Update:

quoted

quoted

quoted

this is a problem which apparently occurred when the user went from

v3.1.5 to v3.2, so this looks likes 3.2 regression.)

I am on 3.2.0 as well.

I didn't spot anything obvious in the logs.
 

It happened for me on a freshly created FS.
"mke2fs -j -O sparse_super -O dir_index -O extents -O filetype -O uninit_bg"
mounted with no additional options for the first time I got an
"EXT4-fs error (device md127): ext4_mb_generate_buddy:739: group 28671, 32765 
clusters in bitmap, 32766 in gd"
after writing about 3TB of data.
I do not have RO snapshots as the OP, but my md sits on to of luks 

containers. 

So we do have the device mapper in common.

After I did an fsck and tried to continue, I didn't get that far.
After another 200GB or so it happened again.
And now it's reproducible:
I can run fsck and then try to continue (using rsync). But as soon as writing 
starts, the process hangs for a long time. At least one minute, probably longer.
Then the ext4_mb_generate_buddy comes again.

I upgraded e2fstools from 1.41.14 to 1.42 and the kernel to 3.2.2.
No difference.
That FS is unusable.

Just for the records: Unlike the contents, the hardware is not new and did 

not 

have any known issues.

  Greetings,
    WIMPy

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