Fulcrum Sync Fail at around ~50%

StartOS Support
1

Hi! I migrated to new start9 0.4.0.1 and with that I decided to switch from Electrs to Fulcrum. After 24h the initial sync stalled around 42% after which it really slowed down, it took another 24h for the next 8%. I read from this start9 community topic that it’s a good idea to shut other services down to speed up the sync. I switched some processes off and I restarted Fulcrum probably a few times. Not sure if that did anything. I also stopped all the processes at one point including Bitcoin Knots. Anyway, after last restart syncing sped up about 5x to 2.5-3 blocks per second until around 52% when I got this ERROR Log:

Input/output error
2026-08-04T19:04:44-07:00 Shutdown complete
2026-08-04T19:04:44-07:00 Error: Fulcrum exited with code 1
2026-08-04T19:04:44-07:00 at ChildProcess. (/usr/lib/startos/package/index.js:4521:39)
2026-08-04T19:04:44-07:00 at ChildProcess.emit (node:events:524:28)
2026-08-04T19:04:44-07:00 at ChildProcess._handle.onexit (node:internal/child_process:293:12)
2026-08-04T19:04:44-07:00 Health Check primary failed: primary daemon crashed
2026-08-04T19:05:14-07:00 Enabled JSON parser: simdjson
2026-08-04T19:05:14-07:00 simdjson implementations:
2026-08-04T19:05:14-07:00 haswell: Intel/AMD AVX2 [supported]
2026-08-04T19:05:14-07:00 westmere: Intel/AMD SSE4.2 [supported]
2026-08-04T19:05:14-07:00 fallback: Generic fallback implementation [supported]
2026-08-04T19:05:14-07:00 active implementation: haswell
2026-08-04T19:05:14-07:00 jemalloc: version 5.3.0-0-g54eaed1
2026-08-04T19:05:14-07:00 Qt: version 6.8.2
2026-08-04T19:05:14-07:00 rocksdb: version 9.2.1-08f9322
2026-08-04T19:05:14-07:00 simdjson: version 0.6.0
2026-08-04T19:05:14-07:00 ssl: OpenSSL 3.5.5 27 Jan 2026
2026-08-04T19:05:14-07:00 zmq: libzmq version: 4.3.5, cppzmq version: 4.10.0
2026-08-04T19:05:14-07:00 UPnP: miniupnpc 2.2.8 (API version: 18)
2026-08-04T19:05:14-07:00 Fulcrum 2.1.1 (Release f06d28f) - Wed Aug 5, 2026 02:05:14.114 UTC - starting up …
2026-08-04T19:05:14-07:00 Max open files: 65536
2026-08-04T19:05:14-07:00 Loading database …
2026-08-04T19:05:14-07:00 DB memory: 2048.00 MiB
2026-08-04T19:05:14-07:00 Coin: BTC
2026-08-04T19:05:14-07:00 Chain: main
2026-08-04T19:05:14-07:00 Verifying headers …
2026-08-04T19:05:14-07:00 FATAL: Caught exception: BlkInfo txNums do not add up to expected value of 0 != 314017769.
2026-08-04T19:05:14-07:00 The database may be corrupted. Delete the datadir and resynch it.
2026-08-04T19:05:14-07:00 Stopping Controller …
2026-08-04T19:05:14-07:00 Closing storage …
2026-08-04T19:05:14-07:00 Flush of all column families returned: IO error: While pread offset 0 len 3892: /data/fulc2_db/034022.sst:

I have DIY setup: ThinkPad X13 Yoga Gen 1: Intel Core i7, 16GB RAM with 2TB ScanDisk external portable SSD Hard Drive. Storage had 1TB available space.

If I need to to delete datadir, could somebody please point that out where it is and how to delete it. Appreciated!

2

Hi there

Good news first — nothing important is lost. Fulcrum’s database is just an index built from your node, no funds or keys in it, so it’s always safe to wipe and rebuild.

The FATAL “delete the datadir and resync” message means the DB got left half-written. But the line that matters is the “Input/output error” just before the crash — that’s a disk-level failure, and on an external USB SSD it usually means the drive briefly dropped out or lost power under the heavy sync load. That’s the likely reason it corrupted.

Simplest fix, no command line: uninstall Fulcrum in the StartOS UI, then reinstall it. Uninstalling clears its data, and the fresh install starts a clean resync.

Two things for the resync:

  • Keep Bitcoin Knots fully synced and running the whole time — Fulcrum indexes from it.

  • Once Fulcrum starts, leave it alone. Stopping/restarting it mid-sync is a common way to corrupt the DB. Shutting other services down to free resources is fine, just don’t stop Fulcrum itself until it’s done.

If it corrupts again after a clean, uninterrupted resync, that points at the SSD or its connection rather than Fulcrum — post your exact drive and how it’s plugged in and we’ll take a look.

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