ci: reduce storage of snapshots in cross-restore test

This is so we use less IO overall sending snapshot to/from S3.

- Punch holes in the memory snapshots
- Decrease guest memory from 1GB to 256MB as it's not important to the
  test.

This decreases around 10x:

Before: 27GB * 22 runs ~ 594GB
After: 2.7GB* 22 runs ~ 59.4GB

Signed-off-by: Pablo Barbáchano <[email protected]>

.buildkite/pipeline_cross.py

@@ -22,8 +22,11 @@
     instances_aarch64 = ["m7g.metal"]
     commands = [
         "./tools/devtool -y test --no-build -- -m nonci -n4 integration_tests/functional/test_snapshot_phase1.py",
-        "mkdir -pv snapshots/{instance}_{kv}",
-        "mv -v test_results/test_snapshot_phase1/* snapshots/{instance}_{kv}",
+        # punch holes in mem snapshot tiles and tar them so they are preserved in S3
+        "find test_results/test_snapshot_phase1 -type f -name mem |xargs -t -n1 fallocate -d",
+        "mv -v test_results/test_snapshot_phase1 snapshot_artifacts",
+        "mkdir -pv snapshots",
+        "tar cSvf snapshots/{instance}_{kv}.tar snapshot_artifacts",
     ]
     pipeline.build_group(
         "📸 create snapshots",
@@ -79,8 +82,8 @@
         k_val = pytest_keyword_for_instance.get(dst_instance, "")
         step = {
             "command": [
-                f"buildkite-agent artifact download snapshots/{src_instance}_{src_kv}/* .",
-                f"mv -v snapshots/{src_instance}_{src_kv} snapshot_artifacts",
+                f"buildkite-agent artifact download snapshots/{src_instance}_{src_kv}.tar .",
+                f"tar xSvf snapshots/{src_instance}_{src_kv}.tar",
                 *pipeline.devtool_test(
                     pytest_opts=f"-m nonci -n4 {k_val} integration_tests/functional/test_snapshot_restore_cross_kernel.py",
                 ),

tests/integration_tests/functional/test_snapshot_phase1.py

@@ -46,7 +46,7 @@ def test_snapshot_phase1(
         cpu_template_name = f"custom_{cpu_template_any['name']}"
     vm.basic_config(
         vcpu_count=2,
-        mem_size_mib=1024,
+        mem_size_mib=256,
         cpu_template=static_cpu_template,
     )