On 09/01/11 18:30, Hugo Mills wrote:
No, subvolumes are a part of the whole filesystem. In btrfs, there
is only one filesystem. There are 6 main B-trees that store metadata
in btrfs (plus a couple of others). One of those is the "filesystem
tree" (or FS tree), which contains all the metadata associated with
the normal POSIX directory/file namespace (basically all the inode and
xattr data). When you create a subvolume, a new FS tree is created,
but it shares *all* of the other btrfs B-trees.
There is only one filesystem, but there may be distinct namespaces
within that filesystem that can be mounted as if they were
filesystems. Think of it more like NFSv4, where there's one overall
namespace exported per server, but clients can mount subsections of
it.
I think this explanation is still missing the key piece that has
confused me despite trying very hard to understand it by reading the
wiki. You talk about "Distinct Namespaces", but what I learnt from
further up the thread is that this "namespace" is also inside the the
"namespace" that makes up the whole filesystem. I mount the whole
filesystem, and all my subvolumes are automatically there (at least that
is what I find in practice). Its this duality of namespace that is the
difficult concept. I am still not sure of there is a default subvolume,
and the other subvolumes are defined within its namespace, or whether
there is an overall filesystem namespace and subvolumes defined within
it and if you mount the default subvolume you would then loose the
overall filesystem namespace and hence no longer see the subvolumes. I
find the wiki also confusing because it talks about subvolumes having to
be at the first level of the filesystem, but again further up this
thread there is an example which is used for real of it not being at the
first level, but at one level down inside a directory.
What it means is that I don't have a mental picture of how this all
works, and all use cases could then be worked out by following this
mental picture. I think it would be helpful if the Wiki contained some
of the use cases that we have been talking about in this thread - but
with more detailed information - like the actual commands used to mount
the filesystems like this, and information as to in what circumstances
you would perform each action.
The main awkward piece of btrfs terminology is the use of "RAID" to
describe btrfs's replication strategies. It's not RAID, and thinking
of it in RAID terms is causing lots of confusion. Most of the other
things in btrfs are, I think, named relatively sanely.
I don't find this AS confusing, although there is still information
missing which I asked in another post that wasn't answered. I still
can't understand if its possible to initialise a filesystem in degraded
mode. If you create the filesystem so that -m RAID1 and -d RAID1 but
only have one device - it implies that it writes two copies of both
metadata and data to that one device. However if you successfully
create the filesystem on two devices and then fail one and mount it -o
degraded it appears to suggest it will only write the one copy.
I was considering how to migrate from an existing mdmadm Raid1 /lvm
arrangement
I suppose I could fail one device of the mdm pair and initialise the
btrfs filesystem with this one device as the first half of a raid1
mirror and the other as a usb stick, then remove the usb stick and mount
the filesystem -o degraded. Copy data to it from the still working half
available lv and then dispose of mdmadm device completely and add in the
freed up device using btrfs device add
--
Alan Chandler
http://www.chandlerfamily.org.uk
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