Hi everyone,

I recently performed an update on my server, and right after that, my Cache pool (a Samsung SSD 870, formatted as BTRFS) started showing as "Unmountable: unsupported or no file system."

Since my Docker appdata lives on this drive, I haven't formatted anything. I started the array in Maintenance Mode, changed the file system type from auto back to BTRFS so the tool would appear, and ran a read-only btrfs check.

Here is the output showing the free space tree errors:

[1/8] checking log skipped (none written)

[2/8] checking root items

[3/8] checking extents

[4/8] checking free space tree

Space key logical 1048576 length 4194304 has no corresponding block group

Space key logical 5242880 length 8388608 has no corresponding block group

[5/8] checking fs roots

[6/8] checking only csums items (without verifying data)

[7/8] checking root refs

[8/8] checking quota groups skipped (not enabled on this FS)

Opening filesystem to check...

Checking filesystem on /dev/sdn1

UUID: 925a6593-cd05-4d1b-a51e-6362c3d381ad

found 470118817792 bytes used, error(s) found

total csum bytes: 437582556

total tree bytes: 2232909824

total fs tree bytes: 1423835136

total extent tree bytes: 222789632

btree space waste bytes: 560886030

file data blocks allocated: 4158107447296

referenced 453169106944

I have attached my diagnostics zip file to this post as well.

Could someone please advise on the safest command to repair this space cache issue so I can get my drive mounted again without losing my data?

Thanks in advance for the help!

keep-diagnostics-20260604-1240.zip

Edited June 5Jun 5 by Hypner
Solved

How to Fix BTRFS Cache Pool Tree Errors (parent transid verify failed)

If your BTRFS cache pool suddenly drops and you see it as unmountable because of metadata tree corruption, don’t panic. Slow down—rushing increases the risk of losing data.

Here’s how you get things working again:

First, stop the Array using the Unraid webGUI.

Under Array Operation, enable Maintenance Mode, then start the array. Running in this mode loads disk drivers but doesn’t let Unraid try to mount the broken filesystem.

Open up the Unraid Terminal. Figure out your cache drive identifier—it’ll look like /dev/sdX1 or maybe /dev/nvmeXnXp1.

Now, run a non-destructive check. This checks for metadata problems without making changes:

btrfs check /dev/sdX1

If the check points out tree or parent transid mismatches, and that’s what’s stopping the filesystem from mounting, go ahead and run the repair. This part can take a while, so don’t interrupt it:

btrfs check --repair /dev/sdX1

When the repair finishes, go back to the Array Operations page, stop Maintenance Mode, and start the array normally.

Your cache pool should mount again, with all your original data right where you left it. I rebooted to be safe. This worked in my case no guarentee it will work in yours.