Re: [PATCH 2/6] cxlflash: Fix to avoid virtual LUN failover failure

From: Uma Krishnan <ukrishn@linux.vnet.ibm.com>
To: linux-scsi@vger.kernel.org,
	James Bottomley <James.Bottomley@HansenPartnership.com>,
	"Martin K. Petersen" <martin.petersen@oracle.com>,
	"Matthew R. Ochs" <mrochs@linux.vnet.ibm.com>,
	"Manoj N. Kumar" <manoj@linux.vnet.ibm.com>,
	Brian King <brking@linux.vnet.ibm.com>
Cc: linuxppc-dev@lists.ozlabs.org, Ian Munsie <imunsie@au1.ibm.com>,
	Andrew Donnellan <andrew.donnellan@au1.ibm.com>
Subject: Re: [PATCH 2/6] cxlflash: Fix to avoid virtual LUN failover failure
Date: Thu, 17 Dec 2015 16:19:59 -0600	[thread overview]
Message-ID: <5673350F.5040501@linux.vnet.ibm.com> (raw)
In-Reply-To: <1449788029-23093-1-git-send-email-ukrishn@linux.vnet.ibm.com>

On 12/10/2015 4:53 PM, Uma Krishnan wrote:
> From: "Matthew R. Ochs" <mrochs@linux.vnet.ibm.com>
>
> Applications which use virtual LUN's that are backed by a physical LUN
> over both adapter ports may experience an I/O failure in the event of
> a link loss (e.g. cable pull).
>
> Virtual LUNs may be accessed through one or both ports of the adapter.
> This access is encoded in the translation entries that comprise the
> virtual LUN and used by the AFU for load-balancing I/O and handling
> failover scenarios. In a link loss scenario, even though the AFU is
> able to maintain connectivity to the LUN, it is up to the application
> to retry the failed I/O. When applications are unaware of the virtual
> LUN's underlying topology, they are unable to make a sound decision of
> when to retry an I/O and therefore are forced to make their reaction to
> a failed I/O absolute. The result is either a failure to retry I/O or
> increased latency for scenarios where a retry is pointless.
>
> To remedy this scenario, provide feedback back to the application on
> virtual LUN creation as to which ports the LUN may be accessed. LUN's
> spanning both ports are candidates for a retry in a presence of an I/O
> failure.
>
> Signed-off-by: Matthew R. Ochs <mrochs@linux.vnet.ibm.com>
> ---

Reviewed-by: Uma Krishnan <ukrishn@linux.vnet.ibm.com>