feat: enforce tiered storage reservations

internal/server/store/free_unix.go

@@ -0,0 +1,22 @@
+//go:build !windows
+
+package store
+
+import (
+	"errors"
+	"math"
+	"syscall"
+)
+
+func filesystemFreeBytes(path string) (uint64, error) {
+	var status syscall.Statfs_t
+	if err := syscall.Statfs(path, &status); err != nil {
+		return 0, err
+	}
+	blockSize := uint64(status.Bsize)
+	availableBlocks := uint64(status.Bavail)
+	if blockSize != 0 && availableBlocks > math.MaxUint64/blockSize {
+		return 0, errors.New("filesystem free-space value overflowed")
+	}
+	return availableBlocks * blockSize, nil
+}