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

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Date:   Wed, 13 Jan 2021 09:44:22 +0200
From:   Avi Kivity <avi@...lladb.com>
To:     Andres Freund <andres@...razel.de>,
        "Darrick J. Wong" <darrick.wong@...cle.com>
Cc:     Andreas Dilger <adilger@...ger.ca>,
        Christoph Hellwig <hch@...radead.org>,
        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 1/12/21 11:36 PM, Andres Freund wrote:
> Hi,
>
> On 2021-01-12 13:14:45 -0800, Darrick J. Wong wrote:
>> ALLOCSP64 can only allocate pre-zeroed blocks as part of extending EOF,
>> whereas a new FZERO flag means that we can pre-zero an arbitrary range
>> of bytes in a file.  I don't know if Avi or Andres' usecases demand that
>> kind of flexibilty but I know I'd rather go for the more powerful
>> interface.
> Postgres/I don't at the moment have a need to allocate "written" zeroed
> space anywhere but EOF. I can see some potential uses for more flexible
> pre-zeroing in the future though, but not very near term.
>

Same here.


I also agree that it's better not to have the kernel fall back 
internally on writing zeros, letting userspace do that. The assumption 
is that WRITE SAME will be O(1)-ish and so can bypass scheduling 
decisions, but if we need to write zeros, better let the application 
throttle the rate.