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From:   Andreas Dilger <adilger@dilger.ca>
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Subject: Re: fallocate(FALLOC_FL_ZERO_RANGE_BUT_REALLY) to avoid unwritten
 extents?
Date:   Mon, 18 Jan 2021 20:44:04 -0700
In-Reply-To: <6d982635-d978-e044-4cca-c140401eb0d3@scylladb.com>
Cc:     Andres Freund <andres@anarazel.de>,
        "Darrick J. Wong" <darrick.wong@oracle.com>,
        Christoph Hellwig <hch@infradead.org>,
        linux-fsdevel@vger.kernel.org, linux-xfs@vger.kernel.org,
        linux-ext4@vger.kernel.org, linux-block@vger.kernel.org
To:     Avi Kivity <avi@scylladb.com>
References: <20201230062819.yinrrp6uwfegsqo3@alap3.anarazel.de>
 <20210104181958.GE6908@magnolia>
 <20210104191058.sryksqjnjjnn5raa@alap3.anarazel.de>
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 <C8811877-48A9-4199-9F28-20F5B071AE36@dilger.ca>
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 <1C33DEE4-8BE9-4BF3-A589-E11532382B36@dilger.ca>
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On Jan 13, 2021, at 12:44 AM, Avi Kivity <avi@scylladb.com> wrote:
>=20
> On 1/12/21 11:36 PM, Andres Freund wrote:
>> Hi,
>>=20
>> 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.
>>=20
>=20
> 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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