Received-SPF: pass (google.com: domain of linux-ext4-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Subject: Re: fallocate(FALLOC_FL_ZERO_RANGE_BUT_REALLY) to avoid unwritten
 extents?
To:     Andres Freund <andres@anarazel.de>,
        "Darrick J. Wong" <darrick.wong@oracle.com>
Cc:     Andreas Dilger <adilger@dilger.ca>,
        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
References: <20201230062819.yinrrp6uwfegsqo3@alap3.anarazel.de>
 <20210104181958.GE6908@magnolia>
 <20210104191058.sryksqjnjjnn5raa@alap3.anarazel.de>
 <f6f75f11-5d5b-ae63-d584-4b6f09ff401e@scylladb.com>
 <20210112181600.GA1228497@infradead.org>
 <C8811877-48A9-4199-9F28-20F5B071AE36@dilger.ca>
 <20210112184339.GA1238746@infradead.org>
 <1C33DEE4-8BE9-4BF3-A589-E11532382B36@dilger.ca>
 <20210112211445.GC1164248@magnolia>
 <20210112213633.fb4tjlgvo6tznfr4@alap3.anarazel.de>
From:   Avi Kivity <avi@scylladb.com>
Message-ID: <6d982635-d978-e044-4cca-c140401eb0d3@scylladb.com>
Date:   Wed, 13 Jan 2021 09:44:22 +0200
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101
 Thunderbird/78.6.0
MIME-Version: 1.0
In-Reply-To: <20210112213633.fb4tjlgvo6tznfr4@alap3.anarazel.de>
Content-Type: text/plain; charset=utf-8; format=flowed
Content-Transfer-Encoding: 7bit
Content-Language: en-US
Precedence: bulk

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.