine on the client side if X was sent over a different TCP channel than Y, because a packet might be dropped (requiring a retransmit) for X, and similar things. If blk-mq can deal with that, we're good and nothing further needs to be done. If not, this should be evaluated by someone more familiar with the internals of the kernel block subsystem than me. > Is it really documented that way, and if yes, why? The documentation does use the term write barrier, but only right before the same explanation as above. It does so because I assumed that that is what a write barrier is, and that this would make things easier to understand. If you tell me that that term is wrong as used there, I can easily remove it (it's not critical to the rest of the documentation). Regards, -- < ron> I mean, the main *practical* problem with C++, is there's like a dozen people in the world who think they really understand all of its rules, and pretty much all of them are just lying to themselves too. -- #debian-devel, OFTC, 2016-02-12 -- To unsubscribe from this list: send the line "unsubscribe linux-block" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
