From: Andreas Dilger <adilger@dilger.ca>
To: Christoph Hellwig <hch@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
Subject: Re: fallocate(FALLOC_FL_ZERO_RANGE_BUT_REALLY) to avoid unwritten extents?
Date: Tue, 12 Jan 2021 11:39:58 -0700 [thread overview]
Message-ID: <C8811877-48A9-4199-9F28-20F5B071AE36@dilger.ca> (raw)
In-Reply-To: <20210112181600.GA1228497@infradead.org>
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On Jan 12, 2021, at 11:16 AM, Christoph Hellwig <hch@infradead.org> wrote:
>
> 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.
>>>
>>
>> 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.
>
> 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.
>
> 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.
Cheers, Andreas
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next prev parent reply other threads:[~2021-01-12 18:41 UTC|newest]
Thread overview: 19+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-12-30 6:28 fallocate(FALLOC_FL_ZERO_RANGE_BUT_REALLY) to avoid unwritten extents? Andres Freund
2021-01-04 18:19 ` Darrick J. Wong
2021-01-04 19:10 ` Andres Freund
2021-01-04 19:57 ` Avi Kivity
2021-01-12 18:16 ` Christoph Hellwig
2021-01-12 18:39 ` Andreas Dilger [this message]
2021-01-12 18:43 ` Christoph Hellwig
2021-01-12 18:51 ` Andreas Dilger
2021-01-12 21:14 ` Darrick J. Wong
2021-01-12 21:36 ` Andres Freund
2021-01-13 7:44 ` Avi Kivity
2021-01-19 3:44 ` Andreas Dilger
2021-01-04 19:17 ` Theodore Ts'o
2021-01-04 19:24 ` Matthew Wilcox
2021-01-04 20:29 ` Andres Freund
2021-01-04 22:40 ` Eric Sandeen
2021-01-06 22:52 ` Dave Chinner
2021-01-06 23:40 ` Andres Freund
2021-01-08 20:32 ` Dave Chinner
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