On Thu, May 17 2012 at 5:09am -0400, Jun'ichi Nomura <j-nomura@xxxxxxxxxxxxx> wrote: > Hi, > > On 05/16/12 21:28, Bernd Schubert wrote: > > Looking through the call chain, I see the underlying problem seems to be in scsi_host_is_busy(). > > > >> static inline int scsi_host_is_busy(struct Scsi_Host *shost) > >> { > >> if ((shost->can_queue > 0 && shost->host_busy >= shost->can_queue) || > >> shost->host_blocked || shost->host_self_blocked) > >> return 1; > >> > >> return 0; > >> } > > multipath_busy() was introduced because, without that, > a request would be prematurely sent down to SCSI, > lose the chance of additional merges and result in > bad performance. > > However, when it is target/host that is busy, I think dm should > send the request down and let SCSI, which has better knowledge > about the shared resource, do appropriate starvation control. > > Could you try the attached patch? > > --- > Jun'ichi Nomura, NEC Corporation > > If sdev is not busy but starget and/or host is busy, > it is better to accept a request from stacking driver. > Otherwise, the stacking device could be starved by other device > sharing the same target/host. Great suggestion. It should be noted that DM mpath is the only caller of blk_lld_busy (and scsi_lld_busy). So even though this patch may _seem_ like the tail (mpath) wagging the dog (SCSI), it is reasonable to change SCSI's definition of a LLD being "busy" if it benefits multipath. -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel