From mboxrd@z Thu Jan 1 00:00:00 1970 From: Martin Steigerwald Subject: Re: btrfs mount flags Date: Sat, 14 Apr 2012 00:05:08 +0200 Message-ID: <201204140005.08619.Martin@lichtvoll.de> References: (sfid-20120413_182119_379057_8476B95A) Mime-Version: 1.0 Content-Type: Text/Plain; charset="iso-8859-1" Cc: Kai Krakow To: linux-btrfs@vger.kernel.org Return-path: In-Reply-To: List-ID: Am Freitag, 13. April 2012 schrieb Kai Krakow: > Hello! Hi! > Is there any documentation about btrfs mount flags wrt: > > 1. which flags are one-time options and are permanent, > 2. which flags are global per btrfs partition, > 3. which flags are local per subvolume mount? > > I'm asking because while googling I found very confusing info about > autodefrag. Some say it is a global flag, others say to get autodefrag > one has to supply it for each subvolume mount to have autodefrag for > that subvolume. > > Then there's space_cache which is a one-time option and does not need > to be given on successive mounts. Otoh there's inode_cache which I > would expect to be handled the same but it doesn't look like it's > implemented that way. I like the distinction in Ext3/Ext4. There are mount option and filesystem features that can be activated at mkfs time or (some) later with tune2fs. Now I would expect a one time mount option to be a filesystem feature that I enabled with btrfstune or so. Cause I always understood mount options as temporary unless I write them into /etc/fstab. But then tune2fs / Ext also supports default mount options, so you could encode into the fs as well. There even is a btrfstune command and it has a switch to enable something: merkaba:~> btrfstune usage: btrfstune [options] device -S value enable/disable seeding but no manual page here. Seeding is mentioned at: Seed Device support It is now possible to create a filesystem to seed other Btrfs filesystems. The original filesystem and devices are included as a readonly starting point to the new FS. All modifications go onto different devices and the COW machinery makes sure the original is unchanged. http://btrfs.ipv5.de/index.php?title=Changelog#Seed_Device_support Anyway the documentation of mount options is on: http://btrfs.ipv5.de/index.php?title=Mount_options it seems. There only for "space_cache" it is mentioned that it is permanent. Not for "inode_cache". The mount manpage which usually has mount options of various filesystems does not mention BTRFS yet. I am not sure whether each filesystem should have an own manpage anyway. Whatever is used, I would like to have a clear distinction between permanent and volatile settings. And I think a permanent option is something which should be set and queryable by a tuning command. Ciao, -- Martin 'Helios' Steigerwald - http://www.Lichtvoll.de GPG: 03B0 0D6C 0040 0710 4AFA B82F 991B EAAC A599 84C7