Do Linux root partitions need to be primary volumes?

In order to determine which partition to boot in Step 1, the MBR looks for the "active boot" flag in the partition table. This will be the partition to which the MBR will pass control in Step 2. When installing Microsoft OS's, the installation routines have a habit of rewriting the MBR when the OS is installed. This Microsoft MBR will search only the four entries in the primary partition table and will not look beyond that into the extended partition--hence the common misperception that you can only boot from primary partitions. However, this simplistic MBR can be replaced with a more sophisticated boot loader and then there is no reason a logical partition needs to be treated any differently than a primary partition. It is possible to boot from a logical partition, though not if you use Microsoft's MBR. Note you don't have to use a Microsoft MBR just because you installed a Microsoft OS--see Step 1, the MBR is just a dumb loader and does not need to be specific to any particular OS.