BTRFS Unmountable No file system (Cache Pool, Raid0)

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Posted (edited)

Hi all,

I have 2 sata SSDs I had setup as a raid0 btrfs cache pool.

After one of my VMs (with the disk being on cache) being frozen I had to restart the array and now I cannot mount my cache drive(s) even though the array starts. So dockers and VMs are disabled.


It seems like one of disks started failing I just saw unraid posting a CRC error.

Is there any way I can mount this back long enough so I can get the data out of it? It would save me some headache.


What I've tried so far

1- Unassign disks start array, reassign and start array

2- btrfs check --readonly (it shows same result for both drives)

Opening filesystem to check...
Checking filesystem on /dev/sdi1
UUID: f4c7a0bf-b527-47f1-ada2-5da96e590ef7
[1/7] checking root items
[2/7] checking extents
data backref 2013043068928 root 5 owner 623 offset 100331225088 num_refs 0 not found in extent tree
incorrect local backref count on 2013043068928 root 5 owner 623 offset 100331225088 found 1 wanted 0 back 0x62aa9a0
incorrect local backref count on 2013043068928 root 13 owner 623 offset 100331225088 found 0 wanted 1 back 0x62aaad0
backref disk bytenr does not match extent record, bytenr=2013043068928, ref bytenr=0
backpointer mismatch on [2013043068928 65536]
ERROR: errors found in extent allocation tree or chunk allocation
[3/7] checking free space tree
cache and super generation don't match, space cache will be invalidated
[4/7] checking fs roots
[5/7] checking only csums items (without verifying data)
[6/7] checking root refs
[7/7] checking quota groups skipped (not enabled on this FS)
found 538120765440 bytes used, error(s) found
total csum bytes: 86454728
total tree bytes: 452263936
total fs tree bytes: 287948800
total extent tree bytes: 64061440
btree space waste bytes: 81374861
file data blocks allocated: 961713012736
 referenced 534715310080


3- blkid output

/dev/sdh1: UUID="f4c7a0bf-b527-47f1-ada2-5da96e590ef7" UUID_SUB="166aa0d2-ada9-45f3-b388-82897c1caf40" BLOCK_SIZE="4096" TYPE="btrfs"
/dev/sdi1: UUID="f4c7a0bf-b527-47f1-ada2-5da96e590ef7" UUID_SUB="62953cd2-a423-47a2-9a2c-b9f6a3f1af4b" BLOCK_SIZE="4096" TYPE="btrfs"


4- btrfs fi show f4c7a0bf-b527-47f1-ada2-5da96e590ef7

Label: none  uuid: f4c7a0bf-b527-47f1-ada2-5da96e590ef7
        Total devices 2 FS bytes used 501.16GiB
        devid    1 size 465.76GiB used 277.03GiB path /dev/sdi1
        devid    2 size 465.76GiB used 277.03GiB path /dev/sdh1


Edited by nextgenpotato
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Posted (edited)

Well, I was telling myself not to rush and try random things to fix it, risking breaking it further, but I have no patience :)


I was able to fix the mounting issue after some googling, and I'm backing up my data (my VM disk images) as I type this.

Solution was posted on reddit r/btrfs.

Basically all I did was to run this command, which I understand deletes the btrfs log marking the drive bad, so it's mountable again at your own risk, which is what I wanted.

btrfs rescue zero-log /dev/sdi1


And I don't think the root cause here was drive failing (it has CRC count of 1)


I was running qcow2 VM disks on this btrfs raid0 config.

I've seen a few times the windows 10 VM reporting it's disk being full, while in reality it was far from being full. And it would go back to normal after a restart.

And when I checked the disk image size it was maxed out as well, being qcow2 it had to be closer to the size of disk user, rather than size allocated.

So I think there is a bug somewhere making the file system confused in this particular scenario.


I ordered a new nvme drive to replace this configuration and I will go back to boring old XFS without any raid config :)


Edited by nextgenpotato
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