From owner-freebsd-fs@freebsd.org Fri Sep 29 06:26:18 2017 Return-Path: Delivered-To: freebsd-fs@mailman.ysv.freebsd.org Received: from mx1.freebsd.org (mx1.freebsd.org [IPv6:2001:1900:2254:206a::19:1]) by mailman.ysv.freebsd.org (Postfix) with ESMTP id 50159E24A23 for ; Fri, 29 Sep 2017 06:26:18 +0000 (UTC) (envelope-from ben.rubson@gmail.com) Received: from mail-wr0-x236.google.com (mail-wr0-x236.google.com [IPv6:2a00:1450:400c:c0c::236]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (Client CN "smtp.gmail.com", Issuer "Google Internet Authority G2" (verified OK)) by mx1.freebsd.org (Postfix) with ESMTPS id DFB0A7FCB9 for ; Fri, 29 Sep 2017 06:26:17 +0000 (UTC) (envelope-from ben.rubson@gmail.com) Received: by mail-wr0-x236.google.com with SMTP id a43so615900wrc.0 for ; Thu, 28 Sep 2017 23:26:17 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20161025; h=mime-version:subject:from:in-reply-to:date :content-transfer-encoding:message-id:references:to; bh=qcq5VfOd0CMFOGS8oqTaDS7Zd78+22TseRCDFp9SL5o=; b=jbZo3J0/65t2EtcXTC/Z06Nh9UuJfgG0AmCp+R5WWOtsXjkTF4IofZyLjOa5zRk9F8 wiNK2V6KVMnLwYvOwuik2DVMMnHFrwQs6GsAefdIwxspAceIWQSDcL0PJnMIjwOQw4Em uV4yKt6iiHTaFhiHUo7OE7OFqSVD/ooISn5ctMPoKF4XaC9VVwAyIbYyU78nhTYYwqaA jmhzHi6p6f2prfBuLDyqOWeJU+xCy6fjUkQeyelr463R0HK3+Z76nY4dIoLzS4IWuWNK 08MtpWrDyBKsPDYF90ZWJ9XR/wtil2nKoRViXw6CNKOuWy5JTBP2ZIWbzGtdIMKyPym9 rjtg== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:mime-version:subject:from:in-reply-to:date :content-transfer-encoding:message-id:references:to; bh=qcq5VfOd0CMFOGS8oqTaDS7Zd78+22TseRCDFp9SL5o=; b=k8LOzXp2Wg5YyVjHoC5ogOJWb4HAuKgA6dJLTU3KNx6YxvCvG6qXczuGpUGSJe74kz 9WSV92CDy41NBPbJiINP97va58DqQQ56hRy0bSjb2itDlRmQwC0bL97HKm+9HSmFyOJO dt5fBrCwVU+VJt3BhJ9mmFM3JJhDjQDsyVVSG74/hVtMF0qZu4mC6zvdZm7f/PnmCTOn Il8orwxJFbbE5FtMNbvefuRskqTIpgkWefjwHpio3aRM4TC7uHfQZH+qDIQISoThbdyn wXcOxerIvpL6Lupy6ikPzz6zg09ALukmdCPJM4Xp0z8tLBv/SrlOJ0fmcIIXe1pZeKcP hfqw== X-Gm-Message-State: AHPjjUgkXomTGA7FnRVqms7xdzk/WvvvvA6OUPT1hwqM8bHv8Y/qgTn4 gtu9mcV3fJstDEJ5UZ2ztdj/p3bv X-Google-Smtp-Source: AOwi7QAqxQLn4NDRsPepUlbXHIjg6r/gTf1XHdroQsv//V3z9EEPuYUdyWtjzGMsivKrljSImYIaRw== X-Received: by 10.223.196.73 with SMTP id a9mr6260851wrg.144.1506666375924; Thu, 28 Sep 2017 23:26:15 -0700 (PDT) Received: from bens-mac.home (LFbn-MAR-1-445-220.w2-15.abo.wanadoo.fr. [2.15.38.220]) by smtp.gmail.com with ESMTPSA id 204sm2178326wml.10.2017.09.28.23.26.14 for (version=TLS1 cipher=ECDHE-RSA-AES128-SHA bits=128/128); Thu, 28 Sep 2017 23:26:15 -0700 (PDT) Content-Type: text/plain; charset=utf-8 Mime-Version: 1.0 (Mac OS X Mail 9.3 \(3124\)) Subject: Re: How to create holes in files ? From: Ben RUBSON In-Reply-To: <1506664468.7204.37.camel@inparadise.se> Date: Fri, 29 Sep 2017 08:26:13 +0200 Content-Transfer-Encoding: quoted-printable Message-Id: <454261D6-0061-4DB9-8184-BDD7061870FB@gmail.com> References: <12ed413d-00d1-4182-bdd5-0e5230d46f01@email.android.com> <4D304428-E4AF-45D4-AB2E-D4B73A7FEE87@gmail.com> <1506664468.7204.37.camel@inparadise.se> To: Freebsd fs X-Mailer: Apple Mail (2.3124) X-BeenThere: freebsd-fs@freebsd.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: Filesystems List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Fri, 29 Sep 2017 06:26:18 -0000 > On 29 Sep 2017, at 07:54, Karli Sj=C3=B6berg = wrote: >=20 > On tor, 2017-09-28 at 22:16 +0200, Ben RUBSON wrote: >>>=20 >>> On 28 Sep 2017, at 20:48, Karli Sj=C3=B6berg >>> wrote: >>>=20 >>> Den 28 sep. 2017 6:47 em skrev Ben RUBSON : >>>>=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.=20 >>>>> 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. >=20 > 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? Of course yes, you're right :) >> And the holes I need to create are not necessarily at the beginning >> of a file. >=20 > 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 to > "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: >=20 > # dd if=3D/dev/zero of=3D/foo/bar.bin bs=3D1M seek=3D7 >=20 > It=C2=B4ll make the file look like this inside (hope the ASCII gods = are with > me): > ____________ > |_______|_|__| Unfortunately here storage blocks will be set/written to 0, they will not be freed/recovered, a file hole will not be created. Below are some commands which demonstrate this. The last 3/4 commands show a _newly_ created file with a hole. # zfs userspace home POSIX User root 36.5K none # dd if=3D/dev/random of=3Df bs=3D1M count=3D4 4+0 records out # zfs userspace home POSIX User root 4.04M none # dd if=3D/dev/zero of=3Df bs=3D1M count=3D2 seek=3D2 2+0 records out # zfs userspace home POSIX User root 4.04M none # rm f # dd if=3D/dev/zero of=3Df bs=3D1M count=3D2 seek=3D2 2+0 records out # zfs userspace home POSIX User root 2.04M none # ls -lh f -rw------- 1 root wheel 4.0M 29 Sep 08:19 f