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Message-ID: <68172a9e-cf68-4962-8229-68e283e894e1@huaweicloud.com> Date: Thu, 8 May 2025 20:17:14 +0800 From: Zhang Yi <yi.zhang@...weicloud.com> To: Christoph Hellwig <hch@....de> Cc: "Darrick J. Wong" <djwong@...nel.org>, linux-fsdevel@...r.kernel.org, linux-ext4@...r.kernel.org, linux-block@...r.kernel.org, dm-devel@...ts.linux.dev, linux-nvme@...ts.infradead.org, linux-scsi@...r.kernel.org, linux-xfs@...r.kernel.org, linux-kernel@...r.kernel.org, tytso@....edu, john.g.garry@...cle.com, bmarzins@...hat.com, chaitanyak@...dia.com, shinichiro.kawasaki@....com, brauner@...nel.org, yi.zhang@...wei.com, chengzhihao1@...wei.com, yukuai3@...wei.com, yangerkun@...wei.com Subject: Re: [RFC PATCH v4 07/11] fs: statx add write zeroes unmap attribute On 2025/5/8 13:01, Christoph Hellwig wrote: > On Wed, May 07, 2025 at 03:33:23PM +0800, Zhang Yi wrote: >> On 2025/5/6 20:11, Christoph Hellwig wrote: >>> On Tue, May 06, 2025 at 07:16:56PM +0800, Zhang Yi wrote: >>>> Sorry, but I don't understand your suggestion. The >>>> STATX_ATTR_WRITE_ZEROES_UNMAP attribute only indicate whether the bdev >>>> and the block device that under the specified file support unmap write >>>> zeroes commoand. It does not reflect whether the bdev and the >>>> filesystems support FALLOC_FL_WRITE_ZEROES. The implementation of >>>> FALLOC_FL_WRITE_ZEROES doesn't fully rely on the unmap write zeroes >>>> commoand now, users simply refer to this attribute flag to determine >>>> whether to use FALLOC_FL_WRITE_ZEROES when preallocating a file. >>>> So, STATX_ATTR_WRITE_ZEROES_UNMAP and FALLOC_FL_WRITE_ZEROES doesn't >>>> have strong relations, why do you suggested to put this into the ext4 >>>> and bdev patches that adding FALLOC_FL_WRITE_ZEROES? >>> >>> So what is the point of STATX_ATTR_WRITE_ZEROES_UNMAP? >> >> My idea is not to strictly limiting the use of FALLOC_FL_WRITE_ZEROES to >> only bdev or files where bdev_unmap_write_zeroes() returns true. In >> other words, STATX_ATTR_WRITE_ZEROES_UNMAP and FALLOC_FL_WRITE_ZEROES >> are not consistent, they are two independent features. Even if some >> devices STATX_ATTR_WRITE_ZEROES_UNMAP are not set, users should still be >> allowed to call fallcoate(FALLOC_FL_WRITE_ZEROES). This is because some >> devices and drivers currently cannot reliably ascertain whether they >> support the unmap write zero command; however, certain devices, such as >> specific cloud storage devices, do support it. Users of these devices >> may also wish to use FALLOC_FL_WRITE_ZEROES to expedite the zeroing >> process. > > What are those "cloud storage devices" where you set it reliably, > i.e.g what drivers? I don't have these 'cloud storage devices' now, but Ted had mentioned those cloud-emulated block devices such as Google's Persistent Desk or Amazon's Elastic Block Device in. I'm not sure if they can accurately report the BLK_FEAT_WRITE_ZEROES_UNMAP feature, maybe Ted can give more details. https://lore.kernel.org/linux-fsdevel/20250106161732.GG1284777@mit.edu/ > >> Therefore, I think that the current point of >> STATX_ATTR_WRITE_ZEROES_UNMAP (possibly STATX_WRITE_ZEROES_UNMAP) should >> be to just indicate whether a bdev or file supports the unmap write zero >> command (i.e., whether bdev_unmap_write_zeroes() returns true). If we >> use standard SCSI and NVMe storage devices, and the >> STATX_ATTR_WRITE_ZEROES_UNMAP attribute is set, users can be assured >> that FALLOC_FL_WRITE_ZEROES is fast and can choose to use >> fallocate(FALLOC_FL_WRITE_ZEROES) immediately. > > That's breaking the abstracton again. An attribute must say something > about the specific file, not about some underlying semi-related feature. OK. > >> Would you prefer to make STATX_ATTR_WRITE_ZEROES_UNMAP and >> FALLOC_FL_WRITE_ZEROES consistent, which means >> fallcoate(FALLOC_FL_WRITE_ZEROES) will return -EOPNOTSUPP if the block >> device doesn't set STATX_ATTR_WRITE_ZEROES_UNMAP ? > > Not sure where the block device comes from here, both of these operate > on a file. I am referring to the block device on which the filesystem is mounted. The support status of the file is directly dependent on this block device. > >> If so, I'd suggested we need to: >> 1) Remove STATX_ATTR_WRITE_ZEROES_UNMAP since users can check the >> existence by calling fallocate(FALLOC_FL_WRITE_ZEROES) directly, this >> statx flag seems useless. > > Yes, that was my inital thought. > >> 2) Make the BLK_FEAT_WRITE_ZEROES_UNMAP sysfs interface to RW, allowing >> users to adjust the block device's support state according to the >> real situation. > > No, it's a feature and not a flag. > I am a bit confused about the feature and the flag, I checked the other features, and it appears that features such as BLK_FEAT_ROTATIONAL allow to be modified, is this flexibility due to historical reasons or for the convenience of testing? Think about this again, I suppose we should keep the BLK_FEAT_WRITE_ZEROES_UNMAP as read-only and add a new flag, BLK_FALG_WRITE_ZEROES_UNMAP_DISABLED, to disable the FALLOC_FL_WRITE_ZEROES. Since the Write Zeroes does not guarantee performance, and some devices may claim to support **UNMAP** Write Zeroes but exhibit extremely slow write-zeroes speeds. Users may want be able to disable it. Thoughts? Thanks, Yi.
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