Date: Fri, 29 Sep 2017 09:31:58 +0200 From: Ben RUBSON <ben.rubson@gmail.com> To: Freebsd fs <freebsd-fs@freebsd.org> Subject: Re: How to create holes in files ? Message-ID: <7E8BB109-C72C-4B5D-A2EF-630C6B34D8E6@gmail.com> In-Reply-To: <20170929080823.175d6abc@raksha.tavi.co.uk> References: <12ed413d-00d1-4182-bdd5-0e5230d46f01@email.android.com> <4D304428-E4AF-45D4-AB2E-D4B73A7FEE87@gmail.com> <1506664468.7204.37.camel@inparadise.se> <454261D6-0061-4DB9-8184-BDD7061870FB@gmail.com> <20170929080823.175d6abc@raksha.tavi.co.uk>
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> On 29 Sep 2017, at 09:08, Bob Eager <rde@tavi.co.uk> wrote: > > On Fri, 29 Sep 2017 08:26:13 +0200 > Ben RUBSON <ben.rubson@gmail.com> wrote: > >> Unfortunately here storage blocks will be set/written to 0, >> they will not be freed/recovered, a file hole will not be created. > > You need to add: conv=sparse to the dd command to turn zero blocks > into unallocated blocks. You're right Bob it works : # dd if=/dev/random of=f bs=1M count=4 4+0 records out # zfs userspace home POSIX User root 4.04M none # dd if=/dev/zero of=f bs=1M count=2 seek=2 conv=sparse 2+0 records out # zfs userspace home POSIX User root 2.04M none # ls -lh f -rw------- 1 root wheel 4.0M 29 Sep 09:25 f Unfortunately, using "conv=notrunc,sparse" to make a hole in the middle of a file does not seem to work, file is truncated : # rm f # dd if=/dev/random of=f bs=1M count=4 4+0 records out # dd if=/dev/zero of=f bs=1M count=1 seek=2 conv=notrunc,sparse 1+0 records out # ls -lh f -rw------- 1 root wheel 3.0M 29 Sep 09:26 f # zfs userspace home POSIX User root 3.04M none Anyway thank you for this tip, this is something to analyse ! Ben
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