Controlling shared disk data in a duplexed computer unit

Details Controlling shared disk data in a duplexed computer unit Controlling shared disk data in a duplexed computer unit

1998-03-13

2002-05-21

Bragdon, Reginald G. (Department: 2186)

Electrical computers and digital processing systems: memory

Storage accessing and control

Specific memory composition

C711S151000, C711S152000

Reexamination Certificate

active

06393518

ABSTRACT:

FIELD OF THE INVENTION
The invention relates to duplexed computer systems and especially to an arrangement with which two computer units can control data on the same disk drive.
BACKGROUND OF THE INVENTION
In fault-tolerant systems that are used for example in switching centres, it is previously known to write data into two separate disk drives. It is also previously known to duplex computers so that when one computer unit fails, the other computer may continue the operation. In such a system, the active unit controls both disk drives. The passive unit is either in reserve or faulty, but it does not usually control the disk drives. It is also previously known to store in the main memory of the computer such areas of the disk drive that were read last, since it is likely that at least some of these areas (for example directories) must be read again soon.
With reference to
FIG. 1
, duplexed computers are usually arranged in such a way that only one (e.g.
10
) of the computers
10
and
20
uses a disk drive
14
and/or a disk drive
24
. The other computer (in this case
20
) is then in reserve or faulty. A first disk drive bus
15
connects both computer units and the first disk drive
14
. Correspondingly, a second disk drive bus
25
connects both computer units and the second disk drive
24
.
FIG. 1
shows an embodiment where each computer unit
10
and
20
comprises two disk drive controllers
13
a
and
13
b
, and
23
a
and
23
b
, respectively. Alternatively, each computer unit
10
and
20
might comprise one disk drive controller having two separate interfaces to the disk drive buses
15
and
25
. The disk drive buses
15
and
25
may be for example SCSI (Small Computer Systems Interface) buses and the disk drives
14
and
24
may correspondingly be disk drives with SCSI interfaces.
FIG. 1
also shows a bidirectional message bus
30
connecting the computers
10
and
20
. Via the message bus
30
the computers
10
and
20
interchange maintenance messages, for example they inquire about each other's condition etc.
The system of
FIG. 1
is shown with two disk drives
14
and
24
since in duplexed systems the disk drives are usually also duplexed. In this exemplary case, the disk drive buses have also been duplexed. As far as the invention is concerned, the disk drives
14
and
24
could also be connected to a common disk drive bus
15
or
25
. However, the invention relates to controlling disk drives regardless of any other disk drives that the system may comprise. The number of disk drives and disk drive buses is therefore not essential to the invention.
Caching the data of the disk drive
14
in the main memory
12
of one computer
10
constitutes technology that is known to a person skilled in the art. The caching is based on the empirical observation that when a disk task is performed to some part of the disk drive
14
, it is likely that another disk task is performed relatively soon to the same part. Therefore it is preferable to store in the main memory
12
the data corresponding to some of the most recently used areas of the disk drive
14
. For the purpose of caching a disk drive, a certain area is usually reserved from the main memory
12
. Alternatively, the disk drive controllers
13
a
and
13
b
may contain a separate cache memory.
A problem occurs with the use of duplexed computers each of which is to be provided with the possibility of using at least one disk drive
14
and
24
while the operation is expedited by caching the disk drive in the main memories
12
and
22
of the computers
10
and
20
. If one computer unit (e.g.
10
) modifies the data on the disk drive
14
, the other computer unit (in this case
20
) is not necessarily aware of it. If this computer
20
relies on the cached data in its main memory without reading the recently updated data from the disk drive
14
, the system gets confused. Due to fault-tolerance, the computer units
10
and
20
have separate main memories, so that the computers
10
and
20
cannot write directly into each other's main memories which areas in the disk drive
14
they have changed.
SUMMARY OF THE INVENTION
On the basis of the above, the object of the invention is to provide a method and a system with which two computer units can cache data in a shared disk drive in a reliable manner. The aims of the invention are achieved with methods and systems that are characterized by what is disclosed in the independent claims. The preferred embodiments are set forth in dependent claims.
The invention is based on the idea that a computer unit desiring to use a disk drive makes sure that another computer does not use the disk drive simultaneously, whereafter the computer checks if the other computer has modified any of the data in the disk drive. If the data in the disk drive has been changed, the cached data corresponding to this data is erased from the main memory. After this, a disk task is performed and the other computer unit is informed of the changed data in the disk drive.
The method and system according to the invention have the primary advantage that shared disk data can be controlled from two computers so that data can still be cached in the main memories of the computers. Also, the invention only requires few additions to the disk control software in order to be realized. The equipment does not usually have to be altered, considering the changes that would be required in any case by the duplexing of the computer system.


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Copy of International Search Report.

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