Hi this happened yesterday and a reboot fixed the problem but today it didn't fix.
Volume appears in crash mode, and read only mode.
I logged in from ssh and looked at the logs, I saw this in dmesg after the reboot:
then in messages log I found many of these
I tried to do a btrfs --repair but it didn't work and didn't took note of the error. now the server is shutdown and I have to decide what to do next
This is a:
DSM 6.2.2-24922 Update 4