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Date: Wed, 9 Apr 2025 12:31:48 +0200
From: Christoph Hellwig <hch@lst.de>
To: Zhang Yi <yi.zhang@huaweicloud.com>
Cc: linux-fsdevel@vger.kernel.org, linux-ext4@vger.kernel.org,
	linux-block@vger.kernel.org, dm-devel@lists.linux.dev,
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	tytso@mit.edu, djwong@kernel.org, john.g.garry@oracle.com,
	bmarzins@redhat.com, chaitanyak@nvidia.com,
	shinichiro.kawasaki@wdc.com, yi.zhang@huawei.com,
	chengzhihao1@huawei.com, yukuai3@huawei.com, yangerkun@huawei.com
Subject: Re: [RFC PATCH -next v3 01/10] block: introduce
 BLK_FEAT_WRITE_ZEROES_UNMAP to queue limits features
Message-ID: <20250409103148.GA4950@lst.de>
References: <20250318073545.3518707-1-yi.zhang@huaweicloud.com> <20250318073545.3518707-2-yi.zhang@huaweicloud.com>
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User-Agent: Mutt/1.5.17 (2007-11-01)

On Tue, Mar 18, 2025 at 03:35:36PM +0800, Zhang Yi wrote:
> From: Zhang Yi <yi.zhang@huawei.com>
> 
> Currently, disks primarily implement the write zeroes command (aka
> REQ_OP_WRITE_ZEROES) through two mechanisms: the first involves
> physically writing zeros to the disk media (e.g., HDDs), while the
> second performs an unmap operation on the logical blocks, effectively
> putting them into a deallocated state (e.g., SSDs). The first method is
> generally slow, while the second method is typically very fast.
> 
> For example, on certain NVMe SSDs that support NVME_NS_DEAC, submitting
> REQ_OP_WRITE_ZEROES requests with the NVME_WZ_DEAC bit can accelerate
> the write zeros operation by placing disk blocks into

Note that this is a can, not a must.  The NVMe definition of Write
Zeroes is unfortunately pretty stupid.

> +		[RO] Devices that explicitly support the unmap write zeroes
> +		operation in which a single write zeroes request with the unmap
> +		bit set to zero out the range of contiguous blocks on storage
> +		by freeing blocks, rather than writing physical zeroes to the
> +		media.

This is not actually guaranteed for nvme or scsi.