From mboxrd@z Thu Jan 1 00:00:00 1970 From: Hubert Kario Subject: Re: Create subvolume from a directory? Date: Tue, 01 May 2012 19:09:21 +0200 Message-ID: <8026880.aVIK1HJLHr@bursa01> References: <4F734250.5030009@libero.it> <4F73B9DC.3040402@cn.fujitsu.com> Mime-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Cc: kreijack@inwind.it, "Matthias G. Eckermann" , Alex , linux-btrfs@vger.kernel.org To: Liu Bo Return-path: In-Reply-To: <4F73B9DC.3040402@cn.fujitsu.com> List-ID: On Thursday 29 of March 2012 09:24:44 Liu Bo wrote: > On 03/29/2012 12:54 AM, Goffredo Baroncelli wrote: > > Could you elaborate which would be the issue ? > > "cp --reflink"-ing a file is not different than snapshotting a file= =2E In > > any case I could mount a snapshot and not the source subvolume. >=20 > We already have a debate about this "cross-link device": > http://comments.gmane.org/gmane.comp.file-systems.btrfs/9864 >=20 > "cp --reflink" will use clone feature, which can share data among fil= es, but > metadata is preserved individually. >=20 > My case is that I can mount both a subvolume and a snapshot via "-o > subvol=3Dxxx" or "-o subvolid=3Dxxx". And how is this different from regular snapshot of subvolume? In the en= d you=20 get two files pointing to same data on the disk while having different=20 metadata. Let me rephrase it: People don't want to be able to do: mount /dev/lvm/btrfs /mnt/a -t btrfs -o subvol=3DvolA mount /dev/lvm/btrfs /mnt/b -t btrfs -o subvol=3DvolB cp --reflink=3Dalways /mnt/a/file /mnt/b Just like you can't do hardlinks over `mount --bind` mountpoints, you=20 shouldn't be able to cp reflink over mountpoints. That's expected as th= is=20 *does* break VFS semantics. *But* people want to be able to do this:=20 mount /dev/lvm/btrfs /mnt/ -t btrfs btrfs subvol create /mnt/subvol big-file-creator > /mnt/subvol/BIG-file btrfs subvol snapshot /mnt/subvol /mnt/subvol-bak big-file-editor /mnt/subvol/BIG-file rm /mnt/subvol-bak/BIG-file cp --reflink=3Dalways /mnt/subvol/BIG-file /mnt/subvol-bak/BIG-file This does not cross any VFS boundaries. Regards, --=20 Hubert Kario QBS - Quality Business Software 02-656 Warszawa, ul. Ksawer=F3w 30/85 tel. +48 (22) 646-61-51, 646-74-24 www.qbs.com.pl -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" = in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html