Synology RS3621xs+ volume crashed recovering metadata
A volume crash indicating metadata recovery failure on the Synology RS3621xs+ is typically caused by a corrupted Btrfs filesystem layer triggered by a failing storage drive. This is often linked to SMART ID 187 (Reported Uncorrectable Errors) or ID 197 (Current Pending Sector). The immediate fix requires identifying the degraded physical disk via DSM Storage Manager, pulling the faulty unit, and initiating a parity rebuild using a matching replacement drive.
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Diagnostic Matrix for RS3621xs+ Metadata Faults
| Symptom / Log Code | Root Hardware Cause | Verification Command | Confirmed Fix |
|---|---|---|---|
| Volume Crashed / Metadata | SATA/SAS disk failing | smartctl -a /dev/sdX |
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| I/O Error / Silent Corruption | Backplane signal degradation | dmesg | grep -i "ata" |
Reseat cable; replace SFF-8643 Check Continuous NVRNAND Media on Amazon ↗ |
| System fan/power warnings | 12V DC rail instability | cat /sys/class/hwmon/ |
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| High Latency / Slow Write | ECC RAM parity error | dmidecode -t memory |
Swap DIMM; memtest86+ |
Phase 1: Initial Hardware Assessment
Before jumping into software repairs, we need to determine if this is a media-level failure or a controller-level event. Metadata recovery errors in Btrfs often occur because the filesystem can no longer map logical blocks to physical sectors. If your RS3621xs+ is blinking amber on one or more drive bays, your physical path is clear: a drive has exceeded its threshold for uncorrectable errors.
Check the rear of the unit. Ensure that your Cat6A patch cables are properly seated in the 10GbE SFP+ ports or the RJ45 interfaces. While network issues rarely cause metadata corruption, a dropped connection during a high-load write process can lead to partial header writes. Inspect the SFP+ transceiver for signs of heat discoloration or dust ingress. If you are using a DAC cable, swap it out to rule out physical shielding failure.
Phase 2: CLI Diagnostics and SMART Analysis
Log into your RS3621xs+ via SSH using an admin account. Once inside, you need to verify which disk is throwing the I/O errors. The Synology kernel reports block-level errors to the dmesg buffer. Run the following command to check for active kernel panics or disk read errors:
admin@RS3621xs:~$ sudo dmesg | grep -i "error" | tail -n 20
[ 4521.123456] ata3: exception Emask 0x0 SAct 0x0 SErr 0x0 action 0x6 frozen
[ 4521.123480] ata3: hard resetting link
[ 4525.678910] sd 2:0:0:0: [sda] tag#0 FAILED Result: hostbyte=DID_BAD_TARGET driverbyte=DRIVER_OK
[ 4525.678930] sd 2:0:0:0: [sda] tag#0 CDB: Read(10) 28 00 00 00 01 00 00 00 08 00
[ 4525.678950] print_req_error: I/O error, dev sda, sector 256If you identify a failing drive (e.g., /dev/sda), execute smartctl -a /dev/sda to check SMART attributes. Focus specifically on ID 5 (Reallocated Sector Count), ID 187 (Reported Uncorrectable Errors), and ID 197 (Current Pending Sector). If ID 187 is greater than zero, the drive is physically dying and must be replaced immediately. Do not attempt to run btrfs check --repair on a drive that is actively generating I/O errors; you will only worsen the metadata corruption.
Phase 3: Power and Environmental Stability
The RS3621xs+ is sensitive to power fluctuations. As a high-performance 12-bay unit, it draws significant current. Ensure your unit is connected to an APC or CyberPower UPS. If your local power grid experiences micro-outages, your 12V DC rail may dip below operational tolerances, causing controller reset loops during heavy metadata writes. If the NAS spontaneously reboots, check the logs in DSM at Control Panel > Info Center > Log Center.
Confirm that your internal fans are spinning correctly. Overheating can cause the SATA backplane to expand slightly, leading to poor pin contact. If the chassis feels excessively hot, clean the dust filters and use compressed air on the fan array. A system running at 60°C+ is significantly more likely to trigger controller-side metadata errors.
Phase 4: Resolving the Metadata Corruption
Once the bad drive is isolated, follow these steps to restore structural integrity:
- Step 1: Identify the degraded drive serial number in Storage Manager > HDD/SSD.
- Step 2: Physically pull the drive. The RS3621xs+ supports hot-swapping, so you do not need to power down, though it is safer to stop non-essential services.
- Step 3: Insert a compatible replacement (e.g., Seagate IronWolf Pro or WD Red Pro). Ensure the capacity matches or exceeds the original.
- Step 4: Navigate to Storage Manager > Storage, select the crashed volume, and click Repair.
- Step 5: The system will rebuild parity. During this time, metadata will be rewritten from the remaining healthy drives.
If the volume remains in a “crashed” state after a successful rebuild, the filesystem journal may be locked. In extreme scenarios, you may need to use btrfs restore to pull data off the partition to an external storage array before reformatting the volume. Always ensure you have a verified backup on a secondary device, such as a localized tape drive or a cloud-synced repository, before performing advanced CLI repairs.
Frequently Asked Questions
Can I continue to access data while the volume is in a “crashed” state?
No. Once a Synology volume crashes, the kernel mounts the filesystem in read-only mode or unmounts it entirely to prevent further corruption. You must address the underlying drive failure or metadata mismatch before the volume can be remounted or repaired via the DSM Storage Manager interface.
What is the significance of SMART ID 197 when troubleshooting metadata?
SMART ID 197 identifies “Current Pending Sectors.” These are blocks that the drive controller cannot read correctly but has not yet reallocated. If the Btrfs metadata happens to sit on one of these sectors, the NAS will flag a metadata corruption error and initiate an automatic volume crash for safety.
Does a metadata recovery failure always mean the hard drive is physically dead?
Not always. While drive failure is the most common cause, logical corruption from unexpected power loss or firmware bugs can trigger it. If your SMART logs show zeros across the board, perform a full file system consistency check via the DSM UI before assuming a hardware replacement is mandatory.