WUBILDR, WUBILDR.MBR and GRLDR

It's fairly common to see messages such as Try (hd0,0) : FAT16 when booting Wubi. Usually it's innocuous, but sometimes booting fails. You might see it stuck on Try (hd0,0) : Ext2  


Rarely you'll see something like:
Try (hd0,0) : FAT16: No WUBILDR
Try (hd0,1) : NTSF5: No wubildr
Try (hd0,2) : FAT32: No WUBILDR
Try (hd0,3) : invalid or null
Try (fd,0) : invalid or null
ERROR: Cannot find GRLDR in all devices: Press ctrl+alt+delete to restart


So what does this all mean, and what is GRLDR? 


Wubi uses GRUB4DOS combined with Grub2 to boot Ubuntu. GRUB4DOS has two parts: grldr.mbr and grldr. Wubi has renamed these to wubildr.mbr and wubildr (you are free to rename them as long as you insert the new name of grldr in the .mbr file). 


The wubildr.mbr file corresponds to a grub legacy bootloader in the Master Boot Record. It is limited to scanning all partitions to find the rest of itself - the part that doesn't fit in the tiny MBR. These are the messages you see as it scans all partitions - and since it is based on grub legacy it outputs partitions with a zero index (unlike Grub2 which still uses zero based indexing on the drives, but not on partitions) i.e. hd0,0 in wubildr.mbr corresponds to hd0,1 in Grub2.


No matter where wubildr.mbr is, when it is called, it starts scanning the root on ALL partitions in the order dictated by BIOS to find the wubildr file. If it doesn't find it, you get an error that it can't find GRLDR, not WUBILDR. The reason it says GRLDR is likely a hardcoded message - for Wubi it's always WUBILDR that it can't find.


So what are the reasons for failure to find WUBILDR? 
1. The rather obvious, but unlikely - there is no WUBILDR 
2. Excessive fragmentation apparently
3. The wubildr file is > 137GB from the start of the partition
4. You have an ext4 partition that is read prior to the one containing WUBILDR.

WUBILDR.MBR cannot read ext4 partitions. The GRUB4DOS code it uses is from 2007 and it simply hangs when it encounters one of these. If you have an ext4 partition higher up in the BIOS order than your Windows partition, you're out of luck.

Some interesting points: when you install Wubi it installs WUBILDR.MBR and WUBILDR to the windows drive C:. But it also installs it to other partitions, like recovery partitions. If these happen to be physically before the windows partition, WUBILDR.MBR will find this WUBILDR and never use the one on your C: partition (it uses the first one it finds). And this is interesting as Grub2 updates the WUBILDR on your /host partition - not always the one used. Maybe this is a clue as to why Grub2 updates no longer boot on some installs.

Wubi and grub problem update

It's been one month since the latest grub-pc update in version 10.04.1 Lucid Lynx, started breaking Wubi installs. And that's a good time to reassess the situation.

I'm not sure how I noticed it - maybe I was browsing Wubi bugs that day - but my quick alert to the issues didn't help. Again, I was hoping for a quick response, maybe the update being pulled - I don't know. But apparently launchpad doesn't have a recall mechanism. At the time I thought it would only affect users who didn't install on the same partition as Windows - the usual grub trick that gets users to install the grub bootloader to their drive masterbootrecord MBR caused by a bug in the 'lupin' package (see here, or here).
But quickly it became apparent that it also affected users who installed on the same partition - a problem we already knew about as it was discovered in the Maverick upgrade. But for 10.04.1 the problem is worse, and requires a manual loop mount of the virtual disk and an edit of the grub configuration file from a live CD.

So now we are on the Christmas break, and the developers get a well-needed rest from all the Natty development, but sadly noone has taken the time to patch lupin, for the many existing users on the supported releases. Still, the volunteers on ubuntuforums.org will soldier on, steering Wubi users to the solution... the Wubi megathread. It's interesting to note that since this thread was created 2 weeks ago, it's had over 2600 visits. And that's not including the number of people who had to be helped before it was created.

So... where are we at? There's not much really to be done to warn users unless they happen to post a support request for an unrelated Wubi question. I'm loathe to join the Wubi-bashing crowd  (although it's tempting): there's still a niche market for users that might cause more damage trying to do a proper partitioned Ubuntu installation. In fact, the new Ubiquity installer has it's own issues and has already caught out a few users and overwritten Windows and all other partitions - see bug 659106.
And in any case, despite all the Wubi-bashing, Wubi appears to be growing in popularity, fueled by its new prominent place on the ubuntu.com download page.

So... I guess we just wait it out, and hope that eventually the grub devs will grace us with their attention and provide a fix.

Wubi Mega-help-thread

There is now a one-stop Wubi 'megathread' on ubuntuforums.org to repair the grub issues Wubi users have been experiencing. Recommended reading if you are using Wubi, even if you haven't had any issues yet.

The thread is: http://ubuntuforums.org/showthread.php?t=1639198