diff --git a/nixos/modules/testing/test-instrumentation.nix b/nixos/modules/testing/test-instrumentation.nix
index c1dbfe305dd..f37bbd0246d 100644
--- a/nixos/modules/testing/test-instrumentation.nix
+++ b/nixos/modules/testing/test-instrumentation.nix
@@ -88,10 +88,6 @@ let kernel = config.boot.kernelPackages.kernel; in
 
     boot.consoleLogLevel = 7;
 
-    # Make sure we don't hit page allocation failures if the VM's memory is
-    # heavily fragmented.
-    boot.kernel.sysctl."vm.min_free_kbytes" = 16384;
-
     # Prevent tests from accessing the Internet.
     networking.defaultGateway = mkOverride 150 "";
     networking.nameservers = mkOverride 150 [ ];
diff --git a/nixos/modules/virtualisation/qemu-vm.nix b/nixos/modules/virtualisation/qemu-vm.nix
index b1f36cf3264..e90e319a36b 100644
--- a/nixos/modules/virtualisation/qemu-vm.nix
+++ b/nixos/modules/virtualisation/qemu-vm.nix
@@ -76,14 +76,14 @@ let
           -virtfs local,path=$TMPDIR/xchg,security_model=none,mount_tag=xchg \
           -virtfs local,path=''${SHARED_DIR:-$TMPDIR/xchg},security_model=none,mount_tag=shared \
           ${if cfg.useBootLoader then ''
-            -drive index=0,id=drive1,file=$NIX_DISK_IMAGE,if=${cfg.qemu.diskInterface},cache=writeback,werror=report \
+            -drive index=0,id=drive1,file=$NIX_DISK_IMAGE,if=${cfg.qemu.diskInterface},cache=none,werror=report \
             -drive index=1,id=drive2,file=$TMPDIR/disk.img,media=disk \
             ${if cfg.useEFIBoot then ''
               -pflash $TMPDIR/bios.bin \
             '' else ''
             ''}
           '' else ''
-            -drive index=0,id=drive1,file=$NIX_DISK_IMAGE,if=${cfg.qemu.diskInterface},cache=writeback,werror=report \
+            -drive index=0,id=drive1,file=$NIX_DISK_IMAGE,if=${cfg.qemu.diskInterface},cache=none,werror=report \
             -kernel ${config.system.build.toplevel}/kernel \
             -initrd ${config.system.build.toplevel}/initrd \
             -append "$(cat ${config.system.build.toplevel}/kernel-params) init=${config.system.build.toplevel}/init regInfo=${regInfo} ${kernelConsole} $QEMU_KERNEL_PARAMS" \
@@ -426,19 +426,19 @@ in
         ${if cfg.writableStore then "/nix/.ro-store" else "/nix/store"} =
           { device = "store";
             fsType = "9p";
-            options = "trans=virtio,version=9p2000.L,msize=1048576,cache=loose";
+            options = "trans=virtio,version=9p2000.L,cache=loose";
             neededForBoot = true;
           };
         "/tmp/xchg" =
           { device = "xchg";
             fsType = "9p";
-            options = "trans=virtio,version=9p2000.L,msize=1048576,cache=loose";
+            options = "trans=virtio,version=9p2000.L,cache=loose";
             neededForBoot = true;
           };
         "/tmp/shared" =
           { device = "shared";
             fsType = "9p";
-            options = "trans=virtio,version=9p2000.L,msize=1048576";
+            options = "trans=virtio,version=9p2000.L";
             neededForBoot = true;
           };
       } // optionalAttrs cfg.writableStore
diff --git a/pkgs/build-support/vm/default.nix b/pkgs/build-support/vm/default.nix
index 250f2b0fbce..ebcbf8c87b5 100644
--- a/pkgs/build-support/vm/default.nix
+++ b/pkgs/build-support/vm/default.nix
@@ -118,7 +118,7 @@ rec {
 
     echo "mounting Nix store..."
     mkdir -p /fs/nix/store
-    mount -t 9p store /fs/nix/store -o trans=virtio,version=9p2000.L,msize=262144,cache=loose
+    mount -t 9p store /fs/nix/store -o trans=virtio,version=9p2000.L,cache=loose
 
     mkdir -p /fs/tmp /fs/run /fs/var
     mount -t tmpfs -o "mode=1777" none /fs/tmp
@@ -127,7 +127,7 @@ rec {
 
     echo "mounting host's temporary directory..."
     mkdir -p /fs/tmp/xchg
-    mount -t 9p xchg /fs/tmp/xchg -o trans=virtio,version=9p2000.L,msize=262144,cache=loose
+    mount -t 9p xchg /fs/tmp/xchg -o trans=virtio,version=9p2000.L,cache=loose
 
     mkdir -p /fs/proc
     mount -t proc none /fs/proc
diff --git a/pkgs/build-support/vm/windows/controller/default.nix b/pkgs/build-support/vm/windows/controller/default.nix
index 0beaf401758..1c8e6af83b8 100644
--- a/pkgs/build-support/vm/windows/controller/default.nix
+++ b/pkgs/build-support/vm/windows/controller/default.nix
@@ -48,11 +48,11 @@ let
     mount -t proc none /fs/proc
 
     mount -t 9p \
-      -o trans=virtio,version=9p2000.L,msize=262144,cache=loose \
+      -o trans=virtio,version=9p2000.L,cache=loose \
       store /fs/nix/store
 
     mount -t 9p \
-      -o trans=virtio,version=9p2000.L,msize=262144,cache=loose \
+      -o trans=virtio,version=9p2000.L,cache=loose \
       xchg /fs/xchg
 
     echo root:x:0:0::/root:/bin/false > /fs/etc/passwd