Beginner's guide to BTRFS

[GreyGeek]

You may be missing the point. Let the install create the partitions.

Exactly.
If you already have a swap partition then let it alone. For the rest of your free space combine it into ONE partition and assign it to btrfs with a label of "/". Don't do any other fancy stuff. That's what is messing you up. Let the INSTALLER create @ and @home for you.

[SpecialEd]

...although this time round the partisions will all be there already...

You may be missing the point. Let the install create the partitions.

[Thomas00]

Agree, something went wrong, what it is I don’t know as it looks like what I did was exactly what you guys recommend. I may try the same thing again and hope for a different outcome...although this time round the partisions will all be there already...

[GreyGeek]

oshunluver stated the same conclusion that I had come to. In a correct install of btrfs there is NO need to edit /etc/fstab unless you want to create OTHER subvolumes on the <ROOT_FS> and mount them at boot, as I demoed with @Data in a post above.

My recommendation is to stop wasting any more time on this, clear your partitions as oshunluver says, and install Kubuntu Bionic with btrfs in the correct way.

[Thomas00]

Here's your problem. When installing to btrfs using manual partitioning, the way to do it is to set only the location for / and then the installer will automatically create the subvolumes. When using BTRFS you do not want or need separate partitions for / and /home as this defeats half or more of the reasons to use btrfs in the first place. A "Proper" BTRFS partitioning scheme is to have a swap partition, whatever partition(s) you need to boot (efi or whatever), and a single btrfs partition.

If you don't have a lot of time invested in this install, I would wipe the partitions, re-partition putting the separate btrfs partitions together, then re-install to just the sole btrfs partition and indicate only "/" when manually setting the installation target. Then you will get @ for / and @home for /home

Sorry, I wasn't precise in how I wrote this, what you are proposing is exactly what I did. Both / and /home are on the same partition.
The boot drive has got 3 partitions:
One for the boot loader (sda1), one for the OS (/ and /home) and a third one for swap:

thomas@homeserver:~$ blkid | grep sda
/dev/sda2: LABEL="OS" UUID="9c3b40b6-c716-11e8-a25e-bcaec52979b7" UUID_SUB="9de1419d-bce2-4df0-881a-18b321c698ed" TYPE="btrfs" PARTUUID="73f89684-7bb9-4719-aca5-235a51b8dd2c"
/dev/sda3: UUID="f6a03583-3a40-404f-897e-320de7e33746" TYPE="swap" PARTUUID="03c0c5de-c4ef-4642-bdd2-9a9715bb1880"

btrfs subvolume list comes back with nothing :-(

thomas@homeserver:~$ sudo btrfs subvolume list /
thomas@homeserver:~$

Maybe I just manually create a @ and @home subvolume, move things and see what happens?

As far as a fresh install is concerned, I spent all day yesterday to get the server and services installed, not sure if I go through the effort of a fresh install again or just leave things as they are, nothing won but also not more time wasted. This was supposed to be a quick "btrfs-convert and be done" type project...

[oshunluvr]

And as far as installation was concerned I didn't do anything special. Had to choose manual disk setup because I wanted to change to btrfs, one partition for / and /home with some spare space left for swap which I set up manually afterwards, and that was it. No other special bits whatsoever...

Here's your problem. When installing to btrfs using manual partitioning, the way to do it is to set only the location for / and then the installer will automatically create the subvolumes. When using BTRFS you do not want or need separate partitions for / and /home as this defeats half or more of the reasons to use btrfs in the first place. A "Proper" BTRFS partitioning scheme is to have a swap partition, whatever partition(s) you need to boot (efi or whatever), and a single btrfs partition.

If you don't have a lot of time invested in this install, I would wipe the partitions, re-partition putting the separate btrfs partitions together, then re-install to just the sole btrfs partition and indicate only "/" when manually setting the installation target. Then you will get @ for / and @home for /home

[oshunluvr]

Back to basics: the "@" is just used to help differentiate a subvolume from a regular folder. It's not a "rule" just a mnemonic device.

Subvolumes are like folders in a file cabinet. The btrfs file system is the drawer and the subvolumes are the folders. If one folder needs more space the other just move down the drawer and make room.

So to identify subvolumes, do what GG says and use the subvolume list command.One thing I think GG may have wrong is you may not have any subvolumes at all, which normally the *buntu installers use - so I don't know what happened. It appears that the installation went to the root BTRFS volume instead of into subvolumes. Subvolumes are important because without then you cannot use the snapshot or backup (send|receive) features of btrfs.

[Thomas00]

Really strange. Well, I added the subvol tag to fstab and ran the below after a reboot. No change.

thomas@homeserver:~$ cat /etc/fstab         
UUID=9c3b40b6-c716-11e8-a25e-bcaec52979b7       /               btrfs defaults,ssd,autodefrag,noatime,subvol=@ 0 0
UUID=aa3dceed-ea65-4e14-87c8-ad93810e556c       /media/data/    btrfs   defaults,autodefrag 0 0  
UUID=e8ec2376-3087-42e4-a73d-26983ff37bbc       /media/backup   btrfs   noauto  0       0
UUID=f6a03583-3a40-404f-897e-320de7e33746       none            swap sw 0 0 
thomas@homeserver:~$ sudo mount -t btrfs /dev/disk/by-uuid/9c3b40b6-c716-11e8-a25e-bcaec52979b7 /mnt
thomas@homeserver:~$ ll /mnt
total 48
drwxr-xr-x 1 root   root    224 Oct  3 14:16 ./
drwxr-xr-x 1 root   root    224 Oct  3 14:16 ../
drwxr-xr-x 1 root   root   2430 Oct  3 14:38 bin/
drwxr-xr-x 1 root   root    620 Oct  3 14:39 boot/
...

thomas@homeserver:~$ sudo umount /dev/disk/by-uuid/9c3b40b6-c716-11e8-a25e-bcaec52979b7
thomas@homeserver:~$ sudo btrfs subvolume show /
/
     Name:                   <FS_TREE>
     UUID:                   -
     Parent UUID:            -
     Received UUID:          -
     Creation time:          -
     Subvolume ID:           5
     Generation:             3565
     Gen at creation:        0
     Parent ID:              0
     Top level ID:           0
     Flags:                  -
     Snapshot(s):
thomas@homeserver:~$ sudo btrfs subvolume show /home
ERROR: not a subvolume: /home
ERROR: Failed to get subvol info /home: 1
thomas@homeserver:~$

And as far as installation was concerned I didn't do anything special. Had to choose manual disk setup because I wanted to change to btrfs, one partition for / and /home with some spare space left for swap which I set up manually afterwards, and that was it. No other special bits whatsoever...

[GreyGeek]

I see you got “subvol=@“ and @home options in your fstab. You first mount the actual drive to @ and then further down, not show in your fstab screenshot, you map @ to /mnt, correct?
I’ll try that tomorrow...

Yes, but I didn't write that. The Kubuntu Bionic install wrote it when I installed it.

[Thomas00]

I see you got “subvol=@“ and @home options in your fstab. You first mount the actual drive to @ and then further down, not show in your fstab screenshot, you map @ to /mnt, correct?
I’ll try that tomorrow...

[GreyGeek]

blkid:

# blkid
/dev/sda1: UUID="[B]00fa8116-00d7-4611-9603-434769265d10[/B]" UUID_SUB="b7f30ed6-bdeb-4100-9c68-75d9e0fdbe32" TYPE="btrfs" PARTUUID="e00dfb49-01"
/dev/sdb1: LABEL="sdb1" UUID="17f4fe91-5cbc-46f6-9577-10aa173ac5f6" UUID_SUB="4d5f96d5-c6c6-4183-814b-88118160b615" TYPE="btrfs" PARTUUID="5fa5762c-9d66-4fdf-ba8f-5c699763e636"
/dev/sdc1: LABEL="sdc1" UUID="b3131abd-c58d-4f32-9270-41815b72b203" UUID_SUB="498bde29-97bd-4282-b246-c1d20368b1da" TYPE="btrfs" PARTUUID="6fe79cf9-2de8-4fab-8f68-51d6e092b2a2"

I'll mount my system to /mnt:

root@GreyGeek:~# mount -t btrfs /dev/disk/by-uuid/00fa8116-00d7-4611-9603-434769265d10 /mnt

and do
# btrfs subvolume show /mnt

/
        Name:                   <FS_TREE>
        UUID:                   -
        Parent UUID:            -
        Received UUID:          -
        Creation time:          -
        Subvolume ID:           5
        Generation:             102496
        Gen at creation:        0
        Parent ID:              0
        Top level ID:           0
        Flags:                  -
        Snapshot(s):

However, when I do
# btrfs subvolume show /
I get:

@
        Name:                   @
        UUID:                   76717daa-fcda-824a-b3cb-324e68742ad4
        Parent UUID:            f7f19158-6625-d548-8b60-174fb1fc22b5
        Received UUID:          -
        Creation time:          2018-09-15 10:32:32 -0500
        Subvolume ID:           841
        Generation:             109789
        Gen at creation:        85003
        Parent ID:              5
        Top level ID:           5
        Flags:                  -
        Snapshot(s):
                                snapshots/@20180927

And when I do:
# btrfs subvolume show /home
I get:

@home
        Name:                   @home
        UUID:                   c53c690d-869a-4e40-89c0-5f83130e193c
        Parent UUID:            63924e8d-0ce2-5b46-ae6d-8aa4ec3a98c3
        Received UUID:          -
        Creation time:          2018-09-15 10:32:44 -0500
        Subvolume ID:           842
        Generation:             109799
        Gen at creation:        85004
        Parent ID:              5
        Top level ID:           5
        Flags:                  -
        Snapshot(s):
                                snapshots/@home20180927

because @ and @home are mounted to / and /home in /etc/fstab. The disk is not mounted in /etc/fstab, only the subvolumes. Note that the uuid's given in the show command are not the ones in the /etc/fstab file or given by blkid.

BUT, not giving an argument to the show command
# btrfs subvolume show
gives:

btrfs subvolume show: too few arguments
usage: btrfs subvolume show [options] <subvol-path>|<mnt>

    Show more information about the subvolume

    -r|--rootid   rootid of the subvolume
    -u|--uuid     uuid of the subvolume

    If no option is specified, <subvol-path> will be shown, otherwise
    the rootid or uuid are resolved relative to the <mnt> path.

which is NOT what you get.

My /etc/fstab contains

#
# <file system> <mount point>   <type>  <options>       <dump>  <pass>
# / was on /dev/sda1 during installation
UUID=[B]00fa8116-00d7-4611-9603-434769265d10[/B] /               btrfs   defaults,ssd,autodefrag,noatime,subvol=@ 0       1
# /home was on /dev/sda1 during installation
UUID=[B]00fa8116-00d7-4611-9603-434769265d10[/B] /home           btrfs   defaults,ssd,autodefrag,noatime,subvol=@home 0       2
#/swapfile                                 none            swap    sw              0       0

[Thomas00]

I think I still haven't understood this @ business. How can you tell it's a subvolume? There is no mentioning of it in the btrfs filesystem output?
I do have other btrfs drives which I converted a while back, maybe they get in the way?

UUID=9c3b40b6-c716-11e8-a25e-bcaec52979b7 / btrfs defaults 0 0
UUID=aa3dceed-ea65-4e14-87c8-ad93810e556c /media/data/ btrfs defaults,autodefrag 0 0
UUID=e8ec2376-3087-42e4-a73d-26983ff37bbc /media/backup btrfs noauto 0 0
UUID=f6a03583-3a40-404f-897e-320de7e33746 none swap sw 0 0

Edit1: I tried the sudo btrfs filesystem usage / command on my laptop and it shows similar output to the one on my server, no mentioning of @ or @/home.
Edit2: Ran btrfs subvolume show / and btrfs subvolume show /media/data and both show the same output:

root@homeserver:~# btrfs subvolume show /media/data
/
Name: <FS_TREE>
UUID: -
Parent UUID: -
Received UUID: -
Creation time: -
Subvolume ID: 5
Generation: 41930
Gen at creation: 0
Parent ID: 0
Top level ID: 0
Flags: -
Snapshot(s):

Here's btrfs filesystem show

root@homeserver:~# btrfs filesystem show
Label: 'OS' uuid: 9c3b40b6-c716-11e8-a25e-bcaec52979b7
Total devices 1 FS bytes used 2.89GiB
devid 1 size 45.90GiB used 3.52GiB path /dev/sde2

Label: 'Data' uuid: aa3dceed-ea65-4e14-87c8-ad93810e556c
Total devices 3 FS bytes used 3.12TiB
devid 1 size 3.64TiB used 2.09TiB path /dev/sda
devid 2 size 3.64TiB used 2.09TiB path /dev/sdb
devid 3 size 3.64TiB used 2.09TiB path /dev/sdc

Label: 'Backup' uuid: e8ec2376-3087-42e4-a73d-26983ff37bbc
Total devices 1 FS bytes used 494.88GiB
devid 1 size 1.82TiB used 597.02GiB path /dev/sdd1

[GreyGeek]

Well, it's a btrfs subvolume alright, but it is missing @ and @home in the root file system. What's in your /etc/fstab file?

[Thomas00]

Here you go:

root@homeserver:~# btrfs filesystem usage /
Overall:
Device size: 45.90GiB
Device allocated: 3.52GiB
Device unallocated: 42.38GiB
Device missing: 0.00B
Used: 2.80GiB
Free (estimated): 42.69GiB (min: 42.69GiB)
Data ratio: 1.00
Metadata ratio: 1.00
Global reserve: 16.00MiB (used: 0.00B)

Data,single: Size:3.01GiB, Used:2.69GiB
/dev/sde2 3.01GiB

Metadata,single: Size:520.00MiB, Used:112.23MiB
/dev/sde2 520.00MiB

System,single: Size:4.00MiB, Used:16.00KiB
/dev/sde2 4.00MiB

Unallocated:
/dev/sde2 42.38GiB
root@homeserver:~#

[GreyGeek]

What does
sudo btrfs filesystem usage /
show you?