optimize ext4 mount options that can be toggled at runtime.

Author: the8472

.github/workflows/ci.yml

@@ -117,6 +117,9 @@ jobs:
         with:
           fetch-depth: 2
 
+      - name: remount root filesystem
+        run: src/ci/scripts/tune-rootfs.sh
+
       # Free up disk space on Linux by removing preinstalled components that
       # we do not need. We do this to enable some of the less resource
       # intensive jobs to run on free runners, which however also have

src/ci/scripts/tune-rootfs.sh

@@ -0,0 +1,20 @@
+#!/bin/bash
+
+set -euo pipefail
+IFS=$'\n\t'
+
+source "$(cd "$(dirname "$0")" && pwd)/../shared.sh"
+
+if isLinux && [[ $(findmnt -n /) =~ "ext4" ]] ; then
+    # noauto_da_alloc since auto_da_alloc causes sync IO for some common file creation patterns
+    # lazytime avoids sync IO when (rel)atime updates are applied
+    # barrier=0 disables write cache flushing, which can reduce latency at the cost of durability,
+    #    but CI machines are ephemeral, so we don't need to be crash-proof.
+    #
+    # Ideally we'd set additional options, but those would require
+    # a reboot or unmounting the rootfs.
+    sudo mount -oremount,delalloc,nodiscard,lazytime,barrier=0,noauto_da_alloc /
+    sudo bash -c 'echo "write through" > /sys/block/sda/queue/write_cache'
+
+    mount
+fi