On Fri, Aug 27 2010 at 1:52am -0400, Jun'ichi Nomura <j-nomura@xxxxxxxxxxxxx> wrote: > Hi Mike, > > (08/27/10 13:08), Mike Snitzer wrote: > > But do you agree that the request-based barrier code (added in commit > > d0bcb8786) could be reverted given the new FLUSH work? > > No, it's a separate thing. > If we don't need to care about the case where multiple clones > of flush request are necessary, the special casing of flush > request can be removed regardless of the new FLUSH work. Ah, yes thanks for clarifying. But we've never cared about multiple clone of a flush so it's odd that such elaborate infrastructure was introduced without a need. > > We no longer need waiting now that ordering isn't a concern. Especially > > The waiting is not for ordering, but for multiple clones. > > > so given rq-based doesn't support multiple targets. As you know, from > > dm_table_set_type: > > > > /* > > * Request-based dm supports only tables that have a single target now. > > * To support multiple targets, request splitting support is needed, > > * and that needs lots of changes in the block-layer. > > * (e.g. request completion process for partial completion.) > > */ > > This comment is about multiple targets. > The special code for barrier is for single target whose > num_flush_requests > 1. Different thing. Yes, I need to not send mail just before going to bed.. > > I think we need to at least benchmark the performance of dm-mpath > > without any of this extra, soon to be unnecessary, code. > > If there will be no need for supporting a request-based target > with num_flush_requests > 1, the special handling of flush > can be removed. > > And since there is no such target in the current tree, > I don't object if you remove that part of code for good reason. OK, certainly something to keep in mind. But _really_ knowing the multipath FLUSH+FUA performance difference (extra special-case code vs none) requires a full FLUSH conversion of request-based DM anyway. In general, request-based DM's barrier/flush code does carry a certain maintenance overhead. It is quite a bit of distracting code in the core DM which isn't buying us anything.. so we _could_ just remove it and never look back (until we have some specific need for num_flush_requests > 1 in rq-based DM). Mike -- To unsubscribe from this list: send the line "unsubscribe linux-scsi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
