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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 11:39:58 -0700
In-Reply-To: <20210112181600.GA1228497@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>
 <f6f75f11-5d5b-ae63-d584-4b6f09ff401e@scylladb.com>
 <20210112181600.GA1228497@infradead.org>
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On Jan 12, 2021, at 11:16 AM, Christoph Hellwig <hch@infradead.org> =
wrote:
>=20
> On Mon, Jan 04, 2021 at 09:57:48PM +0200, Avi Kivity wrote:
>>> I don't have a strong opinion on it. A complex userland application =
can
>>> do a bit better job managing queue depth etc, but otherwise I =
suspect
>>> doing the IO from kernel will win by a small bit. And the =
queue-depth
>>> issue presumably would be relevant for write-zeroes as well, making =
me
>>> lean towards just using the fallback.
>>>=20
>>=20
>> The new flag will avoid requiring DMA to transfer the entire file =
size, and
>> perhaps can be implemented in the device by just adjusting metadata. =
So
>> there is potential for the new flag to be much more efficient.
>=20
> We already support a WRITE_ZEROES operation, which many (but not all)
> NVMe devices and some SCSI devices support.  The blkdev_issue_zeroout
> helper can use those, or falls back to writing actual zeroes.
>=20
> 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.

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.

Cheers, Andreas


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