Error - Failed to start remount Root and Kernel file system - Btrfs & space_cache - Kernel 5.15

After fresh installation new ISO 21.2 preview Manjaro Xfce with three partitions: root btrfs, /boot/efi fat32 and /boot ext4 I have error whíle booting: “Failed to start remount Root and Kernel file system”.


I have the same problem. I was about to create a new topic but since the topic is already open I am sharing it in the same thread.
Here is my Journalctl

-- Journal begins at Sun 2021-11-21 20:00:42 +06, ends at Sun 2021-11-21 20:35:38 +06. --
Nov 21 20:00:42 manjaro kernel: BTRFS error (device sda2): cannot disable free space tree
Nov 21 20:00:42 manjaro systemd[1]: Failed to start Remount Root and Kernel File Systems.
Nov 21 20:00:42 manjaro systemd-remount-fs[249]: /usr/bin/mount for / exited with exit status 32.

Here is my fstab

# <file system>             <mount point>  <type>  <options>  <dump>  <pass>
UUID=C53D-05A9                            /boot/efi      vfat    umask=0077 0 2
UUID=c9ef96fc-a75c-4868-b6e3-47a4f0643cfa /              btrfs   subvol=/@,defaults,noatime,space_cache,autodefrag,compress=zstd,discard=async,ssd,commit=120 0 0
UUID=c9ef96fc-a75c-4868-b6e3-47a4f0643cfa /home          btrfs   subvol=/@home,defaults,noatime,space_cache,autodefrag,compress=zstd,discard=async,ssd,commit=120 0 0
UUID=c9ef96fc-a75c-4868-b6e3-47a4f0643cfa /var/cache     btrfs   subvol=/@cache,defaults,noatime,space_cache,autodefrag,compress=zstd,discard=async,ssd,commit=120 0 0
UUID=c9ef96fc-a75c-4868-b6e3-47a4f0643cfa /var/log       btrfs   subvol=/@log,defaults,noatime,space_cache,autodefrag,compress=zstd,discard=async,ssd,commit=120 0 0
tmpfs                                     /tmp           tmpfs   defaults,noatime,mode=1777 0 0

Here is my blkid

/dev/sdb1: UUID="e677fd9e-6f82-420c-969a-731983d5ca0f" LABEL="LOCKED" TYPE="crypto_LUKS" PARTUUID="8d9a9b5e-926d-c44c-b89a-f3e6c692ab56"
/dev/sda2: LABEL="SYSTEM" UUID="c9ef96fc-a75c-4868-b6e3-47a4f0643cfa" UUID_SUB="c563770d-92c8-4889-97c8-646f0b45a274" BLOCK_SIZE="4096" TYPE="btrfs" PARTUUID="110ff177-9869-3949-8a98-9ae6fa136efa"
/dev/sda1: LABEL_FATBOOT="EFI" LABEL="EFI" UUID="C53D-05A9" BLOCK_SIZE="512" TYPE="vfat" PARTUUID="43f4ee4c-6675-e849-a1d2-d4a20d5a8f0e"

1 Like

the good question

is it Btrfs cache V1 or V2 ? ( to do list )

Same issue here. Did a fresh in of Manjaro 21.2pre1 KDE and the error is at startup and reboot "Failed to start : Remount Root and Kernel file systems".

  • blkid output:
/dev/sda: PTUUID="d9592660-72f1-1f4e-9da7-cb93c4c2c282" PTTYPE="gpt"
/dev/sda1: LABEL_FATBOOT="NO_LABEL" LABEL="NO_LABEL" UUID="03CD-08A5" BLOCK_SIZE="512" TYPE="vfat" PARTUUID="8c65b73f-4432-604a-a1ca-50830577a2c4"
/dev/sda2: UUID="abb98884-ee69-4192-ac97-c66cd1533736" UUID_SUB="033667f8-4597-4b2b-9b78-a210bf381f08" BLOCK_SIZE="4096" TYPE="btrfs" PARTLABEL="root" PARTUUID="0493de67-2e6f-9d43-8e42-223e37c7125a"
/dev/sda3: LABEL="swap" UUID="053a267b-1f8d-403b-b4d1-96fb9ee91226" TYPE="swap" PARTUUID="92e3c238-17d1-ec48-bfd7-a814ac8b06ea"
  • /etc/fstab contents:
# /etc/fstab: static file system information.
# Use 'blkid' to print the universally unique identifier for a device; this may
# be used with UUID= as a more robust way to name devices that works even if
# disks are added and removed. See fstab(5).
# <file system>             <mount point>  <type>  <options>  <dump>  <pass>
UUID=03CD-08A5                            /boot/efi      vfat    umask=0077 0 2
UUID=abb98884-ee69-4192-ac97-c66cd1533736 /              btrfs   subvol=/@,defaults,noatime,space_cache,autodefrag,compress=zstd,discard=async,ssd,commit=120 0 0
UUID=abb98884-ee69-4192-ac97-c66cd1533736 /home          btrfs   subvol=/@home,defaults,noatime,space_cache,autodefrag,compress=zstd,discard=async,ssd,commit=120 0 0
UUID=abb98884-ee69-4192-ac97-c66cd1533736 /var/cache     btrfs   subvol=/@cache,defaults,noatime,space_cache,autodefrag,compress=zstd,discard=async,ssd,commit=120 0 0
UUID=abb98884-ee69-4192-ac97-c66cd1533736 /var/log       btrfs   subvol=/@log,defaults,noatime,space_cache,autodefrag,compress=zstd,discard=async,ssd,commit=120 0 0
UUID=053a267b-1f8d-403b-b4d1-96fb9ee91226 swap           swap    defaults,noatime 0 0
tmpfs                                     /tmp           tmpfs   defaults,noatime,mode=1777 0 0



and do a try


Do you think space_cache is deprecated?^^

replace space_cache in fstab with space_cacheV2


Either removing space_cache or setting it to space_cache=v2 in /etc/fstab, both work.

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Don´t remove space_cache! read performance will degrade! since btrfs-progs 5.15 space_cache was replaced with “free-space-tree” (space_cacheV2)! On very large filesystems (many terabytes) and with certain workloads, the performance of the v1 space cache may degrade drastically. This is why the new v2 implementation was created. It uses a new B-tree called the [free space tree].

Starting with [btrfs-progs 5.15], the “free space tree” is the default for all newly created filesystems

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So, please clarify a bit😅

  • space_cacheV2
  • space_cache=v2
  • free-space-tree
    Which one(s) should be used?

free space tree ist the technical term. space_cacheV2 ist the mount option for free space tree.

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Tried it, but space_cacheV2 gives the "Failed to start : Remount Root and Kernel file systems" error, while if I use space_cache=v2, it is fine.

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You are right - sorry my fault - space_cache=v2 is the correct mount option

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Could you search the manpage of command line "# btrfs check --clear-space-cache v2 device-path ?. I’m tested with the same problem on a fresh install of manjaro kde 21.2pre1 and use to this command line to clear the systemctl failed . The file system must be umount to do the command line , so … boot with the live iso manjaro. Perhaps a bug.

If someone wrote this before me, I’m sorry: read the man page before you change anything or you know what you are doing.

There are two implementations of the free space cache.
The original one, referred to as v1, is the safe default. 

Quote from Manpage/btrfs(5) - btrfs Wiki

I have space_cache on my system still active [current state stable branch kernel 5.15.2]
no remount fails etc.

Maybe this is only a problem on reinstalls and not on existing ones?

After last update btrfs-progs has new version 5.15, which set default space_cache to v2.

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