Wandering Thoughts archives

Copying GPT partition tables from disk to disk

There's a number of situations where you want to replicate partition tables from one disk to another disk; for example, if you are setting up mirroring or (more likely) replacing a dead disk in a mirrored setup with a new one. If you're using old fashioned MBR partitioning, the best tool for this is sfdisk and it's done as follows:

sfdisk -d /dev/OLD | sfdisk /dev/NEW

Under some situations you may need 'sfdisk -f'.

If you're using new, modern GPT partitioning, the equivalent of sfdisk is sgdisk. However it gets used somewhat differently and you need two operations:

sgdisk -R=/dev/NEW /dev/OLD
sgdisk -G /dev/NEW

For obvious reasons you really, really don't want to accidentally flip the arguments. You need sgdisk -G to update the new disk's partitions to have different GUIDs from the original disk, because GUIDs should be globally unique even if the partitioning is the same.

The easiest way to see if your disks are using GPT or MBR partitioning is probably to run 'fdisk -l /dev/DISK' and look at what the 'Disklabel type' says. If it claims GPT partitioning, you can then run 'sgdisk -p /dev/DISK' to see if sgdisk likes the full GPT setup or if it reports problems. Alternately you can use 'gdisk -l /dev/DISK' and pay careful attention to the 'Partition table scan' results, but this option is actually kind of dangerous; under some situations gdisk will stop to prompt you about what to do about 'corrupted' GPTs.

Unfortunately sgdisk lacks any fully supported way of dumping and saving a relatively generic dump of partition information; 'sgdisk -b' explicitly creates something which the documentation says should not be restored on anything except the original disk. This is a hassle if you want to create a generic GPT based partitioning setup which you will exactly replicate on a whole fleet of disks (not that we use GPT partitioning on our new iSCSI backends, partly for this reason).

(I suspect that in practice you can use 'sgdisk -b' dumps for this even if it's not officially supported, but enhh. Don't forget to run 'sgdisk -G' on everything afterwards.)

(This is the kind of entry that I write so I have this information in a place where I can easily find it again.)