linux-kernel - Re: [PATCH 09/14] target/configfs: Expose protection device attributes

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Date:	Sun, 12 Jan 2014 14:52:54 +0200
From:	Sagi Grimberg <sagig@....mellanox.co.il>
To:	"Martin K. Petersen" <martin.petersen@...cle.com>
CC:	"Nicholas A. Bellinger" <nab@...erainc.com>,
	target-devel <target-devel@...r.kernel.org>,
	linux-scsi <linux-scsi@...r.kernel.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Christoph Hellwig <hch@....de>, Hannes Reinecke <hare@...e.de>,
	Sagi Grimberg <sagig@...lanox.com>,
	Or Gerlitz <ogerlitz@...lanox.com>,
	Nicholas Bellinger <nab@...ux-iscsi.org>
Subject: Re: [PATCH 09/14] target/configfs: Expose protection device attributes

On 1/12/2014 2:43 PM, Martin K. Petersen wrote:
>>>>>> "Sagi" == Sagi Grimberg <sagig@....mellanox.co.il> writes:
>>> The IP checksum is only supported by DIX between OS and initiator,
>>> not by the target. I guess we could signal to the initiator via a
>>> vendor-private VPD that IP checksum is supported directly. But now
>>> what we have hardware-accelerated T10 CRC I don't think it's a big
>>> deal.
> Sagi> shouldn't it stick around if it is not deprecated yet, the
> Sagi> transport is required to support ip-csum->CRC conversion anyhow.
>
> SBC mandates that the guard tag on the wire and on the target device be
> the T10 CRC. The IP checksum is a DIX-optimization for application-HBA
> exchanges. The only place you should support the IP checksum is in the
> initiator.

Right.

> Note that you could conceivably do a T10 CRC to IP checksum conversion
> on writes received by the target and store the IP checksum on disk. And
> then convert back to T10 CRC when the data is eventually read. But it
> makes no sense to do that given that you will have to do the T10 CRC
> calculation regardless. Even if the backing store is DIX-capable and
> supports the IP checksum.
>

I agree, but for backstore DIF (SW) emulation it will make sense.
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