On Monday 14 July 2008 06:34:08 Bryn M. Reeves wrote: > Gene Czarcinski wrote: > > I am hoping that someone on this mailing list is expert enough to answer > > my question: > > > > Is there any reason that sector 0 on a Logical Volume should not be all > > zeros? > > All depends what you are doing with it. Many file systems and other > block device users avoid using this sector because it may be "shared" > with the firmware on x86 but there's no requirement for it to be > reserved like this. Ext2/3 are fine (they start in the 1st sector) as > would be LVM2 - e.g. if you were using the LV to provide a physical > volume for a virtualised system. > > > My problem occurred when I effectively dd'ed a real disk partition on > > which grub is installed to a Logical Volume. When grub in installed into > > a partition, it not only installs it's boot code but also makes sector 0 > > on that partition have a fake "msdos" partition table and the kernel (and > > other software) does not like that at all!! > > It is not a "fake" msdos partition table, it's a real one. The > conventions for x86 boot loaders require a partition table since the MBR > (master boot record) consists of a 512 byte sector containing 446 bytes > of executable code followed by the four-entry primary partition table. If the partition table was defined in an MBR ... yes, that is a real one. But this is installed in a partition. I suspect that grub does not "know" if it is installing in a partition or the MBR for a real disk. I do note (just looked) and both ntfs and fat32 disks step on sector 0 of a partition with a skelaton partition table and "msdos" partition table signature. > > You can't install grub without an MBR/partition table so when you > attempt to install it to a device that lacks one grub will lay down a > skeleton MBR on the device. device==yes but this was a partition. One thing I want to try is to zap the "signature" and see if grub will still boot. > > > My solution was to use dd to zero sector 0 and then everything worked ... > > could mount the Logical Volume, etc. > > Shouldn't be a problem for anything that doesn't store data in this > sector. Just be sure to limit the command to only wipe sector 0. Thanks for the reply. >From what you said, I understand that most linux filesystems stay away from sector 0. I have verified this for ext2/ext3 and xfs. It is unlikely that any native use of a Logical Volume will use sector 0. But, there could be a situation with a virtual machine where a Logical Volume was being used as a virtual physical volume. Furthermore, with grub, ntfs, and fat32 put a partition table in sector 0 of a partition. Gene _______________________________________________ linux-lvm mailing list linux-lvm@redhat.com https://www.redhat.com/mailman/listinfo/linux-lvm read the LVM HOW-TO at http://tldp.org/HOWTO/LVM-HOWTO/