High availability computer system

Abstract: A high-availability computer system comprises at least two processors, each having its own private disk drive, and a shared disk drive, accessible by both processors. Each of the private disk drives holds system files for establishing a processor as a secondary processor, and the shared disk drive holds system files for establishing a processor as a primary processor. When it boots up, each processor decides whether to use the system files from its private disk so as to become a secondary, or to use the system files from the shared disk so as to become the primary. As as result, after a system failure or reboot, either processor can pick up the characteristics of the the primary processor, and will have all the most up-to-date primary information available to it.

Description: Background to the invention This invention relates to computer systems. More specifically, the invention is concerned with a high-availability computer system comprising two or more processors, one of which acts as a primary processor, and another of which acts as a secondary or backup processor. The intention behind such a system is that, if the primary processor fails, the secondary processor can take over the total identity of the failed processor, thus ensuring that there is no loss in availability of the system to the user. Typically, in such a system, each processor has its own private disk drive or drives, and the system also includes one or more shared disk drives which are shared between the processors. T. Imai et al, "TOM: Total Operating Management on ACOS-4", NEC Research and Development no 92, 1 January 1989, Tokyo, Japan, pages 130-135, describes a computer system comprising two loosely-coupled processors in hot standby status. Each computer is monitored by an inter-host communication (IHC) component of the other. In operation, a first computer manages two schedule files. If the first computer goes down, a hot standby action is executed and then the other computer connects to one of these schedule files and continues running instead of the first computer. The invention is particularly, although not exclusively, concerned with computer systems running under the UNIX operating system (UNIX is a trademark of Unix System Laboratories). Much of a UNIX system's identity is contained within the directory /etc , which is part of the root filesystem and is normally held on the processor's private system disk. This includes information about users and their passwords, the system's network address, details of which filesystems are currently mounted and available to the system, and the startup processes of specific applications. However, a problem with this is that, when the secondary processor takes over from a failed primary processor, it may not have the most up-to-date version of this information available on its private system disk. For example, if users have changed their passwords, the new password information will be available only on the system disk of the failed processor. The object of the present invention is to provide a novel way of solving this problem. Summary of the invention According to the invention there is provided a computer system according to Claim 1. As will be seen, the invention allows either processor to pick up the characteristics of the primary processor, and any changes made to primary information (such as a user changing a login password) will be available to whichever processor is the primary. Brief description of the drawings Figure 1 is a block diagram of a computer system embodying the present invention. Figure 2 is a flowchart showing a boot process performed by the processors in the system. Description of an Embodiment of the Invention One embodiment of the invention will now be described by way of example with reference to the accompanying drawings. Referring to Figure 1, the system comprises two system cabinets 10,11 and a peripheral cabinet 12. Each of the system cabinets includes a processor 13 and one or more private disk drive units 14 accessible only by the local processor. Each processor runs under the UNIX operating system. The peripheral cabinet contains one or more shared disk drive units 15, accessible by either of the two processors. Each individual shared disk drive may be used by only one processor at a time. In a conventional UNIX system, a special directory /etc holds a number of system administration files. These files include for example the /etc/inittab file, which controls the operation of the init program, and the /etc/rc2.d/ ... files which control the startup of services and applications on that processor. In the present system, the /etc directory is moved from its normal position on each private disk, to become a directory / realroot/etc on the same disk. The files in this directory are used when the processor is acting as a secondary processor. The shared disk drives also hold a similar set of system administration files, in a directory /etc . The files in this directory are used when the processor acts as the primary processor. Whenever one of the processors is closed down e.g. because it is to be powered down or rebooted, it performs a close-down sequence. During this sequence, the processor decides whether it is to become the primary processor during the next bootup. For example, if the processor detects that the system is being rebooted because of failure of the other processor, it assumes that it should boot up as the primary processor next time. If the processor decides that it should become the primary at the next bootup, it creates a special dummy marker file on its local private disks to indicate this, before completing its close-down sequence. Whenever one of the processors is started up or rebooted it runs a boot process. The boot process is shown in Figure 2. The boot process first loads the UNIX kernel, and establishes itself as a bare minimum UNIX system. The boot process sets up a symbolic link, so as to map file names in the / etc directory invisibly on to the /realroot/etc directory. The system therefore runs at the bare minimum level using the system files in the /realroot/etc directory, on the local private disk. At this stage, the boot process also starts up a special daemon process, referred to herein as the status daemon. The status daemons in the two processors attempt to establish contact with each other by way of a private area on the shared disks. Each daemon writes status information about its own processor into this private area, and attempts to read status information from the other daemon. In particular, this status information includes information about whether either of the processors is currently using the shared disk on which the shared system files are held. The bare UNIX system then searches the local private disks to see whether a dummy marker file, indicating that the processor should assume the role of primary, is present. If no such dummy marker file is present, the processor completes its bootup process and assumes the role of secondary. If, on the other hand, the dummy marker file is present, the dummy marker file is deleted (so as to ensure that it cannot be used again) and the processor then attempts to assume the role of primary, as follows. It is possible for the dummy marker files in the processors to be left in an inconsistent state as a result of a system crash. Specifically, it is possible that both processors may have such a dummy marker file present in their private disks. It is imperative that both processors do not attempt to access the system files on the shared disks concurrently, since this would result in major filestore corruption. To ensure against this, before the processor attempts to assume the role of primary, it sends a request to its local status daemon. In response to this request, the daemon checks to see whether the shared system files have already been claimed by the other processor. If the daemon indicates that the shared system files are not available (i.e. they have already been claimed by the other processor), then the processor completes its boot-up process and assumes the role of secondary. If the other processor is not running, the local status daemon recognises this, and deduces that any residual status information from the remote daemon is obsolete and that the shared system files are available for use by the local processor. If, on the other hand, the daemon indicates that the shared system files are available, then the processor proceeds as follows. First, it mounts the /etc directory in the shared disks on to a special mount point called /root . It then changes the symbolic link, so as to map file names in the /etc directory on to the /root/etc directory. This means that the secondary /realroot/etc directory is not used, and the primary directory on the shared disk is used instead. The processor then completes its boot-up process as the primary processor, starting up the primary applications as defined in the etc/rc2.d/. .. files on the shared disk. In summary, it can be seen that the processor will either use the system files held in its own private disk, thus assuming the role of secondary, or else will use the system files held on the shared disk, thus assuming the role of primary.

Claims: A computer system comprising at least two processors (10,11), each having its own private disk drive (14), and a shared disk drive (15), accessible by both said processors (10,11); characterised in that: (a) each of the private disk drives (14) holds system files for establishing a processor (10, 11) as a secondary processor, and the shared disk drive (15) holds system files for establishing a processor (10, 11) as a primary processor; (b) each processor (10,11) has means for deciding, when that processor is closed down, whether it should become the primary processor the next time it is booted up, and for storing an indication of this decision in its own private disk drive (14); and (c) each processor (10,11) also has means for using the stored indication, when that processor is booted up, to determine whether to use the system files from its private disk drive (14) so as to become a secondary processor, or to use the system files from the shared disk drive (15) so as to become the primary processor. A computer system according to Claim 1 wherein the system includes interlock means for ensuring that both said processors (10,11) do not simultaneously use said system files on said shared disk drive (15). A computer system according to either preceding claim wherein each of said processors (10,11) runs under the UNIX operating system. A computer system according to Claim 3 wherein initially each of said processors (10,11) forms a symbolic link to map file names in the /etc directory on to the system files in its private disk drive (14). A computer system according to Claim 4 wherein, when a processor (10,11) determines to use said system files on the shared disk drive (15), it mounts the file system containing those files on a predetermined mount point and then modifies said symbolic link to map file names in the /etc directory on to the system files on the shared disk drive (15). A computer system according to any preceding claim wherein said indication comprises a dummy marker file.

Inventors: SPEED PAUL FORSYTH, TAYLOR RICHARD NORCOTT, SPEED, PAUL FORSYTH, TAYLOR, RICHARD NORCOTT
Applicants: INT COMPUTERS LTD, INTERNATIONAL COMPUTERS LIMITED
Application year: 1995
Main classification: G06F11
IPC codes: G06F15, G06F9, G06F11