Re: [PATCH 2/5] fusion: vmware bug fix prevent inifinite retries

[James Bottomley <James.Bottomley@SteelEye.com>]

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