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

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Date:   Tue, 12 Jan 2021 11:39:58 -0700
From:   Andreas Dilger <adilger@...ger.ca>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     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 Jan 12, 2021, at 11:16 AM, Christoph Hellwig <hch@...radead.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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