raid1 mount as read only

[Stefan K <shadow_7@gmx.net>]

Hello,

I've played a little bit with raid1:
my steps was:
1. create a raid1 with btrfs (add device; balance start -mconvert=raid1 -dconvert=raid1 /)
2.  after finishing, i shutdown the server and remove a device and start it again,
3. it works (i used degraded options in fstab)
4. I shutdown the server, add the 'old' device, start it and run a btrfs balance
5. until here everything is fine
6. I shutdown the server, remove the other harddisk, and start it, 
7. it works
8. I shutdown the server, add the 'old' device, start it and try a btrfs balance
9. but now the FS is mounted as read only

I got the following messages in dmesg:
[    6.401740] BTRFS: device fsid b997e926-ab95-46d0-a9be-da52aa09203d devid 1 transid 3570 /dev/sda1
[    6.403079] BTRFS info (device sda1): allowing degraded mounts
[    6.403084] BTRFS info (device sda1): disk space caching is enabled
[    6.403086] BTRFS info (device sda1): has skinny extents
[    6.405878] BTRFS warning (device sda1): devid 2 uuid 09a7c9f6-9a13-4852-bca6-2d9120f388d4 missing
[    6.409108] BTRFS info (device sda1): detected SSD devices, enabling SSD mode

[    6.652411] BTRFS info (device sda1): allowing degraded mounts
[    6.652414] BTRFS info (device sda1): disk space caching is enabled
[    6.652416] BTRFS warning (device sda1): too many missing devices, writeable remount is not allowed

but all devices are available:
btrfs fi sh
Label: none  uuid: b997e926-ab95-46d0-a9be-da52aa09203d
        Total devices 2 FS bytes used 895.41MiB
        devid    1 size 223.57GiB used 4.06GiB path /dev/sda1
        devid    2 size 223.57GiB used 2.00GiB path /dev/sdb1

mount | grep btrfs
/dev/sda1 on / type btrfs (ro,relatime,degraded,ssd,space_cache,subvolid=5,subvol=/)

If I try to remount it via, I got the following in dmesg:
mount -o remount,rw b997e926-ab95-46d0-a9be-da52aa09203d /
[  320.048563] BTRFS info (device sda1): disk space caching is enabled
[  356.200552] BTRFS: error (device sda1) in write_all_supers:3752: errno=-5 IO failure (errors while submitting device barriers.)
[  356.200628] BTRFS info (device sda1): forced readonly
[  356.200632] BTRFS warning (device sda1): Skipping commit of aborted transaction.
[  356.200633] ------------[ cut here ]------------
[  356.200677] WARNING: CPU: 0 PID: 1914 at /build/linux-EbeuWA/linux-4.9.130/fs/btrfs/transaction.c:1850 cleanup_transaction+0x1f3/0x2e0 [btrfs]
[  356.200678] BTRFS: Transaction aborted (error -5)
[  356.200679] Modules linked in: intel_rapl skx_edac edac_core x86_pkg_temp_thermal intel_powerclamp coretemp kvm_intel kvm irqbypass crct10dif_pclmul crc32_pclmul ghash_clmulni_intel mgag200 ttm drm_kms_helper intel_uncore sg drm dcdbas mei_me i2c_algo_bit intel_rapl_perf pcspkr joydev iTCO_wdt lpc_ich iTCO_vendor_support mfd_core shpchp mei evdev ipmi_si ipmi_msghandler acpi_power_meter button ip_tables x_tables autofs4 ses enclosure scsi_transport_sas sd_mod hid_generic usbhid hid btrfs crc32c_generic xor raid6_pq crc32c_intel aesni_intel aes_x86_64 glue_helper lrw gf128mul ablk_helper cryptd ahci xhci_pci libahci tg3 xhci_hcd ptp megaraid_sas libata pps_core i2c_i801 usbcore bnxt_en i2c_smbus libphy scsi_mod usb_common
[  356.200764] CPU: 0 PID: 1914 Comm: btrfs-transacti Not tainted 4.9.0-8-amd64 #1 Debian 4.9.130-2
[  356.200766] Hardware name: Dell Inc. PowerEdge R840/08XR9M, BIOS 1.2.4 10/18/2018
[  356.200768]  0000000000000000 ffffffff9ef33d74 ffffb9535d70fd50 0000000000000000
[  356.200774]  ffffffff9ec7a59e ffff8ea977b261f8 ffffb9535d70fda8 ffff8e9971cdd500
[  356.200778]  00000000fffffffb ffff8ea97114c980 0000000000000000 ffffffff9ec7a61f
[  356.200783] Call Trace:
[  356.200796]  [<ffffffff9ef33d74>] ? dump_stack+0x5c/0x78
[  356.200801]  [<ffffffff9ec7a59e>] ? __warn+0xbe/0xe0
[  356.200803]  [<ffffffff9ec7a61f>] ? warn_slowpath_fmt+0x5f/0x80
[  356.200828]  [<ffffffffc04ac423>] ? cleanup_transaction+0x1f3/0x2e0 [btrfs]
[  356.200835]  [<ffffffff9ecbd210>] ? prepare_to_wait_event+0xf0/0xf0
[  356.200858]  [<ffffffffc04ade78>] ? btrfs_commit_transaction+0x298/0xa10 [btrfs]
[  356.200879]  [<ffffffffc04ae686>] ? start_transaction+0x96/0x480 [btrfs]
[  356.200900]  [<ffffffffc04a8a8c>] ? transaction_kthread+0x1dc/0x200 [btrfs]
[  356.200919]  [<ffffffffc04a88b0>] ? btrfs_cleanup_transaction+0x580/0x580 [btrfs]
[  356.200926]  [<ffffffff9ec9a569>] ? kthread+0xd9/0xf0
[  356.200933]  [<ffffffff9f2190a4>] ? __switch_to_asm+0x34/0x70
[  356.200937]  [<ffffffff9ec9a490>] ? kthread_park+0x60/0x60
[  356.200941]  [<ffffffff9f219137>] ? ret_from_fork+0x57/0x70
[  356.200943] ---[ end trace 17ece06f94583f5a ]---
[  356.200946] BTRFS: error (device sda1) in cleanup_transaction:1850: errno=-5 IO failure
[  356.200997] BTRFS info (device sda1): delayed_refs has NO entry


My question is whats happen and how can I fix this?

uname -a
Linux yamazaki-06 4.9.0-8-amd64 #1 SMP Debian 4.9.130-2 (2018-10-27) x86_64 GNU/Linux

btrfs version
btrfs-progs v4.7.3