On Fri, Jan 19, 2018 at 05:09:46AM +0000, Bart Van Assche wrote: > On Fri, 2018-01-19 at 10:32 +0800, Ming Lei wrote: > > Now most of times both NVMe and SCSI won't return BLK_STS_RESOURCE, and > > it should be DM-only which returns STS_RESOURCE so often. > > That's wrong at least for SCSI. See also https://marc.info/?l=linux-block&m=151578329417076. > > For other scenario's, e.g. if a SCSI initiator submits a > SCSI request over a fabric and the SCSI target replies with "BUSY" then the Could you explain a bit when SCSI target replies with BUSY very often? Inside initiator, we have limited the max per-LUN requests and per-host requests already before calling .queue_rq(). > SCSI core will end the I/O request with status BLK_STS_RESOURCE after the > maximum number of retries has been reached (see also scsi_io_completion()). > In that last case, if a SCSI target sends a "BUSY" reply over the wire back > to the initiator, there is no other approach for the SCSI initiator to > figure out whether it can queue another request than to resubmit the > request. The worst possible strategy is to resubmit a request immediately > because that will cause a significant fraction of the fabric bandwidth to > be used just for replying "BUSY" to requests that can't be processed > immediately. -- Ming
