From: Andres Freund <andres@anarazel.de>
To: "Darrick J. Wong" <darrick.wong@oracle.com>
Cc: 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: Mon, 4 Jan 2021 11:10:58 -0800 [thread overview]
Message-ID: <20210104191058.sryksqjnjjnn5raa@alap3.anarazel.de> (raw)
In-Reply-To: <20210104181958.GE6908@magnolia>
Hi,
On 2021-01-04 10:19:58 -0800, Darrick J. Wong wrote:
> On Tue, Dec 29, 2020 at 10:28:19PM -0800, Andres Freund wrote:
> > Would it make sense to add a variant of FALLOC_FL_ZERO_RANGE that
> > doesn't convert extents into unwritten extents, but instead uses
> > blkdev_issue_zeroout() if supported? Mostly interested in xfs/ext4
> > myself, but ...
> >
> > Doing so as a variant of FALLOC_FL_ZERO_RANGE seems to make the most
> > sense, as that'd work reasonably efficiently to initialize newly
> > allocated space as well as for zeroing out previously used file space.
> >
> >
> > As blkdev_issue_zeroout() already has a fallback path it seems this
> > should be doable without too much concern for which devices have write
> > zeroes, and which do not?
>
> Question: do you want the kernel to write zeroes even for devices that
> don't support accelerated zeroing?
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.
> Since I assume that if the fallocate fails you'll fall back to writing
> zeroes from userspace anyway...
And there's non-linux platforms as well, at least that's the rumor I hear.
> Second question: Would it help to have a FALLOC_FL_DRY_RUN flag that
> could be used to probe if a file supports fallocate without actually
> changing anything? I'm (separately) pursuing a fix for the loop device
> not being able to figure out if a file actually supports a particular
> fallocate mode.
Hm. I can see some potential uses of such a flag, but I haven't really
wished for it so far.
Greetings,
Andres Freund
next prev parent reply other threads:[~2021-01-04 19:11 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 [this message]
2021-01-04 19:57 ` Avi Kivity
2021-01-12 18:16 ` Christoph Hellwig
2021-01-12 18:39 ` Andreas Dilger
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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