diff --git a/server/deploy/zfs-arc.conf b/server/deploy/zfs-arc.conf
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+# ZFS ARC sizing for glados — a mixed-workload host (32 GB RAM).
+#
+# Why these numbers:
+#  - The cold ZFS pool is BACKUP TIER ONLY. Hot data (MC world, Gitea repos,
+#    Directus DB, containers) all live on NVMe LVM, not ZFS — so a large
+#    ARC doesn't help any of the actual workloads.
+#  - Tier-2 backup work is streaming writes (rsync hot→cold every 4h):
+#    streaming writes don't benefit much from cache.
+#  - Reads happen rarely (disaster recovery, Gitea LFS fetches), and a
+#    one-time cold read is fine.
+#  - MC is the memory-hungry workload (10 GB Java heap). Every GB we DON'T
+#    give to ARC is a GB more headroom for MC and future content.
+#
+# 2 GB max / 1 GB min keeps ZFS metadata + small ARC for occasional reads,
+# without crowding MC out of RAM.
+options zfs zfs_arc_max=2147483648
+options zfs zfs_arc_min=1073741824