­I'm testing with btrfs. I made a raid 1 array, filled it to the brim with 
data, made a "drive" fail, added a new "drive", and then the filesystem got 
forced read only when I removed the missing drive.

How can I recover from this situation? What are the steps to get back to a 
healthy, writable btrfs filesystem?
How can I avoid the filesystem to go read only?

PS, this question has also been asked on the arch linux forum, there the [code] 
[/code] tags work.
https://bbs.archlinux.org/viewtopic.php?id=262062

Here's what I did:

Prepare test partitions:
[code]
fdisk /dev/sdc
[  440.514002]  sdc: sdc1 sdc2 sdc3 sdc4 < sdc5 sdc6 sdc7 sdc8 sdc9 sdc10 >
[/code]
Create the filesystem:
[code]
# mkfs.btrfs -L FULL_FAIL -f -d raid1 -m raid1 /dev/sdc5 /dev/sdc6 /dev/sdc7
btrfs-progs v5.9
See http://btrfs.wiki.kernel.org for more information.

Label:              FULL_FAIL
UUID:               490a7330-079b-4b03-a301-47a70a3acdcf
Node size:          16384
Sector size:        4096
Filesystem size:    21.00GiB
Block group profiles:
  Data:             RAID1             1.00GiB
  Metadata:         RAID1           256.00MiB
  System:           RAID1             8.00MiB
SSD detected:       no
Incompat features:  extref, skinny-metadata
Runtime features:   
Checksum:           crc32c
Number of devices:  3
Devices:
   ID        SIZE  PATH
    1     6.00GiB  /dev/sdc5
    2     7.00GiB  /dev/sdc6
    3     8.00GiB  /dev/sdc7

[  563.633284] BTRFS: device label FULL_FAIL devid 1 transid 5 /dev/sdc5 
scanned by mkfs.btrfs (470)
[  563.634228] BTRFS: device label FULL_FAIL devid 2 transid 5 /dev/sdc6 
scanned by mkfs.btrfs (470)
[  563.648238] BTRFS: device label FULL_FAIL devid 3 transid 5 /dev/sdc7 
scanned by mkfs.btrfs (470)
[/code]
Mount the filesystem
[code]
# mkdir /mnt/full_fail
# mount /dev/sdc5 /mnt/full_fail
[  718.454333] BTRFS info (device sdc5): disk space caching is enabled
[  718.454337] BTRFS info (device sdc5): has skinny extents
[  718.454338] BTRFS info (device sdc5): flagging fs with big metadata feature
[  719.635715] BTRFS info (device sdc5): checking UUID tree
[/code]
Fill the filesystem to the brim
[code]
# mkdir /mnt/full_fail/write_test
# chown cedric /mnt/full_fail/write_test
# su cedric
$ cat ~/mkfiles_and_md5.sh
#! /bin/bash
for n in {1..6000}; do
    if [ ! -f file$( printf %03d "$n" ).bin ]; then
        dd if=/dev/urandom of=file$( printf %03d "$n" ).bin bs=1M count=100
        md5sum file$( printf %03d "$n" ).bin >> md5sums.txt
    fi
done

$ cd /mnt/full_fail/write_test/
$ ~/mkfiles_and_md5.sh
[/code]
Confirm the filesystem is indeed filled
[code]
$ df -h
/dev/sdc5        11G   11G   64K 100% /mnt/full_fail

# btrfs fi show
Label: 'FULL_FAIL'  uuid: 490a7330-079b-4b03-a301-47a70a3acdcf
    Total devices 3 FS bytes used 10.01GiB
    devid    1 size 6.00GiB used 6.00GiB path /dev/sdc5
    devid    2 size 7.00GiB used 6.52GiB path /dev/sdc6
    devid    3 size 8.00GiB used 8.00GiB path /dev/sdc7
[/code]
Simulate a failed drive
[code]
# umount /dev/sdc5
# wipefs -a /dev/sdc6
/dev/sdc6: 8 bytes were erased at offset 0x00010040 (btrfs): 5f 42 48 52 66 53 
5f 4d
[/code]
Remount the filesystem in degraded mode
[code]
# mount /dev/sdc5 /mnt/full_fail/
mount: /mnt/full_fail: wrong fs type, bad option, bad superblock on /dev/sdc5, 
missing codepage or helper program, or other error.
[ 2922.031458] BTRFS info (device sdc5): disk space caching is enabled
[ 2922.031462] BTRFS info (device sdc5): has skinny extents
[ 2922.032429] BTRFS error (device sdc5): devid 2 uuid 
abe6f4b2-4fd2-4e66-ae8d-987e93f78392 is missing
[ 2922.032432] BTRFS error (device sdc5): failed to read the system array: -2
[ 2922.063492] BTRFS error (device sdc5): open_ctree failed

# mount /dev/sdc5 /mnt/full_fail/ -o,degraded
[ 3080.872748] BTRFS info (device sdc5): allowing degraded mounts
[ 3080.872751] BTRFS info (device sdc5): disk space caching is enabled
[ 3080.872752] BTRFS info (device sdc5): has skinny extents
[ 3080.873732] BTRFS warning (device sdc5): devid 2 uuid 
abe6f4b2-4fd2-4e66-ae8d-987e93f78392 is missing
[ 3080.890568] BTRFS warning (device sdc5): devid 2 uuid 
abe6f4b2-4fd2-4e66-ae8d-987e93f78392 is missing
[/code]
Add a new "drive"
[code]
# btrfs device add /dev/sdc8 /mnt/full_fail/
[ 3163.872400] BTRFS info (device sdc5): disk added /dev/sdc8

# btrfs fi show
Label: 'FULL_FAIL'  uuid: 490a7330-079b-4b03-a301-47a70a3acdcf
    Total devices 4 FS bytes used 10.01GiB
    devid    1 size 6.00GiB used 6.00GiB path /dev/sdc5
    devid    3 size 8.00GiB used 8.00GiB path /dev/sdc7
    devid    4 size 9.00GiB used 0.00B path /dev/sdc8
    *** Some devices missing
[/code]
Remove the failed "drive"
[code]
# btrfs device remove missing /mnt/full_fail/
ERROR: error removing device 'missing': Read-only file system
[ 3283.230781] BTRFS info (device sdc5): relocating block group 10758389760 
flags data|raid1
[ 3284.989011] BTRFS info (device sdc5): relocating block group 8888778752 
flags data|raid1
[ 3285.172340] BTRFS info (device sdc5): relocating block group 7815036928 
flags data|raid1
[ 3285.355690] BTRFS info (device sdc5): relocating block group 5667553280 
flags data|raid1
[ 3285.563993] BTRFS info (device sdc5): relocating block group 4593811456 
flags data|raid1
[ 3285.789070] BTRFS info (device sdc5): relocating block group 2446327808 
flags data|raid1
[ 3286.064095] BTRFS info (device sdc5): relocating block group 298844160 flags 
data|raid1
[ 3286.213950] ------------[ cut here ]------------
[ 3286.213953] BTRFS: Transaction aborted (error -28)
[ 3286.214032] WARNING: CPU: 1 PID: 2460 at fs/btrfs/free-space-cache.c:281 
btrfs_truncate_free_space_cache+0x1c0/0x1f0 [btrfs]
[ 3286.214034] Modules linked in: cfg80211 8021q garp mrp stp llc hwmon_vid 
f75375s intel_rapl_msr intel_rapl_common x86_pkg_temp_thermal intel_powerclamp 
coretemp kvm_intel kvm radeon irqbypass i2c_algo_bit snd_hda_codec_realtek ttm 
snd_hda_codec_generic crct10dif_pclmul drm_kms_helper crc32_pclmul 
ledtrig_audio ghash_clmulni_intel cec aesni_intel snd_hda_intel rc_core 
syscopyarea sysfillrect sysimgblt snd_intel_dspcfg crypto_simd snd_hda_codec 
psmouse cryptd glue_helper fb_sys_fops hp_wmi serio_raw snd_hda_core 
sparse_keymap atkbd tpm_infineon ppdev gpio_ich iTCO_wdt mei_hdcp snd_hwdep 
rapl mei_wdt intel_pmc_bxt rfkill snd_pcm wmi_bmof libps2 at24 
iTCO_vendor_support tpm_tis snd_timer intel_cstate snd mei_me e1000e 
intel_uncore parport_pc soundcore mei evdev input_leds i8042 tpm_tis_core 
parport pcspkr mac_hid wmi i2c_i801 lpc_ich tpm serio i2c_smbus rng_core drm 
fuse agpgart ip_tables x_tables btrfs blake2b_generic libcrc32c crc32c_generic 
xor hid_generic usbhid hid raid6_pq ata_generic
[ 3286.214074]  pata_acpi ata_piix crc32c_intel ehci_pci ehci_hcd
[ 3286.214080] CPU: 1 PID: 2460 Comm: btrfs Not tainted 5.9.14-arch1-1 #1
[ 3286.214081] Hardware name: Hewlett-Packard HP Compaq 8200 Elite CMT PC/1494, 
BIOS J01 v02.28 03/24/2015
[ 3286.214116] RIP: 0010:btrfs_truncate_free_space_cache+0x1c0/0x1f0 [btrfs]
[ 3286.214119] Code: 55 50 f0 48 0f ba aa 48 0a 00 00 02 72 22 83 f8 fb 74 40 
83 f8 e2 74 3b 89 c6 48 c7 c7 68 0d 5b c0 89 44 24 04 e8 32 ab c8 ec <0f> 0b 8b 
44 24 04 89 c1 ba 19 01 00 00 48 89 ef 89 44 24 04 48 c7
[ 3286.214121] RSP: 0018:ffffa01a016fbc08 EFLAGS: 00010286
[ 3286.214123] RAX: 0000000000000000 RBX: ffff8ef120f6ac00 RCX: 0000000000000000
[ 3286.214124] RDX: 0000000000000001 RSI: ffffffffadb59b0f RDI: 00000000ffffffff
[ 3286.214125] RBP: ffff8ef121335bc8 R08: 00000000000003a9 R09: 0000000000000001
[ 3286.214127] R10: 0000000000000000 R11: 0000000000000001 R12: ffff8ef0d0ff27c8
[ 3286.214128] R13: ffff8ef11f4bfee0 R14: ffff8ef11e5fe000 R15: ffff8ef120f6ac10
[ 3286.214130] FS:  00007f7ca80a6200(0000) GS:ffff8ef125c80000(0000) 
knlGS:0000000000000000
[ 3286.214132] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 3286.214134] CR2: 000055e08d943920 CR3: 000000021e768005 CR4: 00000000000606e0
[ 3286.214135] Call Trace:
[ 3286.214173]  delete_block_group_cache+0x6f/0xb0 [btrfs]
[ 3286.214206]  btrfs_relocate_block_group+0xd3/0x300 [btrfs]
[ 3286.214238]  btrfs_relocate_chunk+0x27/0xc0 [btrfs]
[ 3286.214270]  btrfs_shrink_device+0x211/0x550 [btrfs]
[ 3286.214303]  btrfs_rm_device+0x174/0x570 [btrfs]
[ 3286.214308]  ? __check_object_size+0x136/0x150
[ 3286.214312]  ? _copy_from_user+0x2e/0x60
[ 3286.214344]  btrfs_ioctl+0x2c60/0x3050 [btrfs]
[ 3286.214349]  ? vfs_statx+0x8f/0x140
[ 3286.214353]  ? __do_sys_newstat+0x47/0x80
[ 3286.214358]  ? __x64_sys_ioctl+0x83/0xb0
[ 3286.214360]  __x64_sys_ioctl+0x83/0xb0
[ 3286.214366]  do_syscall_64+0x33/0x40
[ 3286.214369]  entry_SYSCALL_64_after_hwframe+0x44/0xa9
[ 3286.214372] RIP: 0033:0x7f7ca81c4f6b
[ 3286.214375] Code: 89 d8 49 8d 3c 1c 48 f7 d8 49 39 c4 72 b5 e8 1c ff ff ff 
85 c0 78 ba 4c 89 e0 5b 5d 41 5c c3 f3 0f 1e fa b8 10 00 00 00 0f 05 <48> 3d 01 
f0 ff ff 73 01 c3 48 8b 0d d5 ae 0c 00 f7 d8 64 89 01 48
[ 3286.214376] RSP: 002b:00007fffa3200758 EFLAGS: 00000246 ORIG_RAX: 
0000000000000010
[ 3286.214379] RAX: ffffffffffffffda RBX: 00007fffa3202920 RCX: 00007f7ca81c4f6b
[ 3286.214380] RDX: 00007fffa3201780 RSI: 000000005000943a RDI: 0000000000000003
[ 3286.214381] RBP: 0000000000000000 R08: 00007fffa32017b8 R09: 00007f7ca8290a60
[ 3286.214383] R10: 0000000000000016 R11: 0000000000000246 R12: 0000000000000000
[ 3286.214384] R13: 00007fffa3201780 R14: 0000000000000003 R15: 00007fffa3202928
[ 3286.214388] ---[ end trace 95a480e236de7d8e ]---
[ 3286.214392] BTRFS warning (device sdc5): 
btrfs_truncate_free_space_cache:281: Aborting unused transaction(No space left).
[ 3286.214427] BTRFS info (device sdc5): relocating block group 22020096 flags 
system|raid1
[ 3286.215852] BTRFS warning (device sdc5): Skipping commit of aborted 
transaction.
[ 3286.215856] BTRFS: error (device sdc5) in cleanup_transaction:1898: 
errno=-30 Readonly filesystem
[ 3286.215858] BTRFS info (device sdc5): forced readonly
[/code]
My version:
[code]
# uname -a
Linux bcache-test 5.9.14-arch1-1 #1 SMP PREEMPT Sat, 12 Dec 2020 14:37:12 +0000 
x86_64 GNU/Linux
[/code]

­Further testing: Rebooted the system. Now the filesystem is writable again. i 
figured it to be a good idea to run a balance, so the data from the full drives 
gets dumped into the new drive. That didn't work so well, the filesystem gets 
forced read only a few seconds in, and this time a reboot didn't work anymore.

I find it strange, as the new "drive" is larger (9GB) than the failed "drive" 
(7GB) it replaces.

I'm out of idea's.

In detail:
After rebooting the system, the filesystem can be remounted read / write:
[code]
# btrfs filesystem show
warning, device 2 is missing
Label: 'FULL_FAIL'  uuid: 490a7330-079b-4b03-a301-47a70a3acdcf
    Total devices 4 FS bytes used 10.01GiB
    devid    1 size 6.00GiB used 6.00GiB path /dev/sdc5
    devid    3 size 8.00GiB used 8.00GiB path /dev/sdc7
    devid    4 size 9.00GiB used 0.00B path /dev/sdc8
    *** Some devices missing

# mount /dev/sdc5 /mnt/full_fail/
mount: /mnt/full_fail: wrong fs type, bad option, bad superblock on /dev/sdc5, 
missing codepage or helper program, or other error.
[  317.151879] BTRFS info (device sdc5): disk space caching is enabled
[  317.151882] BTRFS info (device sdc5): has skinny extents
[  317.152888] BTRFS error (device sdc5): devid 2 uuid 
abe6f4b2-4fd2-4e66-ae8d-987e93f78392 is missing
[  317.152891] BTRFS error (device sdc5): failed to read the system array: -2
[  317.180190] BTRFS error (device sdc5): open_ctree failed

# mount /dev/sdc5 /mnt/full_fail/ -o,degraded
[  332.192932] BTRFS info (device sdc5): allowing degraded mounts
[  332.192935] BTRFS info (device sdc5): disk space caching is enabled
[  332.192937] BTRFS info (device sdc5): has skinny extents
[  332.193941] BTRFS warning (device sdc5): devid 2 uuid 
abe6f4b2-4fd2-4e66-ae8d-987e93f78392 is missing
[  332.206721] BTRFS warning (device sdc5): devid 2 uuid 
abe6f4b2-4fd2-4e66-ae8d-987e93f78392 is missing
[  332.248353] BTRFS info (device sdc5): checking UUID tree

# df -h
/dev/sdc5        15G   11G  832K 100% /mnt/full_fail

# touch /mnt/full_fail/test
# ls -l /mnt/full_fail
total 0
-rw-r--r-- 1 root   root    0 Dec 27 12:46 test
drwxr-xr-x 1 cedric root 8118 Dec 27 11:52 write_test
[/code]
Start the balance
[code]
# btrfs balance start /mnt/full_fail
WARNING:

    Full balance without filters requested. This operation is very
    intense and takes potentially very long. It is recommended to
    use the balance filters to narrow down the scope of balance.
    Use 'btrfs balance start --full-balance' option to skip this
    warning. The operation will start in 10 seconds.
    Use Ctrl-C to stop it.
10 9 8 7 6 5 4 3 2 1
Starting balance without any filters.
ERROR: error during balancing '/mnt/full_fail': Read-only file system
There may be more info in syslog - try dmesg | tail
[  610.428524] BTRFS info (device sdc5): balance: start -d -m -s
[  610.428628] BTRFS info (device sdc5): relocating block group 10758389760 
flags data|raid1
[  610.695382] BTRFS info (device sdc5): relocating block group 9962520576 
flags data|raid1
[  610.962003] BTRFS info (device sdc5): relocating block group 8888778752 
flags data|raid1
[  611.236969] BTRFS info (device sdc5): relocating block group 7815036928 
flags data|raid1
[  617.437051] BTRFS info (device sdc5): relocating block group 6741295104 
flags data|raid1
[  617.737177] BTRFS info (device sdc5): relocating block group 5667553280 
flags data|raid1
[  618.070493] BTRFS info (device sdc5): relocating block group 4593811456 
flags data|raid1
[  618.345070] BTRFS info (device sdc5): relocating block group 3520069632 
flags data|raid1
[  618.611753] BTRFS info (device sdc5): relocating block group 2446327808 
flags data|raid1
[  618.869989] BTRFS info (device sdc5): relocating block group 1372585984 
flags data|raid1
[  619.128457] BTRFS info (device sdc5): relocating block group 298844160 flags 
data|raid1
[  619.303350] ------------[ cut here ]------------
[  619.303353] BTRFS: Transaction aborted (error -28)
[  619.303436] WARNING: CPU: 1 PID: 493 at fs/btrfs/free-space-cache.c:281 
btrfs_truncate_free_space_cache+0x1c0/0x1f0 [btrfs]
[  619.303437] Modules linked in: cfg80211 8021q garp mrp stp llc hwmon_vid 
f75375s intel_rapl_msr intel_rapl_common x86_pkg_temp_thermal intel_powerclamp 
radeon coretemp kvm_intel i2c_algo_bit kvm ttm irqbypass crct10dif_pclmul 
crc32_pclmul drm_kms_helper ghash_clmulni_intel aesni_intel 
snd_hda_codec_realtek snd_hda_codec_generic cec hp_wmi ledtrig_audio 
crypto_simd snd_hda_intel cryptd rc_core sparse_keymap snd_intel_dspcfg 
snd_hda_codec glue_helper iTCO_wdt psmouse rfkill syscopyarea serio_raw 
mei_hdcp intel_pmc_bxt gpio_ich mei_wdt rapl sysfillrect ppdev 
iTCO_vendor_support mei_me intel_cstate snd_hda_core atkbd tpm_infineon 
snd_hwdep libps2 sysimgblt snd_pcm wmi_bmof at24 intel_uncore snd_timer e1000e 
fb_sys_fops snd mei soundcore lpc_ich i8042 pcspkr wmi tpm_tis serio 
tpm_tis_core i2c_i801 input_leds tpm parport_pc evdev parport rng_core 
i2c_smbus mac_hid drm fuse agpgart ip_tables x_tables btrfs blake2b_generic 
libcrc32c crc32c_generic xor hid_generic usbhid hid raid6_pq ata_generic
[  619.303478]  pata_acpi crc32c_intel ehci_pci ata_piix ehci_hcd
[  619.303484] CPU: 1 PID: 493 Comm: btrfs Not tainted 5.9.14-arch1-1 #1
[  619.303485] Hardware name: Hewlett-Packard HP Compaq 8200 Elite CMT PC/1494, 
BIOS J01 v02.28 03/24/2015
[  619.303520] RIP: 0010:btrfs_truncate_free_space_cache+0x1c0/0x1f0 [btrfs]
[  619.303524] Code: 55 50 f0 48 0f ba aa 48 0a 00 00 02 72 22 83 f8 fb 74 40 
83 f8 e2 74 3b 89 c6 48 c7 c7 68 ed 2c c0 89 44 24 04 e8 32 cb b6 ce <0f> 0b 8b 
44 24 04 89 c1 ba 19 01 00 00 48 89 ef 89 44 24 04 48 c7
[  619.303525] RSP: 0018:ffffa0ca80283cf8 EFLAGS: 00010282
[  619.303527] RAX: 0000000000000000 RBX: ffff94c2e1d92c00 RCX: 0000000000000000
[  619.303528] RDX: 0000000000000001 RSI: ffffffff8f759b0f RDI: 00000000ffffffff
[  619.303530] RBP: ffff94c2e3830548 R08: 000000000000039c R09: 0000000000000001
[  619.303531] R10: 0000000000000000 R11: 0000000000000001 R12: ffff94c2e1249058
[  619.303532] R13: ffff94c2e04d5b60 R14: ffff94c2e1775800 R15: ffff94c2e1d92c10
[  619.303535] FS:  00007fd0ea4a3200(0000) GS:ffff94c2e5c80000(0000) 
knlGS:0000000000000000
[  619.303536] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[  619.303538] CR2: 00007f75dcb4bf30 CR3: 00000002207e6003 CR4: 00000000000606e0
[  619.303539] Call Trace:
[  619.303577]  delete_block_group_cache+0x6f/0xb0 [btrfs]
[  619.303610]  btrfs_relocate_block_group+0xd3/0x300 [btrfs]
[  619.303642]  btrfs_relocate_chunk+0x27/0xc0 [btrfs]
[  619.303674]  btrfs_balance+0x779/0xef0 [btrfs]
[  619.303709]  btrfs_ioctl_balance+0x292/0x340 [btrfs]
[  619.303716]  __x64_sys_ioctl+0x83/0xb0
[  619.303721]  do_syscall_64+0x33/0x40
[  619.303725]  entry_SYSCALL_64_after_hwframe+0x44/0xa9
[  619.303727] RIP: 0033:0x7fd0ea5c1f6b
[  619.303731] Code: 89 d8 49 8d 3c 1c 48 f7 d8 49 39 c4 72 b5 e8 1c ff ff ff 
85 c0 78 ba 4c 89 e0 5b 5d 41 5c c3 f3 0f 1e fa b8 10 00 00 00 0f 05 <48> 3d 01 
f0 ff ff 73 01 c3 48 8b 0d d5 ae 0c 00 f7 d8 64 89 01 48
[  619.303732] RSP: 002b:00007ffdffa33088 EFLAGS: 00000246 ORIG_RAX: 
0000000000000010
[  619.303734] RAX: ffffffffffffffda RBX: 00007ffdffa33120 RCX: 00007fd0ea5c1f6b
[  619.303736] RDX: 00007ffdffa33120 RSI: 00000000c4009420 RDI: 0000000000000003
[  619.303737] RBP: 0000000000000003 R08: 000055a1cc6a86e0 R09: 00007fd0ea68da60
[  619.303738] R10: 0000000000000231 R11: 0000000000000246 R12: 0000000000000001
[  619.303739] R13: 00007ffdffa34dae R14: 0000000000000000 R15: 0000000000000000
[  619.303743] ---[ end trace 38946790256c33d0 ]---
[  619.303747] BTRFS warning (device sdc5): 
btrfs_truncate_free_space_cache:281: Aborting unused transaction(No space left).
[  619.303783] BTRFS info (device sdc5): relocating block group 22020096 flags 
system|raid1
[  619.305161] BTRFS warning (device sdc5): Skipping commit of aborted 
transaction.
[  619.305166] BTRFS: error (device sdc5) in cleanup_transaction:1898: 
errno=-30 Readonly filesystem
[  619.305168] BTRFS info (device sdc5): forced readonly
[  619.306680] BTRFS info (device sdc5): 12 enospc errors during balance
[  619.306683] BTRFS info (device sdc5): balance: ended with status: -30
[/code]
Remounting also gives a read only filesystem
[code]
# btrfs filesystem show
Label: 'FULL_FAIL'  uuid: 490a7330-079b-4b03-a301-47a70a3acdcf
    Total devices 4 FS bytes used 10.01GiB
    devid    1 size 6.00GiB used 6.00GiB path /dev/sdc5
    devid    3 size 8.00GiB used 8.00GiB path /dev/sdc7
    devid    4 size 9.00GiB used 0.00B path /dev/sdc8
    *** Some devices missing

# umount /mnt/full_fail
# mount /dev/sdc5 /mnt/full_fail/ -o,degraded
[  856.434698] BTRFS info (device sdc5): allowing degraded mounts
[  856.434702] BTRFS info (device sdc5): disk space caching is enabled
[  856.434704] BTRFS info (device sdc5): has skinny extents
[  856.451425] BTRFS warning (device sdc5): devid 2 uuid 
abe6f4b2-4fd2-4e66-ae8d-987e93f78392 is missing
[  856.474289] BTRFS info (device sdc5): checking UUID tree
[  856.543382] BTRFS info (device sdc5): balance: resume -dusage=90 -musage=90 
-susage=90
[  856.543469] ------------[ cut here ]------------
[  856.543471] BTRFS: Transaction aborted (error -28)
[  856.543550] WARNING: CPU: 0 PID: 532 at fs/btrfs/free-space-cache.c:281 
btrfs_truncate_free_space_cache+0x1c0/0x1f0 [btrfs]
[  856.543551] Modules linked in: cfg80211 8021q garp mrp stp llc hwmon_vid 
f75375s intel_rapl_msr intel_rapl_common x86_pkg_temp_thermal intel_powerclamp 
radeon coretemp kvm_intel i2c_algo_bit kvm ttm irqbypass crct10dif_pclmul 
crc32_pclmul drm_kms_helper ghash_clmulni_intel aesni_intel 
snd_hda_codec_realtek snd_hda_codec_generic cec hp_wmi ledtrig_audio 
crypto_simd snd_hda_intel cryptd rc_core sparse_keymap snd_intel_dspcfg 
snd_hda_codec glue_helper iTCO_wdt psmouse rfkill syscopyarea serio_raw 
mei_hdcp intel_pmc_bxt gpio_ich mei_wdt rapl sysfillrect ppdev 
iTCO_vendor_support mei_me intel_cstate snd_hda_core atkbd tpm_infineon 
snd_hwdep libps2 sysimgblt snd_pcm wmi_bmof at24 intel_uncore snd_timer e1000e 
fb_sys_fops snd mei soundcore lpc_ich i8042 pcspkr wmi tpm_tis serio 
tpm_tis_core i2c_i801 input_leds tpm parport_pc evdev parport rng_core 
i2c_smbus mac_hid drm fuse agpgart ip_tables x_tables btrfs blake2b_generic 
libcrc32c crc32c_generic xor hid_generic usbhid hid raid6_pq ata_generic
[  856.543591]  pata_acpi crc32c_intel ehci_pci ata_piix ehci_hcd
[  856.543598] CPU: 0 PID: 532 Comm: btrfs-balance Tainted: G        W         
5.9.14-arch1-1 #1
[  856.543599] Hardware name: Hewlett-Packard HP Compaq 8200 Elite CMT PC/1494, 
BIOS J01 v02.28 03/24/2015
[  856.543633] RIP: 0010:btrfs_truncate_free_space_cache+0x1c0/0x1f0 [btrfs]
[  856.543637] Code: 55 50 f0 48 0f ba aa 48 0a 00 00 02 72 22 83 f8 fb 74 40 
83 f8 e2 74 3b 89 c6 48 c7 c7 68 ed 2c c0 89 44 24 04 e8 32 cb b6 ce <0f> 0b 8b 
44 24 04 89 c1 ba 19 01 00 00 48 89 ef 89 44 24 04 48 c7
[  856.543638] RSP: 0018:ffffa0ca81647d20 EFLAGS: 00010282
[  856.543640] RAX: 0000000000000000 RBX: ffff94c2e0567800 RCX: 0000000000000000
[  856.543642] RDX: 0000000000000001 RSI: ffffffff8f759b0f RDI: 00000000ffffffff
[  856.543643] RBP: ffff94c2e481cb60 R08: 00000000000003cd R09: 0000000000000001
[  856.543644] R10: 0000000000000000 R11: 0000000000000001 R12: ffff94c2e12a35d8
[  856.543646] R13: ffff94c2e3667070 R14: ffff94c2e156d800 R15: ffff94c2e0567810
[  856.543648] FS:  0000000000000000(0000) GS:ffff94c2e5c00000(0000) 
knlGS:0000000000000000
[  856.543650] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[  856.543651] CR2: 000055cab9f6fbc0 CR3: 000000021500e005 CR4: 00000000000606f0
[  856.543653] Call Trace:
[  856.543690]  delete_block_group_cache+0x6f/0xb0 [btrfs]
[  856.543725]  btrfs_relocate_block_group+0xd3/0x300 [btrfs]
[  856.543764]  btrfs_relocate_chunk+0x27/0xc0 [btrfs]
[  856.543803]  btrfs_balance+0x779/0xef0 [btrfs]
[  856.543844]  ? btrfs_balance+0xef0/0xef0 [btrfs]
[  856.543882]  balance_kthread+0x35/0x50 [btrfs]
[  856.543887]  kthread+0x142/0x160
[  856.543891]  ? __kthread_bind_mask+0x60/0x60
[  856.543895]  ret_from_fork+0x22/0x30
[  856.543899] ---[ end trace 38946790256c33d1 ]---
[  856.543903] BTRFS warning (device sdc5): 
btrfs_truncate_free_space_cache:281: Aborting unused transaction(No space left).
[  856.543972] BTRFS info (device sdc5): relocating block group 22020096 flags 
system|raid1
[  856.545589] BTRFS warning (device sdc5): Skipping commit of aborted 
transaction.
[  856.545593] BTRFS: error (device sdc5) in cleanup_transaction:1898: 
errno=-30 Readonly filesystem
[  856.545595] BTRFS info (device sdc5): forced readonly
[  856.547025] BTRFS info (device sdc5): 1 enospc errors during balance
[  856.547027] BTRFS info (device sdc5): balance: ended with status: -30
[/code]
Rebooting the server doesn't help anymore
[code]
mount /dev/sdc5 /mnt/full_fail/ -o,degraded
[  319.244480] BTRFS info (device sdc5): allowing degraded mounts
[  319.244484] BTRFS info (device sdc5): disk space caching is enabled
[  319.244485] BTRFS info (device sdc5): has skinny extents
[  319.246699] BTRFS warning (device sdc5): devid 2 uuid 
abe6f4b2-4fd2-4e66-ae8d-987e93f78392 is missing
[  319.252305] BTRFS warning (device sdc5): devid 2 uuid 
abe6f4b2-4fd2-4e66-ae8d-987e93f78392 is missing
[  319.377704] BTRFS info (device sdc5): balance: resume -dusage=90 -musage=90 
-susage=90
[  319.377792] ------------[ cut here ]------------
[  319.377794] BTRFS: Transaction aborted (error -28)
[  319.377877] WARNING: CPU: 2 PID: 469 at fs/btrfs/free-space-cache.c:281 
btrfs_truncate_free_space_cache+0x1c0/0x1f0 [btrfs]
[  319.377878] Modules linked in: cfg80211 8021q garp mrp stp llc hwmon_vid 
f75375s radeon i2c_algo_bit ttm drm_kms_helper intel_rapl_msr cec 
snd_hda_codec_realtek intel_rapl_common snd_hda_codec_generic ledtrig_audio 
x86_pkg_temp_thermal intel_powerclamp rc_core snd_hda_intel coretemp 
snd_intel_dspcfg syscopyarea snd_hda_codec sysfillrect kvm_intel sysimgblt 
fb_sys_fops snd_hda_core kvm snd_hwdep iTCO_wdt irqbypass snd_pcm psmouse 
crct10dif_pclmul crc32_pclmul intel_pmc_bxt serio_raw ghash_clmulni_intel atkbd 
aesni_intel hp_wmi iTCO_vendor_support crypto_simd mei_hdcp mei_wdt ppdev 
cryptd snd_timer e1000e gpio_ich libps2 snd sparse_keymap glue_helper mei_me 
parport_pc at24 rfkill wmi_bmof soundcore rapl tpm_infineon intel_cstate mei 
parport intel_uncore pcspkr i8042 tpm_tis tpm_tis_core tpm evdev input_leds 
lpc_ich serio i2c_i801 mac_hid i2c_smbus rng_core wmi drm agpgart fuse 
ip_tables x_tables btrfs blake2b_generic libcrc32c crc32c_generic xor 
hid_generic usbhid hid raid6_pq ata_generic
[  319.377919]  pata_acpi crc32c_intel ehci_pci ata_piix ehci_hcd
[  319.377925] CPU: 2 PID: 469 Comm: btrfs-balance Not tainted 5.9.14-arch1-1 #1
[  319.377927] Hardware name: Hewlett-Packard HP Compaq 8200 Elite CMT PC/1494, 
BIOS J01 v02.28 03/24/2015
[  319.377961] RIP: 0010:btrfs_truncate_free_space_cache+0x1c0/0x1f0 [btrfs]
[  319.377965] Code: 55 50 f0 48 0f ba aa 48 0a 00 00 02 72 22 83 f8 fb 74 40 
83 f8 e2 74 3b 89 c6 48 c7 c7 68 9d 35 c0 89 44 24 04 e8 32 1b 4e f3 <0f> 0b 8b 
44 24 04 89 c1 ba 19 01 00 00 48 89 ef 89 44 24 04 48 c7
[  319.377966] RSP: 0018:ffffa9aa0055bd20 EFLAGS: 00010282
[  319.377968] RAX: 0000000000000000 RBX: ffff99e52269a400 RCX: 0000000000000000
[  319.377970] RDX: 0000000000000001 RSI: ffffffffb4159b0f RDI: 00000000ffffffff
[  319.377971] RBP: ffff99e51e5229c0 R08: 000000000000037c R09: 0000000000000001
[  319.377972] R10: 0000000000000000 R11: 0000000000000001 R12: ffff99e5222b72c8
[  319.377974] R13: ffff99e522b0a930 R14: ffff99e521bdb000 R15: ffff99e52269a410
[  319.377976] FS:  0000000000000000(0000) GS:ffff99e525d00000(0000) 
knlGS:0000000000000000
[  319.377977] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[  319.377979] CR2: 00007ff3765f3e20 CR3: 000000020f40e005 CR4: 00000000000606e0
[  319.377980] Call Trace:
[  319.378019]  delete_block_group_cache+0x6f/0xb0 [btrfs]
[  319.378052]  btrfs_relocate_block_group+0xd3/0x300 [btrfs]
[  319.378085]  btrfs_relocate_chunk+0x27/0xc0 [btrfs]
[  319.378117]  btrfs_balance+0x779/0xef0 [btrfs]
[  319.378151]  ? btrfs_balance+0xef0/0xef0 [btrfs]
[  319.378181]  balance_kthread+0x35/0x50 [btrfs]
[  319.378188]  kthread+0x142/0x160
[  319.378191]  ? __kthread_bind_mask+0x60/0x60
[  319.378195]  ret_from_fork+0x22/0x30
[  319.378200] ---[ end trace 75e1a8739f7fa9dd ]---
[  319.378203] BTRFS warning (device sdc5): 
btrfs_truncate_free_space_cache:281: Aborting unused transaction(No space left).
[  319.378266] BTRFS info (device sdc5): relocating block group 22020096 flags 
system|raid1
[  319.379737] BTRFS warning (device sdc5): Skipping commit of aborted 
transaction.
[  319.379742] BTRFS: error (device sdc5) in cleanup_transaction:1898: 
errno=-30 Readonly filesystem
[  319.379744] BTRFS info (device sdc5): forced readonly
[  319.381224] BTRFS info (device sdc5): 1 enospc errors during balance
[  319.381227] BTRFS info (device sdc5): balance: ended with status: -30
[/code]


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