Btrfs device initialisation is quite slow

[Robbie Smith <zoqaeski@gmail.com>]

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Hey all

I've been trying to figure out why my system (home desktop) is taking
so long to boot. Systemd-analyze tells me that my root filesystem
partition (which is btrfs) takes ~11 seconds to become active, and I'm
curious as to why and whether or not I can optimise this.

The primary disk has 4 partitions: a EFI/BIOS boot partion (for GRUB);
a /boot partition (ext4); a swap partition; and the root partition. The
disk itself is not particularly large (320 GB), and I'm using
subvolumes to emulate partitions in btrfs. There are three top-level
subvolumes, for /, /home, and /var, none of which have quotas, and I'm
not at present doing snapshots because I backup every day to an
external drive formatted with ext4. 

I've got a second 5 TB drive for multimedia that is also btrfs, but it
only takes ~3 seconds to come online. I had been using a number of bind
mounts from the multimedia drive to my home folder, so that $HOME/music
and $HOME/videos point to the library, and replacing them with symlinks
reduced the time by ~3 seconds, but it still doesn't account for why
the root device takes so long.

My fstab contains the following:

# /dev/sdc4 LABEL=filesystem
UUID=4ec80601-4799-4fa8-a711-0171c180f25b /
btrfs rw,noatime,space_cache,autodefrag,subvol=rootvol 0 0

# /dev/sdc4 LABEL=filesystem
UUID=4ec80601-4799-4fa8-a711-0171c180f25b /home btrfs rw,noatime,space_cache,autodefrag,subvol=homevol 0 0

# /dev/sdc4 LABEL=filesystem
UUID=4ec80601-4799-4fa8-a711-0171c180f25b /var btrfs rw,noatime,space_cache,autodefrag,subvol=var 0 0

# /dev/sdc2 LABEL=boot
UUID=ca281471-0aac-4090-8660-33b8b9fee5a3 /boot ext4 rw,relatime,data=ordered 0 2

# /dev/sdb1 LABEL=library
UUID=97226949-50e0-4a78-899e-863f5b436bcc /mnt/library btrfs rw,noatime,space_cache,autodefrag 0 0


Can anyone offer any insights or advice?

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