From mboxrd@z Thu Jan 1 00:00:00 1970 From: Goffredo Baroncelli Subject: Re: Can you please define "snapshot" and "subvolume"? Date: Thu, 7 Oct 2010 20:08:23 +0200 Message-ID: <201010072008.23715.kreijack@libero.it> References: Reply-To: kreijack@libero.it Mime-Version: 1.0 Content-Type: Text/Plain; charset=utf-8 To: linux-btrfs@vger.kernel.org Return-path: In-Reply-To: List-ID: On Thursday, 07 October, 2010, Francis Galiegue wrote: > I have difficulties grabbing these two concepts. >=20 > As far as I can tell, a snapshot is an instant, synchronized, > photography of the filesystem at a given point in time; a subvolume i= s > a "subroot" to a btrfs filesystem. >=20 > While I fully understand (and use) the purpose of snapshots, I don't > quite fathom the use case for subvolumes, apart from btrfs-convert... > Why has btrfs grown such a feature in the first place? Can someone > give me a use case for them? By design in btrfs a snapshot is "a instant, synchronized, photography = of" a=20 subvolume. In fact you can snapshot only a subvolume [*].=20 Moreover the subvolumes have the following properties: 1) it is possible to mount a subvolume of a filesystem: if you execute = the=20 following commands: # mount -o subvol=3Dname-of-subvol /dev/sdxx /mn/test the kernel will use the subvolume "name-of-subvol" of the btrfs filesys= tem of=20 the partition /dev/sdxx. Pay attention: this work *only* if the subvolu= me=20 "name-of-subvol" is under the root of the filesystem. 2) a subvolume may be deleted asynchronously by the command "btrfs subv= ol=20 delete ". Pay attention that the deletion is performed not instan= taneous=20 but in background. In fact even though the subvolume disappear instanta= neous=20 the space is freed during the background removing. 3) If you have a subvolume into another one, and you snapshot the latte= r, in=20 the snapshot you cannot see the nested subvolume.=20 =46or example you have the root ('/') under a subvolume and the /home u= nder=20 another subvolume. If you snapshot the root, this snapshot will not con= tain=20 the subvolume /home. This may be useful if you want restore an old snap= shot of=20 the root filesystem without affecting the users homes directories. In the future some attributes (raid mode, compression) may be set per- subvolume basis. Regards G.Baroncelli [*] The btrfsctl utility doesn't return an error when you snapshot a=20 directory. But instead of snapshotting the directory you get a snapshot= of the=20 subvolume which contain the directory. >=20 > --=20 > Francis Galiegue, fgaliegue@gmail.com > "It seems obvious [...] that at least some 'business intelligence' > tools invest so much intelligence on the business side that they have > nothing left for generating SQL queries" (St=C3=A9phane Faroult, in "= The > Art of SQL", ISBN 0-596-00894-5) > -- > 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 >=20 --=20 gpg key@ keyserver.linux.it:Goffredo Baroncelli (ghigo) Key fingerprint =3D 4769 7E51 5293 D36C 814E C054 BF04 F161 3DC5 0512 -- 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