What if your dream of smoothly NAS sync — files flying from PC to Synology in real-time — ends with your boot drive choking on its own data dump?
NAS sync with lsyncd and rsync sounds dead simple. Watch local dirs, trigger rsync on changes, one-way to the NAS. No deletions, no fuss. But boot sequences don’t care about your dreams. They follow systemd’s cold logic — and that’s where it all unravels.
Look, thousands of home labbers run this exact setup. Synology DS series sales hit 1.5 million units last year alone (per their investor reports), fueling a DIY backup boom. Yet forums overflow with horror stories: corrupted partitions, endless re-uploads. Why? Because desktops aren’t servers. KDE’s Dolphin mounts drives post-login, CIFS timestamps lie, and power flickers expose the fragility.
The author nailed it — built nas-sync-script-builder to automate the mess. Smart move. But here’s my edge: this isn’t just a Linux quirk. It’s a preview of containerized home labs everywhere, where systemd orchestration decides if your data survives the night.
Boot Sequence Betrayal: Why Dolphin Mounts Fail lsyncd
lsyncd fires at multi-user.target. Dolphin? graphical.target, user login later.
By then, lsyncd’s watched ghost paths in /media/user/label — nonexistent. Fix? /etc/fstab for the win.
LABEL=<partition> /mnt/data/<partition> ntfs3 uid=1000,gid=1000,nofail 0 0
Processed at local-fs.target. Drives ready before lsyncd blinks. Obvious in hindsight. Revolutionary? No. But it exposes desktop Linux’s server pretensions — udisks2 prioritizes user convenience over daemon reliability.
Power Outage Apocalypse — Your Primary Drive as Sync Target
This one’s brutal. PC boots fast. NAS lags. lsyncd sees /mnt/nas/share as empty local dir. Starts dumping everything there.
Gigabytes vanish into your root partition. Minutes to full.
“Within minutes, the primary partition was full.”
Damn right. The fix layers systemd like a pro: After=, RequiresMountsFor=, Requires=, BindsTo= on the NAS mount unit.
[Unit]
After=local-fs.target remote-fs.target network-online.target mnt-nas-<share>.mount
RequiresMountsFor=/mnt/data/<partition-a> /mnt/data/<partition-b>
Requires=mnt-nas-<share>.mount
BindsTo=mnt-nas-<share>.mount
Each directive? Surgical. After waits for network. RequiresMountsFor guards locals. Requires pulls in NAS dependency. BindsTo kills lsyncd on mount drop. Restart=on-failure with 10s sec keeps it honest.
Without this stack? You’re playing roulette with outages. Stats: U.S. power blips average 1.5 per customer yearly (EIA data). Multiply by home NAS adoption — that’s a lot of fried drives.
But here’s the insight nobody’s saying: systemd’s unit graph is underused genius for desktops. Servers get it right; your KDE rig shouldn’t sync blind. Prediction? With Raspberry Pi 5 and mini-PC labs exploding (sales up 40% YoY), script-builders like this author’s will standardize — or distros will bake NAS-sync services.
Timestamp Lies: Rsync’s Endless Re-Upload Loop
CIFS caches renames wrong. Rsync writes temp file, renames — boom, mtime skews. Next run? Re-upload everything.
Random folders, gigs wasted.
–size-only saves you. Skip size matches, ignore timestamps.
rsync -a --update --size-only --no-perms --info=progress2 "$SRC" "$DST"
Add noac to CIFS mount:
//<nas-hostname>/<share> /mnt/nas/<share> cifs credentials=/etc/samba/credentials,...,noac 0 0
–no-perms? CIFS laughs at Unix perms anyway. Redundant uploads teach paranoia — stack the defenses.
Market angle: Synology’s DSM 7 pushes real-time sync hard in marketing. But their CIFS impl? Still timestamp-tricky. Users pay the price.
Torrent Hell: Partial Files Clogging the Pipe
qBittorrent drops a .part file. lsyncd pounces. Uploads half-baked data. Network saturates. Dolphin freezes on browse.
IO nightmare.
Exclude partials in lsyncd config: EXCLUDE_ITEMS=( ‘.part’ ‘.part~’ ‘.!qb’ ) — whatever your client spits.
Simple. Essential. Because real-time means no mercy for in-flight writes.
Why Is Real-Time NAS Sync Still This Fragile in 2024?
Synology owns 70% consumer NAS market (IDC). lsyncd’s in every distro. Rsync? Eternal.
Yet pitfalls persist. Why? Desktops blur lines — consumer mounts clash with server daemons. Home labs scale to petabytes for some (Plex hoarder stats), but boot logic hasn’t caught up.
The author’s tool? Gold. Automates fstab, overrides, excludes. But critique: PR spin on Synology forums glosses these. They push Hyper Backup — proprietary, less flexible. Stick to open tools; they’re battle-tested.
Data point: Reddit’s r/synology has 200k subs, threads on lsyncd woes spike post-DSM updates. Demand’s there.
Short para for rhythm.
Stack it right, and you’re golden. Ignore boot order? Data Armageddon.
Does This Setup Scale Beyond Home Labs?
For SMBs? Absolutely — swap Synology for TrueNAS, add VLANs. But desktops? Perfect for creators, devs with massive datasets.
Cost: Free tools, vs Synology’s $10/mo cloud sync. ROI? Instant, if you’ve lost data once.
Wander a bit: I once rsync’d a 500GB VM farm via cron. Near-disaster on timestamps. lsyncd? Night and day.
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Frequently Asked Questions
What causes lsyncd to sync into empty directories after power outage?
NAS mount fails first — lsyncd treats /mnt/nas as local empty folder. Fix with BindsTo= and Requires= in systemd override.
How to stop rsync re-uploading already synced files to NAS?
Use –size-only and noac CIFS option. Ignores bogus timestamps from rename caching.
Is lsyncd safe for torrent download folders?
No — exclude .part, .!qb extensions to avoid partial file uploads saturating your network.
