Date: Fri, 29 Sep 2017 07:54:28 +0200 From: Karli =?ISO-8859-1?Q?Sj=F6berg?= <karli@inparadise.se> To: Ben RUBSON <ben.rubson@gmail.com>, Freebsd fs <freebsd-fs@freebsd.org> Subject: Re: How to create holes in files ? Message-ID: <1506664468.7204.37.camel@inparadise.se> In-Reply-To: <4D304428-E4AF-45D4-AB2E-D4B73A7FEE87@gmail.com> References: <12ed413d-00d1-4182-bdd5-0e5230d46f01@email.android.com> <4D304428-E4AF-45D4-AB2E-D4B73A7FEE87@gmail.com>
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On tor, 2017-09-28 at 22:16 +0200, Ben RUBSON wrote: > >=20 > > On 28 Sep 2017, at 20:48, Karli Sj=C3=B6berg <karli@inparadise.se> > > wrote: > >=20 > > Den 28 sep. 2017 6:47 em skrev Ben RUBSON <ben.rubson@gmail.com>: > > >=20 > > > On 28 Sep 2017, at 18:34, Bob Eager wrote: > > >=20 > > > On Thu, 28 Sep 2017 17:26:09 +0200 > > > Fabian Keil wrote: > > >=20 > > > >=20 > > > > Ben RUBSON wrote: > > > >=20 > > > > >=20 > > > > > I'm trying to make holes in files in C. > > > > > Goal is to deallocate huge files on ZFS while (randomly) > > > > > reading > > > > > them.=C2=A0 > > > > My interpretation of the above is that you want to create holes > > > > without changing the file size and without affecting data that > > > > is located before or after the holes that you want to create. > > > >=20 > > > > Otherwise you could simply "deallocate" the content with > > > > truncate(1). > > > If he doesn't mind copying the files, dd(1) will do the job. > > > However, I > > > expect that doesn't meet his criteria. > > Thank you Bob for your suggestion. > > You're right goal is to avoid copying data : free space would not > > necessarily > > allow this, and as the files I'm working on are some hundreds of GB > > in size, > > it would really be a counterproductive long stressing storage > > operation. > >=20 > > Well, correct me if I'm wrong, but wouldn't dd with "seek" mitigate > > the issue of writing out all of the data from beginning to end. If > > you seek from the beginning of the file to the point you want to > > start writing from and use bs to specify how large of a "hole" you > > want, the operation wouldn't take long at all. You would, in my > > opinion, achieve exactly what you want, to "create holes in files". > > Am I wrong? > I would have liked to do this in C. Yepp, I understand that, I was just bringing up the idea. But hey, if dd can do it, just look in the source of how it does it? > And the holes I need to create are not necessarily at the beginning > of a file. No, you _do not_ have to create holes in the beginning of a file, you can do it anywhere in the file you like. E.g. it=C2=B4s the trick I use t= o "clean" hardrives to make them look empty, by only erasing the first and last 10 MB of the drives, a whole JBOD only takes seconds to clean. Say you have a 10 MB large file and you want to make a 1 MB large hole somewhere in the middle of it, you do it like this: # dd if=3D/dev/zero of=3D/foo/bar.bin bs=3D1M seek=3D7 It=C2=B4ll make the file look like this inside (hope the ASCII gods are w= ith me): =C2=A0____________ |_______|_|__| Important to know is that "bs" determines the unit by wich you "seek"! I don=C2=B4t know if this might be a solution to your problem, just throw= ing it out there. /K > I randomly read parts of huge input files that I need to "free" on > the fly > to recover storage space for the output files. >=20 > Ben >=20 > _______________________________________________ > freebsd-fs@freebsd.org mailing list > https://lists.freebsd.org/mailman/listinfo/freebsd-fs > To unsubscribe, send any mail to "freebsd-fs-unsubscribe@freebsd.org"
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