In my case, I am backing up a system spanning five drives formatted
btrfs, on a separate drive containing a separate backup volume and
multiple complete backups, each from a different point in time. This
gives me protection from filesystem corruption, since the backups are on
a separate volume, also protection from accidental deletion and other
such issues by having backups spread over time going back as far as
three months. It also makes things very simple since I can just mount
one of these backup subvolumes in place of the original and immediately
be up and running. Of course, all btrfs volumes could die at once as a
result of some obscure problem such as a poison update or something like
that, and that is why I keep a constantly updated JFS (and period
backups to bluray) copy on hand. I realize this is not foolproof, and
actually plan to extend it further. But huge drives are terribly
inexpensive right now and this is one way I can take advantage of that.
Of course I could have done this using multiple partitions, and I may
one day regret not doing it that way for the very reasons you point out.
However, I believe that I am sufficiently protected at this point to
take the risk. I really figure that if something were to corrupt both
my main system AND the backup volume at the same time, it would probably
knock out separate partitions as well. But ... perhaps not. But I am
indeed aware that one filesystem corruption could knock out all of those
backups in one sweep. As for the umount issue, it is ridiculously easy
to script around, it just seems like somebody, either on the util-linux
side, or on the btrfs side, could provide a more elegant solution, but
it seems to fly in the face of entrenched ideologies on both sides.
Fortunately, my only real problem that I can't script around is the boot
issue and that is hopefully, just a matter of time before it gets
fixed. Thanks for your thoughts, George
On 05/21/2013 01:16 AM, Michael Johnson - MJ wrote:
I realize I am a bit late to the party on this, and but I would like
to understand the details of the workflow you are describing as I am
not seeing the benefit to creating backups on different subvolumes
with btrfs (but that's not to say there aren't reasons).
The way I've gone about things is to have one btrfs volume mounted at
say /mnt/brtfs, backup to it, and then creates a read-only snapshots
in /mnt/btrfs/.snapshots. I think this gives me all the benefits of
what you are describing without any of the hassle.
Now, if my btrfs were to become corrupted, I would loose all of my
backups, but I believe you would be in the same boat using
completely separate subvolumes as they still part of the same
underlying data structure. You would have a similar issue with zfs I
believe.
With more tradition filesystems where the volume management is
separate from the filesystem, having 2 separate instances of say xfs,
on lvm volumes or even different partitions, your filesystem
corruption would not spread from volume A to volume B so that level of
separation makes sense. But with btrfs, treating the subvolumes like
this does not provide such protection, If this is the reason for your
workflow, it may be simply that it used to provide some benefit, but
that benefit is gone. As such I think you can actually simplify your
workflow and utilize btrfs more fully.
That being said, I don't really know your workflow and the reasons for
it, so it is quite possibly a reasonable thing to keep doing. I
simply don't have enough information and know that I have often time
found my self annoyed with a change, spent a lot of time to working
around the change, and then later realized, but I just was stuck in an
old way of thinking and that the new way allowed a more elegant workflow.
But that is all just food for thought. I do agree that if I can say
"mount LABEL=foo" it would be expected that I could also say "umount
LABEL=foo". Perhaps the right thing to do would be to have modify the
btrfs command to allow 'btrfs mount' and 'btrfs umount' similar to the
way zfs works as this would allow some fun magic to happen outside of
util-linux.
In any case, I hope my thoughts are at least a little useful. Cheers!
On Sun, May 19, 2013 at 7:49 AM, George Mitchell <george@xxxxxxxxxxx
<mailto:george@xxxxxxxxxxx>> wrote:
In reply to both of these comments in one message, let me give you
an example.
I use shell scripts to mount and unmount btrfs volumes for backup
purposes. Most of these volumes are not listed in fstab simply
because I do not want to have to clutter my fstab with volumes
that are used only for backup. So the only way I can mount them
is either by LABEL or by UUID. But I can't unmount them by either
LABEL or UUID because that is not supported by util-linux and they
have no intention of supporting it in the future. So I have to
resort to unmounting by directory and it becomes back and forth
between LABEL and directory which becomes very confusing when you
are dealing with complex shell scripts. This is intolerable for
me so I use a kludge that allows me to first translate from LABEL
to device and then unmount by device. To me it just seems klutzy
that one has to resort to these sorts of games to use a file
system that is supposed to be an improvement on what we already
have. A simple virtual volume identifier would resolve that.
Doing the same for subvolumes would be nice, but I could live
without it with no problem. I have worked with nixes for 30 years
beginning with AT&T pre-SRV on DEC-1170s and have seen a lot of
changes since those days, most of them for the better. But, while
functionality is mandatory, convenience is always appreciated and
can help avoid costly mistakes and save time. As I stated in my
original post, I KNOW and appreciate that all of you are working
hard on things that matter far more than this trivial item. But
it is a major convenience and clarity issue for me and I am sure
it will be for others as well. It is only rational that one
should be able to expect to mount by LABEL and unmount by LABEL,
but that doesn't work, and a major part of the reason that doesn't
work is that btrfs does not conform to the pattern of just about
every other file system on the planet in regards to how it treats
mount points. And this is not even to mention all the other
issues involved like a large number of utilities that have no way
of knowing that a given partition is mounted, which would also be
resolved by virtual mount points since many if not most of those
utilities understand and process virtual volume identifiers.
Please just do me a favor and think about this a bit before you
just write it off.
- George
On 05/19/2013 04:04 AM, Martin wrote:
On 10/05/13 15:03, George Mitchell wrote:
One the things that is frustrating me the most at this
point from a user
perspective ... The current method of simply using a
random member device or a LABEL or a UUID is just not
working well for
me. Having a well thought out virtual device
infrastructure would...
Sorry, I'm a bit lost for your comments...
What is your use case and what are you hoping/expecting to see?
I've been following development of btrfs for a while and I'm
looking
forward to use it to efficiently replace some of the very useful
features of LVM2, drbd, and md-raid that I'm using at present...
OK, so the way of managing all that is going to be a little
different.
How would you want that?
Regards,
Martin
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On 05/19/2013 04:15 AM, Roman Mamedov wrote:
On Fri, 10 May 2013 07:03:38 -0700
George Mitchell <george@xxxxxxxxxxx
<mailto:george@xxxxxxxxxxx>> wrote:
One the things that is frustrating me the most at this
point from a user
perspective regarding btrfs is the current lack of virtual
devices to
describe volumes and subvolumes.
From a user perspective btrfs subvolumes have a lot in common
with just
regular directories aka folders, and nothing in common with
(block)devices.
"Describing them with virtual devices" does not seem to make a
whole lot of
sense.
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