fix: Dynamic snapshot sizing to prevent I/O errors

Problem: Fixed 500MB snapshot size was too small for large/active LVs,
causing I/O errors when snapshot space was exhausted during backup.

Solution: Calculate appropriate snapshot size based on LV size:
- Use 10% of LV size for snapshot space
- Minimum 1GB to handle reasonable activity
- Shows snapshot size in logs for transparency

Example:
- 20GB LV → 2GB snapshot
- 100GB LV → 10GB snapshot
- 500MB LV → 1GB snapshot (minimum)

This prevents snapshot overflow and resulting I/O errors during backup.

Changes:
- GUI: Calculate snapshot size before creating
- CLI: Same dynamic sizing logic
- Both show calculated snapshot size in output
- Handles both single LV and VG backup modes

simple_backup_gui.py

@@ -518,8 +518,19 @@ class SimpleBackupGUI:
         snapshot_name = f"{lv_name}_borg_snap"
         self.current_snapshot = f"/dev/{vg_name}/{snapshot_name}"
         
-        self.log(f"Creating snapshot: {snapshot_name}")
-        success, output = self.run_command(f"lvcreate -L1G -s -n {snapshot_name} {source_lv}")
+        # Get LV size to determine appropriate snapshot size
+        success, lv_size_output = self.run_command(f"lvs --noheadings -o lv_size --units b {source_lv}", show_output=False)
+        if success:
+            lv_size_bytes = int(lv_size_output.strip().replace('B', ''))
+            # Use 10% of LV size or minimum 1GB for snapshot
+            snapshot_size_bytes = max(lv_size_bytes // 10, 1024**3)  # Min 1GB
+            snapshot_size_gb = snapshot_size_bytes // (1024**3)
+            snapshot_size = f"{snapshot_size_gb}G"
+        else:
+            snapshot_size = "2G"  # Default fallback
+        
+        self.log(f"Creating snapshot: {snapshot_name} (size: {snapshot_size})")
+        success, output = self.run_command(f"lvcreate -L{snapshot_size} -s -n {snapshot_name} {source_lv}")
         if not success:
             raise Exception(f"Failed to create snapshot: {output}")
         
@@ -609,9 +620,20 @@ class SimpleBackupGUI:
                 snapshot_path = f"/dev/{source_vg}/{snapshot_name}"
                 lv_path = f"/dev/{source_vg}/{lv_name}"
                 
+                # Get LV size to determine appropriate snapshot size
+                success, lv_size_output = self.run_command(f"lvs --noheadings -o lv_size --units b {lv_path}", show_output=False)
+                if success:
+                    lv_size_bytes = int(lv_size_output.strip().replace('B', ''))
+                    # Use 10% of LV size or minimum 1GB for snapshot
+                    snapshot_size_bytes = max(lv_size_bytes // 10, 1024**3)  # Min 1GB
+                    snapshot_size_gb = snapshot_size_bytes // (1024**3)
+                    snapshot_size = f"{snapshot_size_gb}G"
+                else:
+                    snapshot_size = "2G"  # Default fallback
+                
                 self.log(f"Processing LV: {lv_name}")
-                self.log(f"Creating snapshot: {snapshot_name}")
-                success, output = self.run_command(f"lvcreate -L500M -s -n {snapshot_name} {lv_path}")
+                self.log(f"Creating snapshot: {snapshot_name} (size: {snapshot_size})")
+                success, output = self.run_command(f"lvcreate -L{snapshot_size} -s -n {snapshot_name} {lv_path}")
                 if not success:
                     self.log(f"Warning: Failed to create snapshot for {lv_name}: {output}")
                     continue