On Tue, Dec 17, 2013 at 09:30:11AM +0100, Nicolas Michel wrote: > Mhmm. Thanks. I'm begining to understand ;) > Still : how can I see/know that id 5 is mapped to id 0 ? And why doing > this? For what purpose? (is it a default btrfs behavior or is it set > by the Ubuntu installer?)
It's always mapped. Internally, every tree is identified by a number. FS trees (e.g. subvolumes) start with numbers allocated dynamically from 256 upwards. Other trees (chunk tree, extent tree, and all the others) have fixed "well-known" numbers between 1 and 255, and the top-level FS tree is given the number 5. To make things marginally simpler for the user to remember, there's special-case code which looks at subvolume IDs passed from userspace, and converts 0 to 5. This is all btrfs behaviour -- nothing to do with Ubuntu. Hugo. > 2013/12/17 Chris Murphy <li...@colorremedies.com>: > > > > On Dec 16, 2013, at 4:54 PM, Nicolas Michel <be.nicolas.mic...@gmail.com> > > wrote: > > > >> OK. thanks for your pretty fast answer :) > >> > >> Now my last question is: in this case it was "easy" as I know that I > >> created all these subvolumes as parts of volume 0. But in the btrfs > >> subv list / I don't see any information that tells me they belongs to > >> id 0. If I have to debug a server/desktop and I don't know the > >> hierarchy that has been made, how can I know that my tmp subvolume is > >> indeed a child of id 0 ? > > > > When you do a subvol list it shows you what its top level is. Example: > > > > > > > > # btrfs subvol list / > > ID 256 gen 1047 top level 5 path root > > ID 258 gen 983 top level 5 path home > > ID 259 gen 983 top level 5 path data > > ID 276 gen 1012 top level 5 path root_ro > > > > > > root is mounted at / > > home is mounted at /hoome > > data is mounted at /data > > root_ro is not mounted at all > > > > > > #cd /data > > # btrfs subvol create data2 > > Create subvolume './data2' > > # btrfs subvol list / > > ID 256 gen 1047 top level 5 path root > > ID 258 gen 983 top level 5 path home > > ID 259 gen 1048 top level 5 path data > > ID 276 gen 1012 top level 5 path root_ro > > ID 277 gen 1048 top level 5 path data/data2 > > > > # btrfs subvol list /data > > ID 256 gen 1047 top level 5 path root > > ID 258 gen 983 top level 5 path home > > ID 259 gen 1048 top level 5 path data > > ID 276 gen 1012 top level 5 path root_ro > > ID 277 gen 1048 top level 259 path data2 > > > > > > So notice that "top level 5 data/data2" means the same as "top level 259 > > data2" because top level 259 implies data. > > > > You can also use btrfs subvol show <subvol> and it will give you more > > information including whether it's a snapshot, what the parent is; and if > > it's a parent that has snapshots it'll list the snapshots. > > > > # btrfs subvol show /data > > /data > > Name: data > > uuid: bc45f4be-51c9-2848-bb68-d6e922b8e2bd > > Parent uuid: - > > Creation time: 2013-12-12 16:18:13 > > Object ID: 259 > > Generation (Gen): 1048 > > Gen at creation: 11 > > Parent: 5 > > Top Level: 5 > > Flags: - > > Snapshot(s): > > # btrfs subvol show /data/data2 > > /data/data2 > > Name: data2 > > uuid: a66eddf9-107f-0448-a021-da417a982827 > > Parent uuid: - > > Creation time: 2013-12-16 17:11:36 > > Object ID: 277 > > Generation (Gen): 1048 > > Gen at creation: 1048 > > Parent: 259 > > Top Level: 259 > > Flags: - > > Snapshot(s): > > > > > > > > Chris Murphy > > > > > -- === Hugo Mills: hugo@... carfax.org.uk | darksatanic.net | lug.org.uk === PGP key: 65E74AC0 from wwwkeys.eu.pgp.net or http://www.carfax.org.uk --- You can't expect a boy to be depraved until he's gone to --- a good school.
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