From 91fd1c45e124e9df2ac1d375375aa93ab3f5445d Mon Sep 17 00:00:00 2001
From: Sandwich <sandwich@archworks.co>
Date: Sun, 6 Sep 2026 05:39:31 +0200
Subject: [PATCH] fix: buffered disk cache so btrfs checksums stay valid

---
 scripts/setup-vm.sh | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/scripts/setup-vm.sh b/scripts/setup-vm.sh
index 641d46b..f4a7c86 100755
--- a/scripts/setup-vm.sh
+++ b/scripts/setup-vm.sh
@@ -637,8 +637,11 @@ if conformant and E["CONVERT"] == "1":
         if "device='disk'" not in d or ("bus='nvme'" in d and "<serial>" in d):
             return d
         d = re.sub(r"<target dev='([^']*)' bus='(virtio|sata|scsi)'/>", r"<target dev='\1' bus='nvme'/>", d)
+        # cache='none' is O_DIRECT: on btrfs the guest can change a page while the
+        # write is in flight, so the stored checksum never matches and later reads
+        # fail with EIO. Buffered writes hand the filesystem a stable page.
         d = re.sub(r"<driver name='qemu' type='([^']*)'[^/]*/>",
-                   r"<driver name='qemu' type='\1' cache='none' io='native' discard='unmap'/>", d)
+                   r"<driver name='qemu' type='\1' cache='writeback' io='threads' discard='unmap'/>", d)
         d = re.sub(r"\s*<address type='(pci|drive)'[^/]*/>", "", d)
         if "<serial>" not in d:
             serial = E["NVME_SERIAL"] if n[0] == 0 else E["NVME_SERIAL"][:-1] + "0123456789ABCDEF"[n[0] % 16]