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

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Date:   Mon, 18 Jan 2021 20:44:04 -0700
From:   Andreas Dilger <adilger@...ger.ca>
To:     Avi Kivity <avi@...lladb.com>
Cc:     Andres Freund <andres@...razel.de>,
        "Darrick J. Wong" <darrick.wong@...cle.com>,
        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 Jan 13, 2021, at 12:44 AM, Avi Kivity <avi@...lladb.com> wrote:
> 
> 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.
>> 
> 
> 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.

Writing zeroes from userspace has a *lot* more overhead when there is a network
filesystem involved.  It would be better to generate the zeroes on the server,
or directly in the disk than sending GB of zeroes over the network.


Cheers, Andreas






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