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Date: Tue, 4 Feb 2025 14:49:01 +0100
From: Christoph Hellwig <hch@lst.de>
To: "Martin K. Petersen" <martin.petersen@oracle.com>
Cc: Christoph Hellwig <hch@lst.de>, Kanchan Joshi <joshi.k@samsung.com>,
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	linux-nvme@lists.infradead.org, linux-block@vger.kernel.org,
	gost.dev@samsung.com
Subject: Re: [RFC 0/3] Btrfs checksum offload
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References: <CGME20250129141039epcas5p11feb1be4124c0db3c5223325924183a3@epcas5p1.samsung.com> <20250129140207.22718-1-joshi.k@samsung.com> <yq134h0p1m5.fsf@ca-mkp.ca.oracle.com> <20250131074424.GA16182@lst.de> <yq17c66kfxs.fsf@ca-mkp.ca.oracle.com> <20250204051208.GG28103@lst.de> <yq15xlpg9tj.fsf@ca-mkp.ca.oracle.com>
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On Tue, Feb 04, 2025 at 07:52:38AM -0500, Martin K. Petersen wrote:
> I have been told that some arrays use it to disable PI when writing the
> RAID parity blocks. I guess that makes sense if the array firmware is
> mixing data and parity blocks in a single write operation. For my test
> tool I just use WRPROTECT=3 to disable checking when writing "bad" PI.

Hmm, why would you disable PI for parity blocks?  But yes, outside of
Linux there might be uses.  Just thinking of a "perfect" format for
our needs.

> >
> > That would also work fine.  NVMe supports 4byte crc32c + 2 byte app tag +
> > 12 byte guard tag / storage tag and 8-byte crc64 + 2 byte app tag + 6
> > byte guard / storage tag, although Linux only supports the latter so far.
> 
> Yep, the CRC32C variant should be easy to wire up. I've thought about
> the storage tag but haven't really come up with a good use case. It's
> essentially the same situation as with the app tag.

Exactly, it's an app tag by other means.