From 8dd939cb6dcb414e670c87bf1954f17d23a31a3d Mon Sep 17 00:00:00 2001
From: Aaron Paterson <apaterson@pm.me>
Date: Thu, 13 Aug 2026 10:32:52 +0000
Subject: [PATCH] Ask for continuous discard where the runtime makes an ext4

A container's root filesystem and writable layer are sparse files, and
the guest never returns the blocks it frees: deleting inside the guest
frees guest blocks, the host file keeps every block it has ever
touched, so its real size is the high water mark of everything ever
written rather than what is live.

The filesystem hands freed blocks back the moment they are freed when
its mount asks for discard, and the runtime forwards those discards to
hole punches in the backing file. The ask belongs to the file: every
ext4 the runtime creates is such a sparse file, so its mount options
carry discard from the moment the mount exists, and every place the
file is later mounted, container or pod, lower layer or writable,
inherits the ask with the rest of the options. A read-only mount
parses the option and leaves it idle, and a caller who builds a mount
of their own chooses their own options, as they do for everything
else. The swap area has asked for the same thing since it was added,
for the same reason.
---
 Sources/Containerization/ContainerManager.swift            | 7 +++++--
 Sources/Containerization/Image/Unpacker/EXT4Unpacker.swift | 6 +++++-
 2 files changed, 10 insertions(+), 3 deletions(-)

diff --git a/Sources/Containerization/ContainerManager.swift b/Sources/Containerization/ContainerManager.swift
index 27e9fbe47..abd84e492 100644
--- a/Sources/Containerization/ContainerManager.swift
+++ b/Sources/Containerization/ContainerManager.swift
@@ -350,7 +350,7 @@ public struct ContainerManager: Sendable {
                     format: "ext4",
                     source: destination.absolutePath(),
                     destination: "/",
-                    options: []
+                    options: ["discard"]
                 )
             }
             throw err
@@ -364,11 +364,14 @@ public struct ContainerManager: Sendable {
         }
         let filesystem = try EXT4.Formatter(FilePath(path), minDiskSize: size)
         try filesystem.close()
+        // The filesystem lives in a sparse file that only gives freed blocks
+        // back to the host when the guest discards them, so the mount asks
+        // for continuous discard from birth.
         return .block(
             format: "ext4",
             source: path,
             destination: "/",
-            options: []
+            options: ["discard"]
         )
     }
 }
diff --git a/Sources/Containerization/Image/Unpacker/EXT4Unpacker.swift b/Sources/Containerization/Image/Unpacker/EXT4Unpacker.swift
index f69449625..3bfc13ce8 100644
--- a/Sources/Containerization/Image/Unpacker/EXT4Unpacker.swift
+++ b/Sources/Containerization/Image/Unpacker/EXT4Unpacker.swift
@@ -139,11 +139,15 @@ public struct EXT4Unpacker: Unpacker {
             try await filesystem.unpack(reader: reader, progress: progress)
         }
 
+        // The filesystem lives in a sparse file that only gives freed blocks
+        // back to the host when the guest discards them, so the mount asks for
+        // continuous discard from birth. A read-only mount of it parses the
+        // option and leaves it idle.
         return .block(
             format: "ext4",
             source: cleanedPath,
             destination: "/",
-            options: []
+            options: ["discard"]
         )
     }