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I've just been in trouble with crashed btrfs volume, too. Also can't mount the volume & storage manager shows crashed volume "0 bytes / 0 bytes". I found a page about btrfs recovery below, and followed those steps described. https://lists.opensuse.org/opensuse/2017-02/msg00930.html In my case also, "btrfs restore"(Step 8 ) seems to solve the problem. I try to restore 2.5TB volume(used over 2TB), but only I can restore is about 500GB. I followed steps further, on Step11, I could recover almost all chunks (failed only 1 chunk), but chunk tree it self couldn't be recovered. I tried to restore again on Step12, but this step doesn't recover any other files other than that of Step 8. (btrfs recover doesn't overwrite already recovered file, so you can choose same destination to skip already recovered files). ------------<excursion>----------- By the way, My ESXi Setup is a bit complex. (Please read the linked page.) A PSOD crashes the SSD cache, and this makes "User DSM" VM's btrfs volume can't be mount. This was caused by a hardware trouble. (I moved my server (changed location) recently. After that, my system become unstable...) There were 3 PSODs, two didn't crash the volume at all, but 1 PSOD cause severe crash of that btrfs volume and a Win10 VM's NTFS drive. (User files are not lost, but It crashes Win10 system drive severely !) I have to restore Win10 VM from a backup. In my setup, my disk was virtualized, so I can use snapshot and try any btrfs recovery without any risk. (I also copied 2.5TB ext4 vmdk to another volume. It's not required though.) Umm, but there are several trade-offs, and I might not choose same setup if I make another... ------------</excursion>----------- I almost abandoned, but I found this answer on ask ubuntu, which says, I confirmed with 'uname -a' and kernels of DSM6.1 was 3.10. So, I tried to use ubuntu live cd(used 18.04LTS), and it magically mount my crashed btrfs volume ! I could restore almost all files from crashed volume without losing metadata. (Using 'btrfs recover', all the metadatas like permissions, timestamps are gone.) I confirmed that 'btrfs chunk-recover' and mount using newer btrfs system is the key for recovering my crash. (I rolled back to initial state & tried to recover from scratch using Ubuntu Live CD only, but I can't mount volume without 'btrfs chunk-recover' before Step 11. I also tried Step14 without Step11, it also failed.) I recovered using rsync, from crashed btrfs volume to nfs exported "Host DSM" volume (ext4). You need to exclude "@" started folders (see below sample). (Most important is exclude "@sharesnap", because it is snapshot backup files, and it has so many duplicated files and can't restore if you took many snapshots.) Install gnu screen or tmux to make session detachable & have several work windows. (without that, you can't shutdown your PC while restoring) You can monitor read error with "tail -f rsync-recover.log | grep '^rsync:' "(rsync) , "sudo tail -f /var/log/syslog | grep -i btrfs"(btrfs system) (I used script(1) to save error log, because tee doesn't work. It seems something blocks output. Error log is important to find which file is unreliable.) I feel relief when I found I could recover almost all the files. I want to follow "3-2-1 backup" rule, though. : -)

Login (replace URL, PORT, USER, PASS) https://URL:PORT/webapi/auth.cgi?api=SYNO.API.Auth&method=Login&version=1&account=USER&passwd=PASS for ActiveBackup its the following urls Get Activationstatus https://URL:PORT/webapi/entry.cgi?api=SYNO.ActiveBackup.Activation&method=get&version=1 Set ActivationStatus (use your SERIALNUMBER) https://URL:PORT/webapi/entry.cgi?api=SYNO.ActiveBackup.Activation&method=set&version=1&activated=true&serial_number="SERIALNUMBER"  for Office365 its the following urls Get Activationstatus https://URL:PORT/webapi/entry.cgi?api=SYNO.ActiveBackupOffice365&method=get_activation&version=1 Set ActivationStatus (use your SERIALNUMBER) https://URL:PORT/webapi/entry.cgi?api=SYNO.ActiveBackupOffice365&method=set_activation&version=1&serial_number="SERIALNUMBER" for GSuite its the following urls Get Activationstatus https://URL:PORT/webapi/entry.cgi?api=SYNO.ActiveBackupGSuite&method=get_activation&version=1 Set ActivationStatus (use your SERIALNUMBER) https://URL:PORT/webapi/entry.cgi?api=SYNO.ActiveBackupGSuite&method=set_activation&version=1&serial_number="SERIALNUMBER"