# `[2026-09-03]` nh3-dev wedged for ~40 min and it was the BACKUP, not the disk — a stalled cross-site vzdump holding every gue

**nh3-dev wedged for ~40 min and it was the BACKUP, not the disk — a stalled cross-site vzdump holding every guest write via `copy-before-write`.** Symptoms screamed dying disk: 45 writes in flight completing zero, jbd2 + flush kworkers in D state 33 min, io pressure full 96%, load 26, `virtio_ring` in the stack. ⚠ **The discriminator was the ABSENCE of errors** — no SCSI/ATA/IO errors, rpool ONLINE 21%, guest fs 79%, memory fine, and **Dirty only 3.8 MB** (so nothing backed up in page cache; it was stuck BELOW the block layer). ⚠ **The hypervisor was IDLE** — load 0.63, io pressure 0.00, zpool ~0 writes: nothing was reaching the disk because the filter held it. Cause: `vzdump` of VM 102 → **pbs-ana** did 1% at 64 MiB/s then collapsed to **1.4 MiB/s for 35 min**; Proxmox backups interpose a `copy-before-write` filter, so every guest write queues behind the backup's copy-out. FIX = cancel the task (`pvesh delete /nodes/localhost/tasks/<UPID>`); filter detached, inflight 45→0, D-states gone, 191 MB/s dsync restored. ⚠ **`fleecing 0` on the job is why a slow TARGET can stall a GUEST** — fleecing routes copy-before-write to a fast local image instead. Job = `backup-5d8f1221-8f71`, **daily 21:00, `all 1`, storage pbs-ana** → recurs nightly until changed. A prior run of this VM managed 941 MiB/s read, so 1.4 MiB/s is degradation, not normal. → `docs/runbooks/nh3-dev-io-stall.md`