diff --git a/server/disk_avail.go b/server/disk_avail.go
index b879330e850..e27c1a71cc1 100644
--- a/server/disk_avail.go
+++ b/server/disk_avail.go
@@ -11,7 +11,7 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-//go:build !windows && !openbsd && !netbsd && !wasm
+//go:build !windows && !openbsd && !netbsd && !wasm && !illumos && !solaris
 
 package server
 
diff --git a/server/disk_avail_illumos.go b/server/disk_avail_illumos.go
new file mode 100644
index 00000000000..7707bae3593
--- /dev/null
+++ b/server/disk_avail_illumos.go
@@ -0,0 +1,37 @@
+// Copyright 2025 The NATS Authors
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+//go:build illumos || solaris
+
+package server
+
+import (
+	"golang.org/x/sys/unix"
+	"os"
+)
+
+func diskAvailable(storeDir string) int64 {
+	var ba int64
+	if _, err := os.Stat(storeDir); os.IsNotExist(err) {
+		os.MkdirAll(storeDir, defaultDirPerms)
+	}
+	var fs unix.Statvfs_t
+	if err := unix.Statvfs(storeDir, &fs); err == nil {
+		// Estimate 75% of available storage.
+		ba = int64(uint64(fs.Bavail) * uint64(fs.Frsize) / 4 * 3)
+	} else {
+		// Used 1TB default as a guess if all else fails.
+		ba = JetStreamMaxStoreDefault
+	}
+	return ba
+}
diff --git a/server/sysmem/mem_wasm.go b/server/sysmem/mem_unsupported.go
similarity index 93%
rename from server/sysmem/mem_wasm.go
rename to server/sysmem/mem_unsupported.go
index bde3586e56f..bfa32fc2cfc 100644
--- a/server/sysmem/mem_wasm.go
+++ b/server/sysmem/mem_unsupported.go
@@ -11,7 +11,7 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-//go:build wasm
+//go:build illumos || solaris || wasm || zos
 
 package sysmem
 
diff --git a/server/sysmem/mem_zos.go b/server/sysmem/mem_zos.go
deleted file mode 100644
index f8db5c5c77d..00000000000
--- a/server/sysmem/mem_zos.go
+++ /dev/null
@@ -1,21 +0,0 @@
-// Copyright 2022-2025 The NATS Authors
-// Licensed under the Apache License, Version 2.0 (the "License");
-// you may not use this file except in compliance with the License.
-// You may obtain a copy of the License at
-//
-// http://www.apache.org/licenses/LICENSE-2.0
-//
-// Unless required by applicable law or agreed to in writing, software
-// distributed under the License is distributed on an "AS IS" BASIS,
-// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-// See the License for the specific language governing permissions and
-// limitations under the License.
-
-//go:build zos
-
-package sysmem
-
-func Memory() int64 {
-	// TODO: We don't know the system memory
-	return 0
-}