Minor Number Space Management

dev_t consists of a major and a minor number space. Major number space is managed by Solaris and the minor number space is managed by the device driver space. With Sun Cluster, the minor number behaves differently within the user space and the kernel space.

Cluster Wide dev_t

For historical reasons each device node, in addition to its path, is identified by an integral type dev_t. The dev_t is a part of the system interface expected by programmers and system administrators. stat(2) system calls and backup utilities deal directly with dev_ts. dev_t is also a programming interface for device driver writers.

Sun Cluster preserves the assumption that two equal dev_ts point to the same device regardless of the host where the process is executed. This model satisfies the expectations of programs that depend on this feature to establish the equivalence of two devices. Sun Cluster introduces a dual view of minor numbers and the necessary interfaces to implement this dual view. In kernel dev_ts correspond to the major number of the driver in addition to the minor number that the driver has created using ddi_create_minor_node(9F). External minor numbers (viewed from the user space) are managed and assigned unique cluster-wide numbers by the device configuration manager in Sun Cluster.

This dual numbering scheme has one unfortunate side effect, namely that a particular minor number created in the kernel can result in creation of a different minor number in the user space. This discrepancy might be unexpected by user space programs that expect to be able to ascertain some device characteristics from the minor number pattern.

An example of the discrepancy is the use of minor number bit patterns in specifying the particular slice of a disk or the density of a tape device. This class of problems is primarily alleviated by the use of globally unique instance numbers. By encoding the instance number of a device in the minor, the driver can guarantee the creation of cluster-wide unique dev_t values; this avoids minor numbers that do not have the same value between the kernel and the user space.

All dev_t values that are passed in through the standard Solaris entry points such as open, close and ioctl, encode the kernel minor number. The getminor(9F) interface can be used to extract this minor number. However, if the dev_t value is passed as a part of the ioctl data from the user space, the dev_t value has the minor number from the user space encoded. A new DDI interface, ddi_getiminor(9F), has been introduced to ensure that the driver can map between internal and external minor numbers.