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

From: Daniel Axtens <dja@axtens.net>
To: Uma Krishnan <ukrishn@linux.vnet.ibm.com>,
	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: Mon, 14 Dec 2015 16:00:25 +1100	[thread overview]
Message-ID: <87h9jl7hg6.fsf@gamma.ozlabs.ibm.com> (raw)
In-Reply-To: <1449788029-23093-1-git-send-email-ukrishn@linux.vnet.ibm.com>

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> Virtual LUNs may be accessed through one or both ports of the adapter.

Is it possible that there might ever be adapters with a number of ports
other than 2? In particular, is it possible for 3 or 4 port adapters to
exist?

If so, do you need something with a bit more fidelity?

If not, this is a good approach.

Regards,
Daniel

> 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>
> ---
>  drivers/scsi/cxlflash/vlun.c       |  2 ++
>  include/uapi/scsi/cxlflash_ioctl.h | 10 ++++++++++
>  2 files changed, 12 insertions(+)
>
> diff --git a/drivers/scsi/cxlflash/vlun.c b/drivers/scsi/cxlflash/vlun.c
> index a53f583..50f8e93 100644
> --- a/drivers/scsi/cxlflash/vlun.c
> +++ b/drivers/scsi/cxlflash/vlun.c
> @@ -1008,6 +1008,8 @@ int cxlflash_disk_virtual_open(struct scsi_device *sdev, void *arg)
>  	virt->last_lba = last_lba;
>  	virt->rsrc_handle = rsrc_handle;
>  
> +	if (lli->port_sel == BOTH_PORTS)
> +		virt->hdr.return_flags |= DK_CXLFLASH_ALL_PORTS_ACTIVE;
>  out:
>  	if (likely(ctxi))
>  		put_context(ctxi);
> diff --git a/include/uapi/scsi/cxlflash_ioctl.h b/include/uapi/scsi/cxlflash_ioctl.h
> index 831351b..2302f3c 100644
> --- a/include/uapi/scsi/cxlflash_ioctl.h
> +++ b/include/uapi/scsi/cxlflash_ioctl.h
> @@ -31,6 +31,16 @@ struct dk_cxlflash_hdr {
>  };
>  
>  /*
> + * Return flag definitions available to all ioctls
> + *
> + * Similar to the input flags, these are grown from the bottom-up with the
> + * intention that ioctl-specific return flag definitions would grow from the
> + * top-down, allowing the two sets to co-exist. While not required/enforced
> + * at this time, this provides future flexibility.
> + */
> +#define DK_CXLFLASH_ALL_PORTS_ACTIVE	0x0000000000000001ULL
> +
> +/*
>   * Notes:
>   * -----
>   * The 'context_id' field of all ioctl structures contains the context
> -- 
> 2.1.0
>
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