System and method for satisfying I/O requests before a replica has been fully synchronized
Various methods and systems are disclosed for handling I/O requests to a replica that has not yet been fully synchronized. One method involves detecting the transfer of the role of working volume from a first volume to a second volume. The second volume is configured as a replica of the first volume, but the second volume may not be fully synchronized with the first volume at the time that the role of working volume is transferred. Subsequent to the role of working volume being transferred to the second volume, a read request to the second volume is satisfied by accessing a log. The log is configured to store values that have been written to the first volume, and at least one of the values written to the first volume has not been written to the second volume.
1. A method comprising:
detecting a designation of a second volume as a working volume, wherein
prior to the detecting, a first volume is the working volume,
the designation occurs in response to the first volume becoming inaccessible, and
the second volume is configured as a replica of the first volume; and
satisfying a read request to the second volume by accessing a log, subsequent to detecting the designation of the second volume as the working volume, wherein
the log stores values written to the first volume and at least one of the values written to the first volume has not been written to the second volume,
the satisfying the read request comprises retrieving a value from the log, and
the read request is satisfied prior to writing the value to the second volume.
2. The method of claim 1 , further comprising:
appending, to the log, an entry corresponding to a write access, in response to detecting the write access to a first region of the first volume; wherein
the entry stores information identifying the first region and a new value of the first region.
3. The method of claim 2 , further comprising:
storing the log at a bunker site, wherein
the first volume is located at a first site, and
the second volume is located at a second site.
4. The method of claim 3 , further comprising:
generating a map, wherein
the map stores information mapping entries in the log to the second volume;
detecting a read request to a region of the second volume, wherein
the read request is generated by an application;
determining whether a most recent value of the region is stored in the log, based on the map;
accessing the log, if the map indicates that the most recent value of the region is stored in the log; and
returning the most recent value to the application.
5. A system comprising:
a detection module configured to detect a designation of a second volume as a working volume, wherein prior to the detection, a first volume is the working volume, and the designation occurs in response to the first volume becoming inaccessible;
a read redirection module configured to detect a read request to the first volume;
a computer readable storage medium coupled to the read redirection module and storing a log, wherein
the read redirection module is configured to satisfy the read request by retrieving data from the log, instead of from the first volume,
the read request is satisfied prior to writing the data to the first volume,
the log stores values written to a second volume that have not been written to the first volume, and
the first volume is a replica of the second volume.
6. The system of claim 5 , wherein
the read request specifies a first region of the first volume,
the read redirection module is configured to access a map in response to detecting the read request, and
the map identifies whether a most recent value of the first region is stored in the log.
7. The system of claim 5 , wherein
the first volume, log, and the read redirection module are located at a first site,
the second volume is located at a second site, and
the first site is coupled to the second site by a network.
8. The system of claim 7 , wherein
the first volume and the read redirection module are located at a first site,
the log is located at a bunker site,
the second volume is located at a second site, and
a network couples the first site, second site, and bunker site.
9. A system comprising:
means for detecting a designation of a second volume as a working volume, wherein prior to the detecting, a first volume is the working volume,
the designation occurs in response to the first volume becoming inaccessible, and
the second volume is configured as a replica of the first volume;
means for accessing a log in order to satisfy a read request to the second volume, subsequent to detecting the designation of the second volume as the working volume, wherein
the log stores values written to the first volume and at least one of the values written to the first volume has not been written to the second volume,
the satisfying the read request comprises retrieving a value from the log, and
the read request is satisfied prior to writing the value to the second volume; and
a computer readable storage means for storing the log.
10. The system of claim 9 , further comprising:
means for appending an entry corresponding to a write access to the log, in response to detecting the write access to a first region of the first volume; wherein
the entry stores information identifying the first region and a new value of the first region.
11. The system of claim 10 , further comprising:
means for storing the log at a bunker site, wherein
the first volume is located at a first site, and
the second volume is located at a second site.
12. The system of claim 11 , further comprising:
means for generating a map, wherein
the map stores information mapping entries in the log to second first volume;
means for detecting a read request to a region of the second volume, wherein
the read request is generated by an application;
means for determining whether a most recent value of the region is stored in the log, based on the map;
means for accessing the log, if the map indicates that the most recent value of the region is stored in the log; and
means for returning the most recent value to the application.
13. A computer readable storage medium comprising program instructions executable to:
detect a designation of a second volume as a working volume, wherein
prior to detecting the designation, a first volume is the working volume,
the designation occurs in response to the first volume becoming inaccessible, and
the second volume is configured as a replica of the first volume; and
access a log in order to satisfy a read request to the second volume, subsequent to detecting the designation of the second volume as the working volume, wherein
the log stores values written to the first volume and at least one of the values written to the first volume has not been written to the second volume,
the satisfying the read request comprises retrieving a value from the log, and
the read request is satisfied prior to writing the value to the second volume.
14. The computer readable storage medium of claim 13 , wherein
the log comprises an entry,
the entry corresponds to a write access to a first region of the first volume, and
the entry stores information identifying the first region and a new value of the first region.
15. The computer readable storage medium of claim 14 , wherein
the log is stored at a bunker site,
the first volume is located at a first site, and
the second volume is located at a second site.
16. The computer readable storage medium of claim 15 , wherein the program instructions are further executable to:
detect a read request to a region of the second volume, wherein
the read request is generated by an application;
determine whether a most recent value of the region is stored in the log, based on a map, wherein
the map stores information mapping entries in the log to the second volume;
access the log, if the map indicates that the most recent value of the region is stored in the log; and
return the most recent value to the application.
17. A system comprising:
a processor; and
a memory coupled to the processor, wherein the memory stores program instructions executable by the processor to:
detect a designation of a second volume as a working volume, wherein
prior to detecting the designation, a first volume is the working volume,
the designation occurs in response to the first volume becoming inaccessible, and
the second volume is configured as a replica of the first volume; and
access a log in order to satisfy a read request to the second volume, subsequent to detecting the designation of the second volume as the working volume, wherein
the log stores values written to the first volume and at least one of the values written to the first volume has not been written to the second volume,
the satisfying the read request comprises retrieving a value from the log, and
the read request is satisfied prior to writing the value to the second volume.
18. The system of claim 17 , wherein
the log comprises an entry,
the entry corresponds to a write access to a first region of the first volume, and
the entry stores information identifying the first region and a new value of the first region.
19. The system of claim 18 , wherein
the log is stored at a bunker site,
the first volume is located at a first site, and
the second volume is located at a second site.
20. The system of claim 19 , wherein the program instructions are further executable by the processor to:
detect a read request to a region of the second volume, wherein
the read request is generated by an application;
determine whether a most recent value of the region is stored in the log, based on a map, wherein
the map stores information mapping entries in the log to the second volume;
access the log, if the map indicates that the most recent value of the region is stored in the log; and
return the most recent value to the application.