btrbk/doc/FAQ.md

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2015-06-09 16:23:20 +02:00
Why is it not possible to backup '/' (btrfs root) ?
---------------------------------------------------
or in other words: why does this config not work:
/etc/btrbk/btrbk.conf:
volume /
subvolume /
snapshot_name rootfs
The answer for this is that btrbk is designed to never alter your
source subvolume. In the config above, the snapshots would be created
*inside* the source subvolume at: `/rootfs.20150101`.
How should I organize my btrfs filesystem?
------------------------------------------
There's lots of ways to do this, and each one of them has its reason
to exist. Make sure to read the [btrfs SysadminGuide on
kernel.org](https://btrfs.wiki.kernel.org/index.php/SysadminGuide) as
a good entry point.
<!-- TODO: add links to recommendations for ubuntu and other distros -->
## btrfs root
If your linux root filesystem is btrfs, I recommend booting linux from
a btrfs subvolume, and use the btrfs root only as a container for
subvolumes (i.e. NOT booting from "subvolid=0"). This has the big
advantage that you can choose the subvolume in which to boot by simply
switching the `rootflags=subvol=<subvolume>` kernel boot option.
Example (/boot/grub/grub.cfg):
menuentry 'Linux' {
linux /boot/vmlinuz root=/dev/sdb3 ro rootflags=subvol=rootfs quiet
}
menuentry 'Linux (testing)' {
linux /boot/vmlinuz root=/dev/sdb3 ro rootflags=subvol=rootfs_testing
}
Note that btrbk snapshots and backups are read-only, this means you
have to create a run-time (rw) snapshot before booting into it:
# btrfs subvolume snapshot /mnt/btr_pool/backup/btrbk/rootfs-20150101 /mnt/btr_pool/rootfs_testing
How do I convert '/' (subvolid=0) into a subvolume?
---------------------------------------------------
There's several ways to achieve this, the solution described below is
that it guarantees not to create new files (extents) on disk.
## Step 1: make a snapshot of your root filesystem
Assuming that '/' is mounted with `subvolid=0`:
# btrfs subvolume snapshot / /rootfs
Note that this command does NOT make any physical copy of the files of
your subvolumes within "/", it will only add some metadata.
## Step 2: make sure that `/rootfs/etc/fstab` is ok.
Add mount point for subvolid=0 to fstab, something like this:
/etc/fstab:
/dev/sda1 /mnt/btr_pool btrfs subvolid=0,noatime 0 0
## Step 3: boot from the new subvolume "rootfs".
Either:
- add `rootflags=subvol=rootfs` to grub.cfg
- set subvolume "rootfs" as default:
# btrfs subvolume set-default <subvolid> /
## Step 4: after reboot, check if everything went fine:
First check your **system log** for btrfs errors, then:
# btrfs subvolume show /
Name: rootfs
...
Great, this tells us that we just booted into our new snapshot!
# mount /mnt/btr_pool
# btrfs subvolume show /mnt/btr_pool
/mnt/btr_pool is btrfs root
This means that the root volume (subvolid=0) is correctly mounted.
#### Step 5: delete old (duplicate) files
Carefully delete all old files from `/mnt/btr_pool`, except "rootfs"
and all other subvolumes within "/". You can list all these by typing:
# btrfs subvolume list -a /mnt/btr_pool
Make sure you do NOT delete anything within the directories listed
here!
something like:
# cd /mnt/btr_pool
# rm -rf bin sbin usr lib var ...