From 2c32c7ae1aac156f6e6f3cfaf8063fcfa2bee500 Mon Sep 17 00:00:00 2001 From: Ian Wienand Date: Mon, 3 Aug 2015 15:48:52 +1000 Subject: [PATCH] Handle modern sfdisk and correctly align image partition As described in the comments, sfdisk was rewritten for util-linux 2.26 (as shipped in F22) and now interprets arguments a sectors, rather than cylinders. The current partitioning line is "1 - - *" (start/size/type/bootable) which means you start getting: --- /usr/sbin/grub2-install: warning: this msdos-style partition label has no post-MBR gap; embedding won't be possible. /usr/sbin/grub2-install: warning: Embedding is not possible. GRUB can only be installed in this setup by using blocklists. However, blocklists are UNRELIABLE and their use is discoura ged.. /usr/sbin/grub2-install: error: will not proceed with blocklists. --- when building images, because the start is interpreted by the new sfdisk as sector 1 and it crams the partition right next to the MBR. Specifying "-" for the size is undefined in the man page; even reading the source it's not totally clear what "-" for the size does [2]. In any case, the alignment is wrong in sectors or cylinders; we want to be a multiple of 4KiB for best performance. The intent here is to create one single, Linux, bootable, partition taking up the whole disk starting at 1MiB, so "2048 + L *" makes this clear. We use the -uS argument to ensure both versions treat this start-value as a sector offset (newer sfdisk essentially ignores the argument). As described in the comments, bugs in the older sfdisk necessitate usage of "--force". Although we could choose more or less, it seems most common to align to a 1MiB boundary (i.e. starting at sector 2048). libguestfs has some disucssion around --alignment and where it sets it's default to this [3]. The 2.26-era sfdisk also defaults to putting partitions here. 1MiB should be enough for GPT schemes in the future as well. [1] https://github.com/karelzak/util-linux/blob/master/libfdisk/src/script.c#L1050 [2] https://bugzilla.redhat.com/show_bug.cgi?id=1249893 [3] http://libguestfs.org/virt-resize.1.html Change-Id: I2c2966f98d1d5ad4ebb433ea148b3b26c65dc1b5 --- elements/vm/block-device.d/10-partition | 24 ++++++++++++++++++++---- 1 file changed, 20 insertions(+), 4 deletions(-) diff --git a/elements/vm/block-device.d/10-partition b/elements/vm/block-device.d/10-partition index 2b5baba5..c72c40dd 100755 --- a/elements/vm/block-device.d/10-partition +++ b/elements/vm/block-device.d/10-partition @@ -17,10 +17,26 @@ if [[ "$ARCH" =~ "ppc" ]] ; then ; EOF else -# Create 1 partition far enough up the disk to permit grub to be installed on -# the MBR. - sudo sfdisk $IMAGE_BLOCK_DEVICE << EOF -1 - - * + # sfdisk changed around 2.26 and was largely rewritten. The old + # version defaults to c/h/s and interprets arguments as cylinders. + # The new version defaults to sectors. + # + # Luckily, the old version has a -uS flag (ignored by the new + # version) to interpret arguments as sectors. However, it + # incorrectly calculates the c/h/s offset when finding free space, + # and gets upset [1], requiring the use of --force. + # + # TODO: switch this to parted, or something saner (is there such a + # thing when talking about partitioning?) + # + # The below command creates a partition that starts at 1MiB -- + # which is the standard place to start a partition these days -- + # and fills up the disk. The zeros are to ignore the other 3 + # primary partitions (probably not needed, but left for safety). + # + # [1] https://bugs.launchpad.net/ubuntu/+source/util-linux/+bug/1481158 + sudo sfdisk -uS --force $IMAGE_BLOCK_DEVICE << EOF +2048 + L * 0 0; 0 0; 0 0;