Allow specification of filesystem journal size

In many cases, the statically sized 64MB journal is far below the
e2fstools default calculation[0] which calls for a 64MB journal only
on filesystems smaller than 16GB. On bare metal in particular, the
correct default journal size will often be in the 512MB-1GB range.

Since we cannot know what the target system is, this should be a
tunable parameter that the user can set depending on the intended
image usage.

Add a DIB_JOURNAL_SIZE envvar and --mkfs-journal-size parameter
to the image creation so users can override the default journal
size.

[0] https://git.kernel.org/pub/scm/fs/ext2/e2fsprogs.git/tree/lib/ext2fs/mkjournal.c#n333

Change-Id: I65fa13a088eecdfe61636678578577ea2cfb3c0c

diskimage_builder/lib/disk-image-create

@@ -70,6 +70,7 @@ function show_options () {
     echo "       Making this value unnecessarily large will consume extra disk space "
     echo "       on the root partition with extra file system inodes."
     echo "    --min-tmpfs size -- minimum size in GB needed in tmpfs to build the image"
+    echo "    --mkfs-journal-size -- filesystem journal size in MB to pass to mkfs."
     echo "    --mkfs-options -- option flags to be passed directly to mkfs."
     echo "       Options should be passed as a single string value."
     echo "    --no-tmpfs -- do not use tmpfs to speed image build"
@@ -149,6 +150,7 @@ while true ; do
         --image-size) export DIB_IMAGE_SIZE=$2; shift 2;;
         --image-cache) export DIB_IMAGE_CACHE=$2; shift 2;;
         --max-online-resize) export MAX_ONLINE_RESIZE=$2; shift 2;;
+        --mkfs-journal-size) export DIB_JOURNAL_SIZE=$2; shift 2;;
         --mkfs-options) MKFS_OPTS=$2; shift 2;;
         --min-tmpfs) export DIB_MIN_TMPFS=$2; shift 2;;
         --no-tmpfs) shift; export DIB_NO_TMPFS=1;;
@@ -417,15 +419,21 @@ fi
 
 rm -f ${du_output}
 
+if [ -n "$DIB_JOURNAL_SIZE" ]; then
+    journal_size="$DIB_JOURNAL_SIZE"
+else
+    journal_size=64
+fi
+
 if [ "$DIB_ROOT_FSTYPE" = "ext4" ] ; then
   # Very conservative to handle images being resized a lot
   # We set journal size to 64M so our journal is large enough when we
   # perform an FS resize.
-  MKFS_OPTS="-i 4096 -J size=64 $MKFS_OPTS"
+  MKFS_OPTS="-i 4096 -J size=$journal_size $MKFS_OPTS"
   # Grow the image size to account for the journal, only if the user
   # has not asked for a specific size.
   if [ -z "$DIB_IMAGE_SIZE" ]; then
-    du_size=$(( $du_size + 65536 ))
+    du_size=$(( $du_size + ($journal_size * 1024) ))
   fi
 fi
 

doc/source/user_guide/building_an_image.rst

@@ -660,6 +660,13 @@ configuration:
    default can be overridden by passing ``'-i 16384'`` as a
    ``--mkfs-options`` argument.
 
+``--mkfs-journal-size``
+   Only valid for ``FS_TYPE==ext4``.  This value set the filesystem
+   journal size in MB; overriding the default of 64MiB.  Note the
+   image size will be grown to fit the journal, unless
+   ``DIB_IMAGE_SIZE`` is explicitly set.  Can also set
+   ``DIB_JOURNAL_SIZE``.
+
 ``--max-online-resize``
    Only valid for ``FS_TYPE==ext4``; this value sets the maximum
    filesystem blocks when resizing.  Can also set

releasenotes/notes/root-journal-size-618e064d6681699a.yaml

@@ -0,0 +1,4 @@
+---
+features:
+  - The ``--mkfs-journal-size`` option is added to override the default
+    journal size for basic ext4 root partitions.