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To check a NAS for silent data corruption, run the data or filesystem scrub supported by its filesystem, then review the scrub results and system logs. A SMART test checks drive health; it does not compare file contents with checksums. Before starting, identify the NAS model, operating-system version, filesystem, storage layout, and whether checksums are enabled for the data you want to protect. A scrub can detect damage, but it can repair it only when the system has a verified good copy.
What a NAS scrub checks—and what it does not
A scrub reads stored data and checks it against checksums or, depending on the storage system, redundancy information. Btrfs scrub validates checksum-protected data and metadata. OpenZFS scrub checks block checksums across the pool, including blocks that have not recently been read. This makes a scrub useful for finding errors that an ordinary drive-health check may not reveal.
A scrub is not the same as a SMART test, a RAID parity check, or a backup-integrity check. Each answers a different question:
- Filesystem or data scrub: Do the stored blocks match their checksums, and can a valid redundant copy repair a mismatch?
- SMART test: Is the drive reporting signs of a hardware problem?
- RAID scrubbing: Is parity or redundancy data consistent? QNAP describes this for RAID 5/6; it is not identical to checking file data against filesystem checksums.
- Backup integrity check: Does the backup contain the expected files or matching content?
As the OpenZFS project puts it, “ZFS checksums every block, so it knows when data is wrong instead of guessing.” That describes checksum detection; it does not mean every damaged block can be reconstructed.
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Identify your NAS, filesystem, and checksum coverage
Record the NAS model and operating-system version, filesystem (for example, Btrfs, ZFS, or ext4), pool or RAID layout, and checksum settings for the folders you care about. Support and menu labels vary by model and software version, so use the vendor’s current interface and instructions for your device rather than assuming every NAS exposes the same controls.
Checksum coverage matters. On Synology DSM, Btrfs silent-corruption detection depends on the shared-folder data checksum option being enabled. The DSM 7.2 software specification also limits corrupted-data recovery to supported configurations; check the specification and your device’s pool configuration before relying on repair. A scrub cannot compare file data to a checksum that was never recorded.
There is an advanced Btrfs exception: the NOCOW file attribute, set with chattr +C, implicitly enables NODATASUM. Metadata remains validated, but file data does not receive checksum validation. This can matter in application data directories; check the NAS vendor’s handling of the relevant workload before assuming those files are covered.
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Choose the scrub supported by your NAS
These options are not interchangeable. Use the vendor-supported control where available, and check that it applies to your filesystem and pool.
| System or check | What it checks | Repair and scheduling notes |
|---|---|---|
| Btrfs scrub | Checksum-protected data and metadata. | On replicated profiles such as RAID1, a read-write scrub can repair damage using a verified good replica. Btrfs documentation recommends a monthly scrub and estimates about 80% of idle filesystem device bandwidth utilization; schedule it for a quieter period. |
| OpenZFS scrub | Block checksums across the pool. | Can repair when the pool has redundancy. OpenZFS operations documentation (accessed 2026-10-07; no publication date stated) describes monthly as a common baseline for consumer disks and quarterly for enterprise disks. Scrubs are I/O-intensive and cannot overlap with a resilver. |
| QNAP RAID scrubbing | Parity consistency for RAID 5/6, as described by QNAP. | QNAP recommends at least monthly and scheduling outside busy hours because of possible read/write impact. This is a parity check, not automatically a filesystem checksum scrub. |
| QNAP ZFS data scrubbing | ZFS data on QES and QuTS hero, as described in QNAP’s 2022-07-20 scrubbing article. | QNAP recommends at least monthly and suggests avoiding busy hours. Follow the control and instructions for your QNAP software version. |
| Synology DSM scrubbing | DSM’s specification lists filesystem scrubbing for Btrfs and RAID scrubbing for RAID 5, RAID 6, and RAID F1. | DSM runs the operations sequentially when a data scrub is performed. Synology’s Knowledge Center describes additional supported pool types and checksum prerequisites; verify support for the specific model and pool. |
For the Btrfs, OpenZFS, and QNAP guidance above, treat the stated cadence as platform guidance, not a guarantee that a particular schedule prevents all data loss. Follow a NAS vendor’s supported policy if it differs.
Run the scrub and save its results
- Check the pool first. Confirm the intended volume or pool is healthy enough to run the vendor-supported scrub, and note any active rebuild or resilver. OpenZFS scrubs cannot overlap a resilver.
- Open the NAS storage or filesystem controls. Select the data/filesystem scrub for Btrfs or ZFS. On QNAP, distinguish the RAID scrubbing option from ZFS data scrubbing. On Synology DSM, confirm the model, DSM version, and pool type support the operation; exact controls vary.
- Choose a low-activity time. A scrub reads substantial data and can affect performance. Btrfs estimates roughly 80% of idle filesystem device bandwidth utilization, and QNAP warns of read/write impact.
- Let it finish, then save the completion summary. Record whether the operation completed, how many errors were corrected or left uncorrectable, and which files or devices were identified. A task marked complete is not proof that its error count was zero.
For a system administered directly with OpenZFS, the documented shell commands are zpool scrub pool and, after it finishes, zpool status -v pool. Replace pool with the actual pool name. Use the NAS vendor’s interface instead if shell administration is not supported or appropriate for your device.
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Interpret checksum errors and device counters
On Synology, open Log Center > Logs to find files associated with a checksum-mismatch notification, as described in Synology’s support article last updated 2023-08-30. On OpenZFS, zpool status -v can identify files with unrepairable errors. A nonzero CKSUM counter means a device returned data that did not match its checksum. OpenZFS notes that cabling, a controller, or memory can be implicated; the counter alone does not establish which component is at fault.
Save the status and logs before taking corrective action. Clearing counters does not repair file contents. If errors recur, investigate the identified device and the storage path rather than treating a reset counter as a clean bill of health.
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Check drive and memory health separately
If the scrub or logs point to a drive problem, run the NAS vendor’s supported SMART tests. Synology documents scheduling SMART tests and lists defective drives and faulty memory modules among possible causes of checksum mismatch errors; its guidance also recommends a memory test. These are troubleshooting signals, not proof that file contents are correct. If diagnosis indicates a failed drive, verify replacement compatibility for the exact NAS model before installing a compatible replacement NAS hard drive.
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Know when to restore data
Repair requires a valid alternate copy. Btrfs scrub is not a general filesystem checker such as btrfs check: it validates checksums and relies on known-good replicas for repair. OpenZFS says persistent errors on a file mean the data is lost from that pool and should be restored from a backup or snapshot. Do not run low-level repair commands casually on valuable data.
RAID redundancy is not a backup: it may provide a copy for repair, but it does not provide a clean historical version if corruption has already reached the other copies. Synology also cautions that snapshots and synchronized or onsite copies may already contain corrupted data. When possible, restore from a known-good offsite backup, and preserve the affected NAS data and logs until you understand the failure.
Verify the backup before relying on it
Run the integrity check provided by your backup software, not just a check that a job completed. For QNAP HBS 3, the versioned 3v21.x documentation distinguishes quick checks from content checks:
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- Content check: uses hashes to compare file content. Some job types temporarily download files.
Supported job types depend on HBS version and backup type. QNAP warns that checks can require significant resources and that cloud downloads may incur provider fees. Confirm that the backup copy is readable and contains the needed version before using it for a restore.
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