somehow fuse gets stuck... maybe fsync=0 fixed it?

experimental/Makefile

@@ -37,9 +37,13 @@ spack.lock: spack.yaml deps/spack
 # Spack believes it's installing to UPSTREAM_STORE, but in reality it's installing new
 # packages to `DOWNSTREAM_STORE`.
 build: spack.lock deps/usr/bin/overlayfs
-	mkdir -p $(UPSTREAM_STORE) $(DOWNSTREAM_STORE) work merged
+	# Not sure if really required, but to avoid having to bind mount
+	# the merged dir over the bottom layer, add a level of indirection with a
+	# symlink from $(UPSTREAM_STORE) to $(UPSTREAM_STORE)-current
+	mkdir -p $(UPSTREAM_STORE)-current $(DOWNSTREAM_STORE) work merged
+	ln -s $(UPSTREAM_STORE)-current $(UPSTREAM_STORE) || true
 	$(SRUN_BUILD) \
-		$(OVERLAY_SH) $(OVERLAYFS) $(UPSTREAM_STORE) $(DOWNSTREAM_STORE) $(CURDIR)/work $(CURDIR)/merged \
+		$(OVERLAY_SH) $(OVERLAYFS) $(UPSTREAM_STORE)-current $(DOWNSTREAM_STORE) $(CURDIR)/work $(CURDIR)/merged \
 		$(UNSHARE) -rm \
 			$(BIND_SH) $(CURDIR)/merged $(UPSTREAM_STORE) \
 			$(SPACK) \

experimental/bin/overlay.sh

@@ -20,7 +20,7 @@ fusermount -u "$merge" > /dev/null 2>&1 || true
 
 # First process runs the overlay mount
 if [ "$SLURM_LOCALID" = 0 ] || [ -z "${SLURM_LOCALID+x}" ]; then
-    "$overlayfscmd" -o "auto_unmount,big_writes,max_write=1048576,threaded=0,lowerdir=$lower,upperdir=$upper,workdir=$work" "$merge"
+    "$overlayfscmd" -o "fsync=0,noxattrs=1,xattr_permissions=0,lowerdir=$lower,upperdir=$upper,workdir=$work" "$merge"
 fi
 
 # Other processes wait for it to come into existence