linux-ext4 - Re: fallocate(FALLOC_FL_ZERO_RANGE_BUT_REALLY) to avoid unwritten extents?

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Date:   Tue, 12 Jan 2021 18:43:39 +0000
From:   Christoph Hellwig <hch@...radead.org>
To:     Andreas Dilger <adilger@...ger.ca>
Cc:     Christoph Hellwig <hch@...radead.org>,
        Avi Kivity <avi@...lladb.com>,
        Andres Freund <andres@...razel.de>,
        "Darrick J. Wong" <darrick.wong@...cle.com>,
        linux-fsdevel@...r.kernel.org, linux-xfs@...r.kernel.org,
        linux-ext4@...r.kernel.org, linux-block@...r.kernel.org
Subject: Re: fallocate(FALLOC_FL_ZERO_RANGE_BUT_REALLY) to avoid unwritten
 extents?

On Tue, Jan 12, 2021 at 11:39:58AM -0700, Andreas Dilger wrote:
> > XFS already has a XFS_IOC_ALLOCSP64 that is defined to actually
> > allocate written extents.  It does not currently use
> > blkdev_issue_zeroout, but could be changed pretty trivially to do so.
> > 
> >> But note it will need to be plumbed down to md and dm to be generally
> >> useful.
> > 
> > DM and MD already support mddev_check_write_zeroes, at least for the
> > usual targets.
> 
> Similarly, ext4 also has EXT4_GET_BLOCKS_CREATE_ZERO that can allocate zero
> filled extents rather than unwritten extents (without clobbering existing
> data like FALLOC_FL_ZERO_RANGE does), and just needs a flag from fallocate()
> to trigger it.  This is plumbed down to blkdev_issue_zeroout() as well.

XFS_IOC_ALLOCSP64 actually is an ioctl that has been around since 1995
on IRIX (as an fcntl).