Electrical computers and digital processing systems: memory – Storage accessing and control – Access timing
Reexamination Certificate
1998-01-30
2001-05-08
Ellis, Kevin L. (Department: 2751)
Electrical computers and digital processing systems: memory
Storage accessing and control
Access timing
C711S162000
Reexamination Certificate
active
06230246
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to distributed storage systems and more particularly to non-intrusive crash consistent copying in a distributed storage system without client cooperation.
BACKGROUND
Techniques have been developed to efficiently store information in a computer network. For example, the physical storage elements of a group of computer storage servers can be used to form a logical storage device, commonly referred to as a “virtual disk.” The virtual disk is functionally equivalent to a single physical storage element but is actually formed of several physical storage elements.
For reasons analogous to the reasons why information stored at a physical storage element must be backed-up, the information stored at a virtual disk must also be backed-up. Because of the distributed nature of a virtual disk, however, special care must be taken to ensure that the same “version” of the virtual disk is copied from each of the physical storage elements that form the virtual disk.
To ensure consistency between the original and back-up copies of a virtual disk, prior to copying the virtual disk, the entire computer storage system is conventionally placed into a quiescent state. This basically causes the computer storage system to become inactive. The copy operation is permitted only after the computer storage system has reached quiescence. This procedure, although ensuring consistency between copies of a virtual disk, significantly intrudes on the normal operation of the computer storage system, including operations unrelated to the making of the copies. Accordingly, it would be advantageous to be able to copy virtual disks without intruding on normal computer storage system operations.
OBJECT OF THE INVENTION
It is, accordingly, an object of the present invention to provide a technique for copying the contents of a virtual disk without interfering with normal operation of a distributed storage system.
It is another object of the present invention to provide further advantages and features, the details of which shall be described below.
SUMMARY OF THE INVENTION
The present invention advantageously provides an apparatus and an associated method for controlling access to storage elements within a physical storage device when an associated logical storage device, which can be formed of a number of physical storage elements, for example, storage elements distributed across multiple computing nodes connected by a network, is being copied.
In accordance with the invention, a memory stores a write-barrier value and a processor prohibits write operations on an associated storage element. In operation, a write-barrier value of a first state, for example 0, is stored in the memory when the logical storage device is to be copied.
While the stored write-barrier value is in the first state, the execution of write operations to the storage element(s) associated with the memory is prohibited by the processor. Advantageously, the write-barrier value stored in the memory is set to the first state by the processor upon the receipt of a request to copy the logical storage device. The copying of the logical storage device may be automatically initiated by the processor.
After the write-barrier value is set to 0, portions of the logical storage device are copied, and finally, the write-barrier value that was previously set to the first state is set to a second state for example, to 1. While the write-barrier value is set to 1, write operations upon the associated storage element can be executed in a normal fashion, i.e., are no longer prohibited by the processor.
Preferably, the logical storage device can only be copied when the write-barrier value stored in the memory is in the first state, i.e., 0. The logical storage device is copied by a copy-on-write technique which avoids the often cumbersome process of completely copying each file.
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Lee Edward K.
Thekkath Chandramohan A.
Compaq Computer Corporation
Ellis Kevin L.
Oppenheimer Wolff & Donnelly LLP
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