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

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Date:   Tue, 12 Jan 2021 13:14:45 -0800
From:   "Darrick J. Wong" <darrick.wong@...cle.com>
To:     Andreas Dilger <adilger@...ger.ca>
Cc:     Christoph Hellwig <hch@...radead.org>,
        Avi Kivity <avi@...lladb.com>,
        Andres Freund <andres@...razel.de>,
        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 11:51:07AM -0700, Andreas Dilger wrote:
> On Jan 12, 2021, at 11:43 AM, Christoph Hellwig <hch@...radead.org> wrote:
> > 
> > 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).
> 
> I'm not against adding XFS_IOC_ALLOCSP64 to ext4, if applications are actually
> using that.

<shudder> Some of them are, but--

ALLOCSP64 can only allocate pre-zeroed blocks as part of extending EOF,
whereas a new FZERO flag means that we can pre-zero an arbitrary range
of bytes in a file.  I don't know if Avi or Andres' usecases demand that
kind of flexibilty but I know I'd rather go for the more powerful
interface.

--D

> It also makes sense to me that there also be an fallocate() mode for allocating
> zeroed blocks (which was the original request), since fallocate() is already
> doing very similar things and is the central interface for managing block
> allocation instead of having a filesystem-specific ioctl() to do this.
> 
> Cheers, Andreas
> 
> 
> 
> 
>