Mailinglist Archive: opensuse (1620 mails)

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Re: [opensuse] Btrfs.. Whats the difference
On 11/04/2014 06:53 PM, John Andersen wrote:
While installing 13.2 I let it use btrfs for root and xfs for /home partition.

Then you get these nebulous message that says some of the subvolumes of root
are shadowed by other filesystems.
That message means that you have a subvolume named /home defined in the /root
automatically, and also a /home
partition. Better language is needed for that error message. The term
"Duplicate directory" names comes to mind.
"Shadowed" is not very descriptive.

But then I began to wonder why btrfs defines subvolumes instead of letting
the system define sub-directories?

Whats the difference in functionality? Are these sub-volume sizes fixed? Is
it just part of the
snapshot process? What's up with that?

I'm sure the designers had something in mind and its quite possibly
documented somewhere. What follows is my thoughts and experience.

I did the installation as you described. Eventually the /home partition
corrupted irretrievably. Its not a conventional reiserFS restored from
a backup.

I think that the BtrFs model is that the FS should encompass the whole
of the system as one logical volume possibly across many partitions or
platters or spindles. It has that capability.

The subvolume mechanism, the manual says, can be treated like a
directory or a moutned FS. Perhaps that's what is confusing about it.
You can have a real (as in the XFS) mounted on /home or or ....

Well, in any other other FS if you don't mount the partition on /home
you can still create stuff there, and when you do the actual mount it
does overlay. "shadowed". So I asked myself why bother and cleared
out all the subvolumes.

OUCH! That mean that /usr got DELETED. The subvolumes are not just
markers. They really are directories.

Or are they? See https://lwn.net/Articles/579009/


*sigh* Re-install. "Learning experience".

This time I did without subvolumes and I can't say i missed them.


I'm sure that the idea behind BtrFS is that it can optimise the btree
and space in some fantastic way. I'm sure that it really wants the
whole of my 1T drive to be the BtrFS, including /boot and /swap. And
I'm sure it can mirror onto another, and I'm sure that the subvolumes
can be snapshotted and can then be used for a disk-to-disk-to-tape
backup. I'm certain that just as my LVM partitions can grow and grow
across spindles, so can BtrFS.

I am not, however, ready, to try installing a system on a single,
encompassing single BtrFS. I'll let someone else try it and report.
I'll also let someone else experiment with that BtrFS growing across
many spindles.

https://btrfs.wiki.kernel.org/index.php/Main_Page


What I might experiment with, though, is this

<quote
src="http://www.oracle.com/technetwork/articles/servers-storage-admin/advanced-btrfs-1734952.html";>
Redundant Configuration

With Btrfs, you no longer need to use mdadm to create mirrored volumes
or complex RAID configurations. These capabilities are built into the
file system.
</quote>

What I'm not clear on is how to extend an existing FS to become a BtrFS
style RAIDFS.






--
A: Yes.
> Q: Are you sure?
>> A: Because it reverses the logical flow of conversation.
>>> Q: Why is top posting frowned upon?

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