GNU bug report logs - #11443
linux "cp" and ocfs2 reflink/clone/fastcopy/copy on write

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From: Kai Petzke <petzke <at> teltarif.de>
To: 11443 <at> debbugs.gnu.org
Cc: Georg Schmitz <schmitz <at> teltarif.de>
Subject: bug#11443: linux "cp" and ocfs2 reflink/clone/fastcopy/copy on write
Date: Wed, 9 May 2012 16:28:27 +0200 (CEST)

[Message part 1 (text/plain, inline)]

Hello,


there has been work by others about adding support for the OCFS2 "reflink" 
ioctl() call, which is similiar to the btrfs "clone" call, and creates a 
copy-on-write copy of the original, thus allowing to "copy" even gigabyte 
sized files within a tiny fraction of a second, and without using much 
additional file system space. See:
    http://lists.gnu.org/archive/html/coreutils/2011-08/msg00046.html
    http://lists.gnu.org/archive/html/bug-coreutils/2010-04/msg00185.html

I have updated those patches to work against coreutils 8.16, removed those 
bugs, that I spotted. In particular, if the destination file exists, the 
"reflink" ist automatically tried again after removing it, and if not all 
attributes are copied, it is made sure, that the following open() system 
call does not truncate the just created copy.


I strongly suggest including that patch in the coreutils package, even 
though the interface to use to different system calls to achieve the same 
thing is awkward. But, as laid out in the comments in the source, btrfs 
clone and ocfs2 reflink are semantically quite different, so that unifying 
them into one on the kernel side is not likely to happen, soon, if it 
happens at all. If users don't use the --reflink option of "cp", the 
additional code makes no difference, so it doesn't hurt. And if users use 
"--reflink" on either of the supported file systems, they get a huge 
advantage out of it!


Greetings


Kai Petzke
www.teltarif.de - Kommunikation ganz einfach

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[coreutils-8.16-ocfs2-reflink.diff (text/x-patch, attachment)]

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