Date: Fri, 29 Sep 2017 00:21:16 -0700 From: Bakul Shah <bakul@bitblocks.com> To: Ben RUBSON <ben.rubson@gmail.com> Cc: Freebsd fs <freebsd-fs@freebsd.org> Subject: Re: How to create holes in files ? Message-ID: <20170929072131.68594156E525@mail.bitblocks.com> In-Reply-To: Your message of "Thu, 28 Sep 2017 16:39:28 %2B0200." <44E32501-4213-4A09-992C-92DB4EF33C0C@gmail.com> References: <44E32501-4213-4A09-992C-92DB4EF33C0C@gmail.com>
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On Thu, 28 Sep 2017 16:39:28 +0200 Ben RUBSON <ben.rubson@gmail.com> wrote: Ben RUBSON writes: > Hi, > > I'm trying to make holes in files in C. > Goal is to deallocate huge files on ZFS while (randomly) reading them. > > The only thing I found is on Linux, fallocate(2) with FALLOC_FL_PUNCH_HOLE. > > What about FreeBSD ? You can copy such a file using rsync with -S or --sparse option or dd with conv=sparse option. I don't know if these programs use seek with SEEK_HOLE and SEEK_DATA to avoid reading existing holes in the source file as zeroes. Someone mentioned TRIM support but AFAIK this is only used (if available) when a file is freed. Replacing a zero block write with lseek will impact every write (you have to check if the block is all zeroes) so not worth doing in the general case.
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