Mailinglist Archive: opensuse-factory (837 mails)

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Re: [opensuse-factory] Can we see BTRFS as the default in the next version of openSUSE 11.5-12.0
  • From: Carl-Daniel Hailfinger <c-d.hailfinger.devel.2006@xxxxxxx>
  • Date: Mon, 21 Mar 2011 08:50:45 +0100
  • Message-id: <4D870355.3010107@gmx.net>
Am 20.03.2011 17:12 schrieb Jeff Mahoney:
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On 03/20/2011 11:25 AM, Carl-Daniel Hailfinger wrote:
Am 19.03.2011 22:21 schrieb Jeff Mahoney:
Honestly, the feature that has the most interest for me is the
subvolumes. The upshot is that you create one partition, create a btrfs
on it, and then create subvolumes. They all use the same shared storage
pool. You can add additional disks to expand it and all subvolumes have
access to the new space. This means I'm not staring at the 10 GB I have
free on /home while always doing rpm -e kernel-source during zypper
dup's because my / is too small.
Is there any advantage of btrfs over a bunch of ext3 filesystems on
different lvm logical volumes? The lvm pv is a storage pool, and I can
resize each lv (containing one ext3 each) as needed, so the problem you
described does not happen for me.
The problem I described *does* happen to you. You're just so accustomed
to the workaround that it doesn't seem like it's a problem anymore. That
process is still using statically defined volume sizes, but you're
resizing them when you need to shift space. That's not elastic. It's
multiple steps that modify both file system and storage, and it's not a
process I'd recommend to notice users. It's tough to not waste space
when shrinking a file system unless you shrink it lower than you intend,
shrink the block device, then grow it back up to the new limit. Another
step. Another opportunity for user confusion.

With btrfs, outside of the initial mkfs and a single command to create a
subvolume, there are no additional steps. Each logical file system
allocates space directly out of the same pool. df will show the same
amount of free space on all subvolumes. It's easy and allows us the
flexibility to have separate logical volumes without the hassle and risk
of allocating the wrong amount of space or having to jump through hoops
to change it later.

Thank you for the detailed explanation.
I had assumed that btrfs subvolumes would provide fs fault isolation, but it seems I was mistaken.

If you just want the elastic storage and don't need snapshots, emulating the subvolume feature on any filesystem should be as easy as mounting the filesystem to one of the desired mount moints, and then using mount --move to move upper-level directories of the filesystem to other desired mount points.

My usage pattern is having a separate filesystem for / so I can reformat+reinstall the OS instead of upgrading (saves a lot of time) and elastic storage for all my data (in /home and /storage).

Regards,
Carl-Daniel

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http://www.hailfinger.org/

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