Mailinglist Archive: opensuse (1620 mails)

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Re: [opensuse] When not to use Btrfs for /home?
On 11/18/2014 02:47 PM, John Andersen wrote:
On 11/18/2014 05:22 AM, Anton Aylward wrote:
You can set up a relation -- that is a HARD LINK --
between a file in the subvolume and elsewhere on the FS, possibly in
another subvolume.

Are you sure of this?

That thread is from 2010. It is now the end of 2014 and a lot more work
has been done on BtrFS since then.

Hardlinks did when I tried it back when I had a btrFS as everything all
on the one disk and nothing else and a number of subvolumes. Lined from
something under subvolume /home to subvolume /tmp.

The 'causes bugs' doesn't make sense.

If I had a single FS ext4Fs system I could hard link as in the example
above, so why not on BtrFS? It does say in the docs that subvolumes are
like directories :-)

The last post at your reference says

If btrfs is to support deduplication one day, it has to work across
subvolumes - otherwise, it won't be very useful.

I agree with that.

Right now we do have tools, I forget the name, which walks a FS and
looks for identical files to hardlink so as to reduce space used.
Are you saying those tools won't work on BtrFS?

I say they do. I say hard links do work. Here's my evidence.
Right now I have BtrFS as ROOT and it has zypper snapshotting enabled so
I have the following

# btrfs subvolume list /
ID 269 gen 343531 top level 5 path .snapshots
ID 541 gen 341419 top level 269 path .snapshots/1/snapshot
ID 542 gen 341419 top level 269 path .snapshots/2/snapshot
ID 544 gen 341419 top level 269 path .snapshots/3/snapshot
ID 545 gen 341419 top level 269 path .snapshots/4/snapshot
ID 546 gen 343373 top level 269 path .snapshots/5/snapshot
ID 547 gen 343378 top level 269 path .snapshots/6/snapshot
Mainbox:~ #

# ls -li /etc/hosts /.snapshots/6/snapshot/etc/hosts
863349 -rw-r--r-- 1 root root 1268 Jun 13 18:37 /etc/hosts
863349 -rw-r--r-- 1 root root 1268 Jun 13 18:37

# sum /etc/hosts /.snapshots/6/snapshot/etc/hosts
57395 2 /etc/hosts
57395 2 /.snapshots/6/snapshot/etc/hosts

# cksum /etc/hosts /.snapshots/6/snapshot/etc/hosts
1962092743 1268 /etc/hosts
1962092743 1268 /.snapshots/6/snapshot/etc/hosts

See, same size, same inode.
I believe that constitutes a hard link.

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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