Squashfs snap mounts on Ubuntu (and similar read-only filesystems like
erofs on Home Assistant OS) always report near-full usage and trigger
false disk alerts. The filter logic existed in Proxmox monitoring but
wasn't applied to host agents.
Changes:
- Extract read-only filesystem filter to shared pkg/fsfilters package
- Apply filter in hostmetrics.collectDisks() for host/docker agents
- Apply filter in monitor.ApplyHostReport() for backward compatibility
- Convert internal/monitoring/fs_filters.go to wrapper functions
This prevents squashfs, erofs, iso9660, cdfs, udf, cramfs, romfs, and
saturated overlay filesystems from generating alerts. Filtering happens
at both collection time (agents) and ingestion time (server) to ensure
older agents don't cause false alerts until they're updated.
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