Kiyoshi Ueda wrote: > This patch converts dm-multipath target to request-based from bio-based. > > Basically, the patch just converts the I/O unit from struct bio > to struct request. > In the course of the conversion, it also changes the I/O queueing > mechanism. The change in the I/O queueing is described in details > as follows. > > I/O queueing mechanism change > ----------------------------- > In I/O submission, map_io(), there is no mechanism change from > bio-based, since the clone request is ready for retry as it is. > However, in I/O complition, do_end_io(), there is a mechanism change > from bio-based, since the clone request is not ready for retry. > > In do_end_io() of bio-based, the clone bio has all needed memory > for resubmission. So the target driver can queue it and resubmit > it later without memory allocations. > The mechanism has almost no overhead. > > On the other hand, in do_end_io() of request-based, the clone request > doesn't have clone bios, so the target driver can't resubmit it > as it is. To resubmit the clone request, memory allocation for > clone bios is needed, and it takes some overheads. > To avoid the overheads just for queueing, the target driver doesn't > queue the clone request inside itself. > Instead, the target driver asks dm core for queueing and remapping > the original request of the clone request, since the overhead for > queueing is just a freeing memory for the clone request. > > As a result, the target driver doesn't need to record/restore > the information of the original request for resubmitting > the clone request. So dm_bio_details in dm_mpath_io is removed. > > > multipath_busy() > --------------------- > The target driver returns "busy", only when the following case: > o The target driver will map I/Os, if map() function is called > and > o The mapped I/Os will wait on underlying device's queue due to > their congestions, if map() function is called now. > > In other cases, the target driver doesn't return "busy". > Otherwise, dm core will keep the I/Os and the target driver can't > do what it wants. > (e.g. the target driver can't map I/Os now, so wants to kill I/Os.) > > > Signed-off-by: Kiyoshi Ueda <k-ueda@xxxxxxxxxxxxx> > Signed-off-by: Jun'ichi Nomura <j-nomura@xxxxxxxxxxxxx> Acked-by: Hannes Reinecke <hare@xxxxxxx> Cheers, Hannes -- Dr. Hannes Reinecke zSeries & Storage hare@xxxxxxx +49 911 74053 688 SUSE LINUX Products GmbH, Maxfeldstr. 5, 90409 Nürnberg GF: Markus Rex, HRB 16746 (AG Nürnberg) -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
