Electrical computers and digital processing systems: memory – Storage accessing and control – Control technique
Reexamination Certificate
2000-12-13
2004-06-01
Bragdon, Reginald G. (Department: 2188)
Electrical computers and digital processing systems: memory
Storage accessing and control
Control technique
Reexamination Certificate
active
06745305
ABSTRACT:
The present invention relates to high availability computer storage systems and, more particularly, to methods for mirroring disk drives within a network.
BACKGROUND OF THE INVENTION
The increased reliance by business on On-Line Transaction Processing and Decision Support Systems has increased the demand for high availability systems since these systems are critical to the functioning of day-to-day activities in many businesses. These systems are not only essential for the support of normal daily operations, but they also store critically important customer and corporate data. Continuous availability is no longer an ideal; it is a necessity for many companies. Longer work days, expansion into new markets and customer demand for more efficient service create an expanded requirement for increased system availability. Users are demanding a means of ensuring very high availability of their applications and the access to data that permits them to accomplish their tasks and provide the highest levels of customer service. Interruption of workflow due to system failure is expensive and it can cause the loss of business. The need to increase computer system availability is becoming one of businesses key concerns.
Implementation of client/server computing is growing throughout today's businesses—for key business applications as well as electronic mail, distributed databases, file transfer, retail point-of-sale, inter-networking, and other applications. It is possible for companies to gain competitive advantages from client/server environments by controlling the cost of the technology components through economies of scale and the use of clustered computing resources. There is a boost in productivity when businesses have high availability and easy access to information throughout the corporate enterprise.
An important component of a high-availability client/server system is a reliable, fault-tolerant data storage system. In some networked or “clustered” multiple server arrangements, the physical data storage system may be a shared RAID (Redundant Array of Inexpensive Disks) disk array system, or a shared pair of disk drives or disk arrays operating in a mirrored arrangement.
A disk mirroring system, also referred to as a RAID Level 1 system, typically consists of two equal-capacity disk drives that mirror one another. One disk drive contains a duplicate of all the files contained on the other disk drive, each drive essentially functioning as a backup for the other disk drive. Should one of the disk drives fail all the data and applications contained on the failed drive are available from the remaining disk drive. The mirror drives may be co-located such as in a single personal computer or server computer, or may reside at different locations within a network. The drives may be logical drives, partitions or drive arrays. They do not need to be equivalent in all aspects or of equal capacity.
Most disk mirroring procedures utilizing shared drives will write or update both mirror drives synchronously. In systems performing disk mirroring with non-shared drives over a network, writes directed to a primary drive are received and forwarded to the secondary mirror drive. Upon receipt from the secondary drive of an acknowledgement signal indicating a successful update of the secondary drive, the write to the primary is completed.
A computer system including multiple servers and a pair of shared disk drives is shown in FIG.
1
.
FIG. 1
provides a diagram of clustered or networked computers having a primary server
101
and a secondary server
103
in a fail-over pair arrangement. Primary server
101
is the preferred application server of the pair, and secondary server
103
preferably provides fail-over protection for the primary server. The primary and secondary servers are coupled through a network bus system
105
to a plurality of client computers
107
though
109
. The primary and secondary servers
101
and
103
each shares access to a pair of disk storage devices
111
and
113
. Disk storage devices
111
and
113
are SCSI (Small Computer Systems Interface) disk drives or disk arrays connected to servers
101
and
103
through a pair of SCSI busses
115
and
117
.
Primary disk storage device
111
and secondary disk storage device
113
are two equal-capacity storage devices that mirror each other. Each storage device contains a duplicate of all files contained on the other storage device, and a write or update to one storage device updates both devices in the same manner. In the event that either storage device fails, the data contained therein remains available to the system from the operational mirror storage device.
After the disk mirrors are established, the drives on the primary and secondary servers are synchronized, and both servers are up and running, disk mirroring conventionally proceeds as follows:
1. After the initial mirror is established, the system locks out all user access to the secondary drive
113
. Reads and writes are not allowed to the secondary drive
113
. The primary server drive
111
is accessible for both reads and writes.
2. Whenever the primary server
101
receives a write request for mirrored drive
111
the request is sent to the secondary mirrored drive
113
first. The secondary system
103
executes the write request on its mirrored drive and then sends the status of the write back to the primary server
101
. The primary server
101
does nothing on the write request until the secondary server
103
returns its status.
3. When the secondary server
103
returns a successful status, the primary server
101
executes the write to its mirrored drive
111
. Should an error occur while the secondary server
103
executes its mirrored drive write, the write process on the secondary server is terminated. The primary server then completes the write request on its mirrored drive and the status of the mirror then changes from Normal to Broken.
4. The secondary mirrored drive
113
is locked to all users once the mirror is created. Locking the secondary mirrored drive
113
ensures that data on both the primary and secondary mirrored drives is not corrupted through an inadvertent write to the secondary mirrored drive. When a mirror is deleted, the secondary mirrored drive is unlocked and full access is again allowed to both the primary drive
111
and secondary drive
113
.
In disk mirroring applications, particularly disk mirroring applications utilizing non-shared drives over a network, it can be a very expensive operation in terms of processing time and network bandwidth to write a disk block from one disk drive to its mirror disk drive. A method for synchronizing mirrored disk volumes and writing data to a mirrored drive that reduces the overall expense of disk mirroring operations, particularly when mirroring non-shared disk drives over a network, is desired.
OBJECTS OF THE INVENTION
It is therefore an object of the present invention to provide a new and method for mirroring storage devices that reduces the expense of synchronizing mirrored disk volumes and writing data to a mirrored drive.
It is another object of the present invention to provide such a method for mirroring storage devices over a network that reduces the processing time and network bandwidth required to write a disk block from a primary disk drive to a secondary mirror disk drive.
It is yet another object of the present invention to provide a new and useful method for mirroring storage devices that optimizes disk mirror synchronization operations.
It is a still further object of the present invention to provide a new and useful method for mirroring storage devices that optimizes disk mirror write operations.
SUMMARY OF THE INVENTION
There is provided, in accordance with the present invention, methods for duplicating data stored on a first storage device on a second storage device, and for duplicating data written to a first data storage device on a second data storage device.
The method for duplicating data stored on a first storage device on a second storage device comprises th
Bragdon Reginald G.
NCR Corporation
Stover James M.
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