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From:   Andreas Dilger <adilger@dilger.ca>
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Subject: Re: fallocate(FALLOC_FL_ZERO_RANGE_BUT_REALLY) to avoid unwritten
 extents?
Date:   Tue, 12 Jan 2021 11:51:07 -0700
In-Reply-To: <20210112184339.GA1238746@infradead.org>
Cc:     Avi Kivity <avi@scylladb.com>, Andres Freund <andres@anarazel.de>,
        "Darrick J. Wong" <darrick.wong@oracle.com>,
        linux-fsdevel@vger.kernel.org, linux-xfs@vger.kernel.org,
        linux-ext4@vger.kernel.org, linux-block@vger.kernel.org
To:     Christoph Hellwig <hch@infradead.org>
References: <20201230062819.yinrrp6uwfegsqo3@alap3.anarazel.de>
 <20210104181958.GE6908@magnolia>
 <20210104191058.sryksqjnjjnn5raa@alap3.anarazel.de>
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 <20210112181600.GA1228497@infradead.org>
 <C8811877-48A9-4199-9F28-20F5B071AE36@dilger.ca>
 <20210112184339.GA1238746@infradead.org>
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On Jan 12, 2021, at 11:43 AM, Christoph Hellwig <hch@infradead.org> =
wrote:
>=20
> 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.
>>>=20
>>>> But note it will need to be plumbed down to md and dm to be =
generally
>>>> useful.
>>>=20
>>> DM and MD already support mddev_check_write_zeroes, at least for the
>>> usual targets.
>>=20
>> 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.
>=20
> 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.

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


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