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bug#11443: linux "cp" and ocfs2 reflink/clone/fastcopy/copy on write

From: Kai Petzke
Subject: bug#11443: linux "cp" and ocfs2 reflink/clone/fastcopy/copy on write
Date: Wed, 9 May 2012 16:28:27 +0200 (CEST)
User-agent: Alpine 2.00 (LNX 1167 2008-08-23)


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:

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!


Kai Petzke
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