System and method for performing consistent resynchronization between synchronized copies
A method involves detecting that a first storage device is inaccessible. The information stored on the first storage device is a synchronized copy of information stored on a second storage device. In response to detecting that the first storage device is inaccessible, modification logging is initiated. Modification logging involves detecting modifications to information stored on the second storage device and storing information that indicates an order in which the modifications occur to the second storage device. The stored information can indicate the order explicitly (e.g., by including a timestamp or sequence number) or indirectly (e.g., the order of entries stored in a log can indicate the order of the modifications represented by those entries). The stored information can also include the new values generated by the modifications. The stored information can then be used to resynchronize the information stored on the first and second storage devices.
1. A method comprising:
applying, to a first storage device, a plurality of first modifications to information stored on a second storage device;
performing change tracking, wherein the change tracking comprises
identifying regions of the first storage device that have values differing from values stored on the second storage device as a result of the first modifications without identifying the order in which different regions are modified, and
setting a bit in a bitmap for each identified region, wherein
the bitmap comprises a plurality of bits,
each bit corresponds to a respective region of the first storage device, and
the bit is cleared when the respective region corresponding to the bit is synchronized with the second storage device;
detecting that connectivity to the first storage device has been lost, wherein
modification logging is inhibited prior to the detecting;
in response to the detecting, initiating the modification logging, wherein
the modification logging continues until connectivity has been restored and the first storage device has been synchronized with respect to the second storage device,
the modification logging is performed at a smaller granularity of information than the change tracking, and
the modification logging comprises:
detecting a plurality of second modifications to information stored on the second storage device, wherein
information stored on the first storage device is a synchronized copy of the information stored on the second storage device, prior to the time at which the plurality of second modifications are applied to the second storage device, and
storing information indicating an order in which the second modifications occur to the second storage device; and
in response to the detecting, inhibiting the change tracking, wherein
the change tracking is inhibited as long as the modification logging continues.
2. The method of claim 1 , wherein the storing the information indicating the order in which the second modifications occur further comprises:
storing a new value written to the second storage device in an entry in a log, wherein
an order of entries within the log identifies the order in which the second modifications occur.
3. The method of claim 2 , further comprising:
detecting that connectivity to the first storage device has been restored, subsequent to detecting that connectivity to the first storage device has been lost; and
synchronizing the first storage device with respect to the second storage device, wherein the synchronizing comprises:
copying each of a plurality of values from the log to the first storage device, in the order in which the values were stored in the log.
4. The method of claim 2 , wherein the modification logging further comprises
identifying regions of the first storage device that have values differing from values stored on the second storage device as a result of the plurality of second modifications to information stored on the second storage device, without identifying the order in which different regions are modified, and
storing information corresponding to the identified regions in the log.
5. The method of claim 3 , further comprising:
detecting a first write operation to the second storage device, wherein
the first write operation is generated by an application, and
performance of the first write operation writes a new value to a region of the second storage device; and
waiting to signal completion of the first write operation to the application until a copy of the new value is stored in the log.
6. The method of claim 1 , further comprising:
inhibiting modification logging, in response to detecting that connectivity to the first storage device has been restored and the first storage device is synchronized with respect to the second storage device.
7. The method of claim 6 , further comprising:
identifying second modifications to the second storage device in a bitmap, if modification logging is inhibited.
8. The method of claim 6 , wherein
the first storage device is located at a remote site,
the second storage device is located at a primary site, and
the remote site is coupled to the primary site by a network.
9. A system comprising:
a first storage device;
a second storage device, wherein information stored in the second storage device is a synchronized copy of information stored in the first storage device; and
a synchronization module coupled to the first storage device and the second storage device, the synchronization module comprising:
a change tracking module configured to perform change tracking, wherein the change tracking module is configured to
identify regions of the second storage device that have values differing from values stored on the first storage device without identifying the order in which different regions are modified, and
set a bit in a bitmap for each identified region, wherein
the bitmap comprises a plurality of bits,
each bit corresponds to a respective region of the first storage device, and
the bit is cleared when the respective region corresponding to the bit is synchronized with the second storage device, and
a logging module configured to perform modification logging, wherein the logging module is configured to store information indicating an order in which first modifications occur to the first storage device, wherein
the modification logging is performed at a smaller granularity of information than the change tracking,
the logging module is enabled in response to the synchronization module detecting that connectivity to the second storage device has been lost, wherein the logging module is disabled prior to the time at which connectivity has been detected to have been lost,
the logging module remains enabled until connectivity has been restored and the second storage device has been synchronized with respect to the first storage device, and
the synchronization module inhibits the change tracking module and the application, to the second storage device, of second modifications to information stored on the first storage device, as long as the logging module remains enabled.
10. The system of claim 9 , wherein the logging module is configured to:
store a new value written to the first storage device in an entry in a log, wherein
an order of entries within the log indicates the order in which the first modifications to the first storage device occur.
11. The system of claim 10 , wherein the synchronization module is configured to:
detect that connectivity to the second storage device has been restored, subsequent to detecting that connectivity to the second storage device has been lost; and
synchronize the second storage device with respect to the first storage device by copying each of a plurality of values from the log to the second storage device, in the order in which the values were stored.
12. The system of claim 10 , wherein the synchronization module is configured to:
disable the logging module, in response to detecting that connectivity to the second storage device has been restored and the second storage device is synchronized with respect to the first storage device.
13. A system comprising:
means for applying to a first storage device a plurality of first modifications to information stored on a second storage device;
means for performing change tracking, wherein the change tracking comprises
identifying regions of the first storage device that have values differing from values stored on the second storage device without identifying the order in which different regions are modified, and
setting a bit in a bitmap for each identified region, wherein
the bitmap comprises a plurality of bits,
each bit corresponds to a respective region of the first storage device, and
the bit is cleared when the respective region corresponding to the bit is synchronized with the second storage device;
means for detecting that connectivity to the first storage device has been lost, wherein modification logging is inhibited prior to the time at which connectivity has been detected to have been lost;
means for performing the modification logging, wherein
the modification logging is configured to be performed at a smaller granularity of information than the change tracking,
the means for performing modification logging are enabled in response to detecting that connectivity to the first storage device has been lost,
the means for performing modification logging remains enabled until connectivity has been restored and the first storage device has been synchronized with respect to the second storage device, and
the modification logging comprises:
detecting a plurality of second modifications to information stored on the second storage device, wherein
information stored on the first storage device is a synchronized copy of the information stored on the second storage device, prior to the time at which the plurality of second modifications are applied to the second storage device, and
storing information indicating an order in which the second modifications occur to the second storage device; and
means for inhibiting the means for applying and the means for performing change tracking as long as the means for performing the modification logging remains enabled, in response to detecting that connectivity to the first storage device has been lost.
14. The system of claim 13 , wherein storing the information indicating the order in which the second modifications occur comprises:
storing a new value written to the second storage device in an entry in a log, wherein
an order of entries within the log identifies the order in which the second modifications occur.
15. The system of claim 14 , further comprising:
means for detecting that connectivity to the first storage device has been restored, subsequent to detecting that connectivity to the first storage device has been lost; and
means for synchronizing the first storage device with respect to the second storage device, wherein the synchronizing comprises:
copying each of a plurality of values from the log to the first storage device, in the order in which the values were stored in the log.
16. The system of claim 13 , further comprising:
means for inhibiting modification logging, in response to detecting that connectivity to the first storage device has been restored and the first storage device is synchronized with respect to the second storage device.
17. A computer readable storage medium comprising program instructions executable to:
apply to a first storage device a plurality of first modifications to information stored on a second storage device, and
perform change tracking, wherein the change tracking comprises
identifying regions of the first storage device that have values differing from values stored on the second storage device without identifying the order in which different regions are modified, and
setting a bit in a bitmap for each identified region, wherein
the bitmap comprises a plurality of bits,
each bit corresponds to a respective region of the first storage device, and
the bit is cleared when the respective region corresponding to the bit is synchronized with the second storage device;
detect that connectivity to the first storage device has been lost, wherein
modification logging is inhibited prior to the time at which connectivity has been detected to have been lost;
perform the modification logging, wherein
the modification logging is performed at a smaller granularity of information than the change tracking,
the modification logging is enabled in response to detecting that the first storage device is inaccessible,
the modification logging remains enabled until connectivity has been restored and the first storage device has been synchronized with respect to the second storage device, and
the modification logging comprises:
detecting a plurality of second modifications to information stored on the second storage device, wherein
information stored on the first storage device is a synchronized copy of the information stored on the second storage device, prior to the time at which the plurality of second modifications are applied to the second storage device; and
storing information indicating an order in which the second modifications occur to the second storage device; and
inhibit the modification application as long as the modification logging is enabled, in response to the detecting.
18. The computer readable storage medium of claim 17 , wherein storing the information indicating the order in which the second modifications occur comprises:
storing a new value written to the second storage device in an entry in a log, wherein
an order of entries within the log identifies the order in which the second modifications occur.
19. The computer readable storage medium of claim 18 , wherein the program instructions are further executable to:
detect that connectivity to the first storage device has been restored, subsequent to detecting that connectivity to the first storage device has been lost; and
synchronize the first storage device with respect to the second storage device, wherein synchronizing the first storage device comprises:
copying each of a plurality of values from the log to the first storage device, in the order in which the values were stored in the log.
20. The computer readable storage medium of claim 17 , wherein the program instructions are further executable to:
inhibit modification logging, in response to detecting that the first storage device is accessible and synchronized with respect to the second storage device.