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DuoBolt

DuoBolt for NAS

A NAS accumulates duplicates faster than any other storage you own. Years of phone backups, photo libraries synced from three devices, a spouse’s media library, a second copy “just in case”, a torrent that overlapped with a streaming rip — it all stacks up on the same volume and grows invisibly for years.

This page covers how DuoBolt handles NAS scanning specifically: why most duplicate finders fail on network storage, how DuoBolt’s engine was built around the problem, and a step-by-step guide for scanning Synology, QNAP, TrueNAS, Unraid, and any SMB/NFS share.

Why NAS Is the Hardest Place to Find Duplicates

Section titled “Why NAS Is the Hardest Place to Find Duplicates”

Scanning a NAS is fundamentally different from scanning a local SSD. Three things conspire against most duplicate finders:

Cold, spun-down disks

A power-saving NAS parks its HDDs after idle minutes. First-access latency jumps from milliseconds to seconds as each drive spins up. Tools that read files sequentially stall on every new directory.

Network I/O latency

Every file open, stat, and read is a round-trip over Ethernet. A local scan doing 50,000 files/sec might drop to 500/sec over SMB. Legacy tools never see saturation — they’re always waiting on the wire.

Single-threaded hashing

Most duplicate finders still hash one file at a time with MD5 or SHA-256. On a modern 8-core machine scanning a NAS, seven cores sit idle while the eighth waits on a slow network read.

The result: older tools routinely time out on terabyte-scale NAS volumes. In our benchmark suite, both dupeGuru and Gemini 2 exceeded the 15-minute cutoff on a 1 TB Synology snapshot. Czkawka completed, but took 85 seconds cold where DuoBolt took 64.

How DuoBolt’s Engine Handles NAS

Section titled “How DuoBolt’s Engine Handles NAS”

DuoBolt was designed from day one with NAS and terabyte-scale storage in mind. Four architectural decisions compound to deliver the speed advantage:

  1. Per-root parallelism

    Multiple scan roots run in parallel instead of sequentially. If you point DuoBolt at /Volumes/NAS/photos and /Volumes/NAS/videos, both walk concurrently, so the bottleneck becomes disk and network throughput — not tool pacing.

  2. Streaming chunked I/O

    Reads are chunked and streamed, overlapping disk access with hashing. While BLAKE3 processes one chunk, the next is already on the wire. CPUs stay busy; the network is never the limiter waiting for a free core.

  3. Multi-core BLAKE3 hashing

    BLAKE3 is tree-structured and multi-core by design. On an M1 Pro scanning a NAS, DuoBolt uses all performance cores in parallel to process chunks, saturating whatever I/O the NAS can deliver.

  4. Head+tail prehash

    Before any full-content hash, DuoBolt hashes just the first and last N KiB of each candidate file. Non-matches are eliminated from the candidate pool before touching the rest of the file — a massive I/O saving on large media files that look similar by size but differ at the edges.

Benchmark: 1 TB Synology DS920+

Section titled “Benchmark: 1 TB Synology DS920+”

Cold scan — 64 seconds

1.01 TB volume, HDDs spun down before the test. Mix of 605 GB video, 246 GB photos, 162 GB music. DuoBolt discovers, prehashes, full-hashes, and groups everything in just over a minute.

Warm scan — 20 seconds

Same volume, caches warmed. DuoBolt’s cache layer short-circuits unchanged files, pushing the scan under 20 seconds on subsequent runs.

For side-by-side comparison with Czkawka, dupeGuru, and Gemini 2 on the same hardware, see the full benchmark results.

Cache Layer — Why Warm Scans Finish in Seconds

Section titled “Cache Layer — Why Warm Scans Finish in Seconds”

The difference between a 64-second cold scan and a 20-second warm scan is not magic — it is DuoBolt’s cache layer working as designed. When DuoBolt hashes a file, it records the full-content BLAKE3 hash keyed by path, size, and modification time. On subsequent scans, files whose (size, mtime) signature is unchanged skip hashing entirely and pull their hash from the cache.

For a NAS this matters more than for any other target. Every hash retrieved from cache is one file DuoBolt does not read over the network — the single biggest win on a medium dominated by SMB or NFS latency.

Configure the cache under Settings → Storage. Toggle the hash cache on or off, and set separate minimum sizes for local and network volumes — the network one defaults lower on purpose, because re-reading a small file over SMB costs far more than caching it does. Every control, with its default.

The Settings Storage panel: the hash cache toggle above minimum file size sliders for local and network volumes, set to 64 and 32 KiB, followed by the buttons that clear each cache and compact the cache file.

The Minimum file size (network) slider is the one to reach for on a NAS — lower it to cache smaller files on slow NAS shares, raise it to keep the cache lean. Compact cache file reports what the cache is holding and returns the room it no longer uses, without dropping anything it has cached.

Scanning a NAS Step-by-Step

Section titled “Scanning a NAS Step-by-Step”

  1. Mount the share on your machine

    • macOS: Finder → Cmd+K → smb://your-nas.local/volume (or afp://, nfs://)
    • Windows: File Explorer → right-click “This PC” → Map Network Drive

  2. Open DuoBolt and click Add Folder

    Navigate to the mounted share and select the directory you want to deduplicate (e.g., /Volumes/NAS/photos).

  3. Configure filters (optional but recommended)

    • Min size: set to 1 MiB or higher to skip thumbnails and metadata files
    • Ignore hidden files and Ignore system files on
    • Exclude directory extensions: .photoslibrary, .aplibrary, .tmbundle (macOS managed bundles)

  4. Run the scan

    First scan is “cold” — expect the times shown in our benchmark. Subsequent runs benefit from DuoBolt’s cache layer.

  5. Review before deleting

Supported NAS Platforms

Section titled “Supported NAS Platforms”

DuoBolt works with any share the host OS can mount — in the typical setup it runs on a desktop or server that has the NAS share mounted, not on the NAS itself. Tested and known-good:

  • Synology — DS, RS, FS series (DSM 6 and 7)
  • QNAP — TS, TVS, TBS series (QTS and QuTS)
  • TrueNAS — CORE and SCALE (SMB and NFS)
  • Unraid — via SMB shares
  • Asustor, Terramaster, Ugreen — any SMB/AFP/NFS share
  • Custom Linux file servers — Samba, NFS, or SSHFS mounts

For SMB shares, DuoBolt works transparently through the mount — there is no protocol-level optimization needed.

Protocol Choice — SMB, NFS, AFP, SSHFS

Section titled “Protocol Choice — SMB, NFS, AFP, SSHFS”

DuoBolt does not speak network protocols directly. It scans through whatever the host operating system mounts, so the protocol choice lives entirely on the mount side — but it materially affects scan throughput, especially on long cold scans.

  • SMB (SMB3) — the default on macOS and Windows for Synology, QNAP, TrueNAS, Unraid, and any modern share. Supports multichannel, encryption, and signing. The best all-around choice for most users and the configuration our benchmarks run on.
  • NFS (v4) — common on Linux hosts and ZFS-backed NAS. Slightly lower per-operation latency than SMB in many setups, at the cost of trickier UID/GID permission semantics. If both host and NAS run Linux, NFS frequently edges out SMB on large scans.
  • AFP — deprecated. Apple dropped it as Time Machine’s default in macOS 11. Read-only compatibility remains but it is slow; avoid as a primary mount.
  • SSHFS — convenient (any machine with SSH access works), but the per-file round-trip overhead is punishing on scans of tens of thousands of small files. Usable for ad-hoc scans; a poor default.
  • iSCSI / block-level mounts — appear as local disks to the OS and to DuoBolt. Fast, but the target is exclusive to one client at a time.

When in doubt: SMB3. NFS only if you have a specific reason.

What to Exclude from a NAS Scan

Section titled “What to Exclude from a NAS Scan”

Not everything on a NAS that looks like a duplicate is one. Filesystems, backup tools, and media apps routinely store intentional hardlinks, snapshots, or managed metadata that inflate scan counts without representing real redundancy. Knowing what to skip shortens the scan and produces cleaner results.

Synology:

  • @eaDir/ — hidden thumbnail and metadata directory created inside every media folder. Useless to hash. Exclude it by directory extension: eaDir.
  • #recycle/ — per-share Recycle Bin. Often holds intentional duplicates of “deleted” files; decide based on your cleanup goals.
  • @appstore/, @database/, @tmp/ — system-managed, always skip.

Filesystem snapshots (ZFS, BTRFS, Synology Btrfs):

  • .snapshots/, .zfs/snapshot/ — point-in-time copies of the entire volume. They look like massive duplication but represent historical state. Always exclude; otherwise every snapshot doubles the apparent duplicate count.
  • APFS snapshots are invisible at the file level on macOS hosts, so no action needed.

Backup bundles:

  • .backupdb/ (Time Machine) — relies on OS-level hardlinks. DuoBolt’s symlink/hardlink collapse handles this correctly by default, but excluding the directory saves a lot of scan time.
  • .AppleDouble/ and ._* AppleDouble sidecars — generated by macOS on non-HFS filesystems. Covered by Ignore system files.

Managed library bundles (macOS):

  • *.photoslibrary, *.aplibrary, *.itlp, *.imovielibrary — appear as single files in Finder but are massive directory trees internally. Exclude by directory extension: photoslibrary, aplibrary, itlp, imovielibrary.

Cross-platform noise:

  • Thumbs.db, desktop.ini, .DS_Store — always safe to skip. Covered by Ignore system files.

All of the above can be handled in one pass by toggling Ignore system files, Ignore hidden directories, and Exclude directory extensions in the scan setup.

Performance Tips for NAS Scans

Section titled “Performance Tips for NAS Scans”
  • Warm the cache on the first pass. Cold-scan timings are not representative of steady state; the second run is what day-to-day workflow feels like.
  • Set a sensible minimum file size. Thumbnails, sidecar metadata, and app config files are rarely worth hashing and inflate scan counts without reclaiming space.
  • Exclude managed library bundles (.photoslibrary, .aplibrary, .itlp). These look massive but are not user-facing duplicates.
  • Scan during off-peak hours if the NAS is also handling Time Machine backups, streaming, or other heavy workloads — network contention kills scan throughput.
  • Re-run an audit from Scan History. It keeps the exact setup a scan used, so a monthly NAS pass is one click rather than a set-up job.

Next Steps

Section titled “Next Steps”
Download DuoBolt macOS and Windows — free to scan, free to start cleaning
Performance Benchmarks Full comparison against Czkawka, dupeGuru, and Gemini 2
Similar photos Group look-alike shots once the exact duplicates are gone
FAQ Safe deletion, permissions, and common NAS scanning questions