Replacing a Btrfs drive - best practices?

[Ken D'Ambrosio]

On 2017-11-07 00:11, Matthew Crews wrote:
> As a follow-up, here is the method I used that worked fairly well.
> This allowed me to move my Btrfs /home partition from a stand-alone
> disk to a new Btrfs raid1 array.
> 
> 
> 1. Make a Raid1 Btrfs filesystem
> 
> # mkfs.btrfs -m raid1 -d raid1 /dev/sdc /dev/sdd
> where sdc and sdd are my new drives, sda is my existing / partition
> and sdb is my existing /home
> 
> 
> 2. Mount the Raid1 array
> 
> # mkdir /media/homebackup
> # mount /dev/sdc /media/homebackup
> 
> 
> 3. Make a snapshot of my existing /home
> 
> # mount /dev/sdb1 /mnt
> # btrfs subvolume snapshot /mnt/@home /mnt/@home_backup
> 
> Ubuntu Btrfs documentation recommends mounting Btrfs volumes to /mnt
> for ease of working with. See:
> https://help.ubuntu.com/community/btrfs
> 
> Might as well make a backup of / (root) too. Just in case something
> gets mucked up in fstab later.
> # mount /dev/sda1 /mnt
> # btrfs subvolume snapshot /mnt/@ /mnt/@_backup
> 
> 4. Send the snapshot to the new drive
> 
> # btrfs send /mnt/@home_backup | pv -b -a | btrfs receive 
> /media/homebackup
> 
> This part can take several hours, depending on the amount of data that
> needs to be sent.
> 
> 
> 5. Make new snapshot, wdevid    2 size 1.82TiB used 1.21TiB path 
> /dev/sdd
> 
> Label: none  uuid: 56b012cf-a870-4105-88e7-6f5806738e8d
> 	Total devices 1 FS bytes used 1.21TiB
> 	devid    1 size 1.82TiB used 1.58TiB path /dev/sdb1
> 
> hich resets to writable
> 
> # btrfs subvolume snapshot /media/homebackup/@home_backup
> /media/homebackup/@home
> 
> 
> 6. Edit /etc/fstab
> 
> The simple way is to change the UUID to point to the new drive(s), and
> have the old drive point to a new location. Something like:
> 
> UUID=XXX /media/homebackup btrfs defaults,subvol=@home,noauto 0 2
> UUID=XXX /home btrfs noatime,nodiratime,autodefrag,subvol=@home 0 3
> 
> 
> 7. Reboot and verify
> 
> If all goes according to plan, the new raid array will mount as /home
> upon boot, and the old drive will not be mounted. In my case:
> 
> matthew at matt-linux-desktop:~$ df -k
> Filesystem              1K-blocks       Used Available Use% Mounted on
> /dev/sda1               470807552    8777432 460347848   2% /
> /dev/sdc               1953514584 1302434304 650683152  67% /home
> /dev/sdc               1953514584 1302434304 650683152  67% /mnt
> 
> matthew at matt-linux-desktop:~$ sudo lsblk -f
> NAME           FSTYPE   LABEL UUID                                 
> MOUNTPOINT
> sda
> ├─sda1         btrfs          dde09d67-fe28-4a79-ac1d-3dec0ef486ca /
> ├─sda2
> └─sda5         swap           21c78577-95f6-49fb-be38-862013ec9252
>   └─cryptswap1 swap           e3c2d3de-e2d0-4fba-9fda-d929aea3167b 
> [SWAP]
> sdb
> └─sdb1         btrfs          56b012cf-a870-4105-88e7-6f5806738e8d
> sdc            btrfs          d3c8cb5d-5394-477b-bb1f-5cc78316800e /mnt
> sdd            btrfs          d3c8cb5d-5394-477b-bb1f-5cc78316800e
> 
> sdb1 is currently not mounted, while sdc is mounted at /home and /mnt.
> Due to a quirk with Btrfs raid arrays, sdd will appear to be
> unmounted. However you can verify that the raid array is working:
> 
> matthew at matt-linux-desktop:~$ sudo btrfs filesystem show
> Label: none  uuid: d3c8cb5d-5394-477b-bb1f-5cc78316800e
> 	Total devices 2 FS bytes used 1.21TiB
> 	devid    1 size 1.82TiB used 1.21TiB path /dev/sdc
> 	devid    2 size 1.82TiB used 1.21TiB path /dev/sdd
> 
> 
> 
> 
> Hope this helps anyone else out there!

An excellent write-up and outcome!  Thanks for taking the time to reply 
in detail -- that's truly a Google-worthy e-mail there.

I should, alas, mention one thing for said Googlers, though: btrfs-based 
RAID-0 and RAID-1 (and RAID-10) are superfine for deployment; 
btrfs-based RAID-5/RAID-6, however... not so much.  When I need to use 
those, I use mdadm to create the RAID, then install standalone btrfs on 
the mdadm virtual disk.  This doesn't allow the RAID to leverage btrfs's 
features, which is a bummer, but it *does* make sure your data's safe.

Glad everything went swimmingly!

-Ken