On Wed, 2007-01-10 at 08:10 -0800, James Bottomley wrote: > On Tue, 2007-01-09 at 16:32 -0500, Edward Goggin wrote: > > The attached (untested) patch shows a VMware and scsi transport agnostic > > approach which introduces a new host status (DID_QUALIFIED_REQUEUE) to > > be used by mptscsih.c (and other LLDs) instead of DID_BUS_BUSY. A host > > status of DID_QUALIFIED_REQUEUE will return ADD_TO_MLQUEUE from > > scsi_decide_disposition IFF the REQ_FAILFAST bit is not set in the > > cmd_flags field of the SCSI command's request structure. > > > > The approach depends on both VMware Linux guests not setting > > REQ_FAILFAST and non-VMware Linux hosts with an IBM RDAC/MPP multi- > > pathing driver doing so. This requirement is not a problem for VMware > > since its guest operating systems have no need to configure block device > > multi-pathing. This requirement shouldn't be a problem for the IBM > > RDAC/MPP driver either since it should already be setting the > > REQ_FAILFAST attribute of I/Os for which it is providing multi-pathing, > > similar to what the Linux dm-multipath driver already does. > > Not in the driver, please ... > > the SAM status BUSY is a well known one for array controllers to return > while contemplating a failover. Thus, if we think this is the issue, > the mid-layer should be the entity to pass the status through on > REQ_FAILFAST not the driver (i.e. pass SAM_STAT_BUSY through unmodified > and alter the mid-layer). However, I'd be unhappy about doing this: > BUSY is a standard return for a lot of controllers for transient > resource conditions, which wouldn't necessarily be alleviated on path > failover. > > James > I share your concern about affecting the semantics of SAM_STAT_BUSY. This is why the patch introduces a new host status instead of simply changing the code for BUSY in scsi_decide_disposition to requeue only if not REQ_FAILFAST. The new host status is meant to provide the option for LLDs to enact conditional requeuing of the cmd (conditional based on the REQ_FAILFAST setting) which is not throttled by the cmd's retry count -- a capability which is not there currently. > - 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
