Mikulas Patocka <mpatocka <at> redhat.com> writes:

The existence of a snapshot changes I/O completion times significantly, so 
it may be a race condition in ext4 that gets triggered which changed 
timings.

The idea that timing might cause issues on a FS is disturbing.

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.

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.

Just for the records: Unlike the contents, the hardware is not new and did not 
have any known issues.

  Greetings,
    WIMPy