On 2017-10-31 14:51, Andrei Borzenkov wrote:
31.10.2017 20:45, Austin S. Hemmelgarn пишет:
On 2017-10-31 12:23, ST wrote:
Hello,
I've recently learned about btrfs and consider to utilize for my needs.
I have several questions in this regard:
I manage a dedicated server remotely and have some sort of script that
installs an OS from several images. There I can define partitions and
their FSs.
1. By default the script provides a small separate partition for /boot
with ext3. Does it have any advantages or can I simply have /boot
within / all on btrfs? (Note: the OS is Debian9)
It depends on the boot loader. I think Debian 9's version of GRUB has
no issue with BTRFS, but see the response below to your question on
subvolumes for the one caveat.
2. as for the / I get ca. following written to /etc/fstab:
UUID=blah_blah /dev/sda3 / btrfs ...
So top-level volume is populated after initial installation with the
main filesystem dir-structure (/bin /usr /home, etc..). As per btrfs
wiki I would like top-level volume to have only subvolumes (at least,
the one mounted as /) and snapshots. I can make a snapshot of the
top-level volume with / structure, but how can get rid of all the
directories within top-lvl volume and keep only the subvolume
containing / (and later snapshots), unmount it and then mount the
snapshot that I took? rm -rf / - is not a good idea...
There are three approaches to doing this, from a live environment, from
single user mode running with init=/bin/bash, or from systemd emergency
mode. Doing it from a live environment is much safer overall, even if
it does take a bit longer. I'm listing the last two methods here only
for completeness, and I very much suggest that you use the first (do it
from a live environment).
Regardless of which method you use, if you don't have a separate boot
partition, you will have to create a symlink called /boot outside the
subvolume, pointing at the boot directory inside the subvolume, or
change the boot loader to look at the new location for /boot.
From a live environment, it's pretty simple overall, though it's much
easier if your live environment matches your distribution:
1. Create the snapshot of the root, naming it what you want the
subvolume to be called (I usually just call it root, SUSE and Ubuntu
call it @, others may have different conventions).
2. Delete everything except the snapshot you just created. The safest
way to do this is to explicitly list each individual top-level directory
to delete.
3. Use `btrfs subvolume list` to figure out the subvolume ID for the
subvolume you just created, and then set that as the default subvolume
with `btrfs subvolume set-default /path SUBVOLID`. Once you do this,
you will need to specify subvolid=5 in the mount options to get the real
top-level subvolume.
Note that current grub2 works with absolute paths (relative to
filesystem root). It means that if a) /boot/grub is on btrfs and b) it
is part of snapshot that becomes new root, $prefix (that points to
/boot/grub) in the first-stage grub2 image will be wrong. So to be on
safe side you would want to reinstall grub2 after this change.
Generally yes, though you can also make a symlink pointing to the boot
directory under the new subvolume (snapshot), and things should work
correctly as far as I know (this works on Gentoo, not sure about other
distros though).
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