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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.
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