MANAGING COPY-ON-WRITES TO SNAPSHOTS

• US 20110191555A1
• Filed: 01/29/2010
• Published: 08/04/2011
• Est. Priority Date: 01/29/2010
• Status: Active Grant

• Current Assignee: Veritas Technologies, LLC (Whitehouse Group Ltd.)
• Original Assignee: Symantec Corporation (NortonLifeLock Inc.)
• Inventors: Narayanan, Priyesh
• Granted Patent: US 9,176,853 B2
• Time in Patent Office: Days
• US Class Current: 711/162
• CPC Class Codes: G06F 11/1451 by selection of backup contents
• CPC Class Codes: G06F 11/1466 to make the backup process non-disruptive
• CPC Class Codes: G06F 12/00 Accessing, addressing or allocating within memory systems or architectures digital input from, or digital output to record carriers, e.g. to disk storage units, G06F3/06
• CPC Class Codes: G06F 12/16 Protection against loss of memory contents contains no material, see G06F11/00
• CPC Class Codes: G06F 2201/84 Using snapshots, i.e. a logical point-in-time copy of the data
• CPC Class Codes: G06F 3/0614 Improving the reliability of storage systems
• CPC Class Codes: G06F 3/065 Replication mechanisms
• CPC Class Codes: G06F 3/067 Distributed or networked storage systems, e.g. storage area networks [SAN], network attached storage [NAS]

Abstract

An attempt to write to a block of data in a main volume of data is detected. An indicator associated with the block of data is accessed before a copy-on-write operation to a snapshot volume is performed for the block of data. The indicator is used to determine whether the copy-on-write operation is to be performed for the block of data.

Claims

1. A computer-readable storage medium having computer-executable instructions for causing a computer system to perform a method comprising:

detecting an attempt to write to a block of data in a first volume of data, wherein a copy-on-write snapshot volume is associated with said first volume and wherein said snapshot volume includes a pointer to said block of data;

accessing an indicator associated with said block of data before a copy-on-write operation to said snapshot volume is performed for said block of data; and

determining whether said copy-on-write operation is to be performed for said block of data based on said indicator.

2. The computer-readable storage medium of claim 1 wherein said block of data is accessed prior to said copy-on-write operation, wherein further said copy-on-write operation is therefore not performed for said block of data.

3. The computer-readable storage medium of claim 1 wherein said snapshot volume is for a portion of said first volume and said block of data is not in said portion, wherein further said copy-on-write operation is therefore not performed for said block of data.

4. The computer-readable storage medium of claim 1 wherein said snapshot volume is for a file spanning a subset of said blocks of data and said block of data is not in said subset, wherein further said copy-on-write operation is therefore not performed for said block of data.

5. The computer-readable storage medium of claim 1 further comprising maintaining a first bitmap of said first volume, wherein said indicator comprises a bit value in said first bitmap.

6. The computer-readable storage medium of claim 5 further comprising maintaining a second bitmap of said first volume, wherein said second bitmap tracks which of said blocks have been written to said snapshot volume.

7. The computer-readable storage medium of claim 1 wherein said method further comprises updating said indicator if said block of data is successfully read before it is copy-on-written, wherein said block of data need not be copy-on-written in response to a subsequent write to that block.

8. A computer-readable storage medium having computer-executable instructions for causing a computer system to perform a method comprising:

accessing a volume of data comprising a plurality of blocks of data, wherein a copy-on-write snapshot is associated with said volume; and

designating a subset of said blocks of data as blocks not of interest, wherein said snapshot does not include a copy of blocks of data in said subset;

wherein a copy-on-write, to said snapshot, of a block of data included in said subset is prevented, and wherein a copy-on-write, to said snapshot, of a block of data not included in said subset is allowed.

9. The computer-readable storage medium of claim 8 wherein said subset comprises a block of data that has been successfully read.

10. The computer-readable storage medium of claim 8 wherein said snapshot is for a portion of said volume and said subset is not in said portion.

11. The computer-readable storage medium of claim 8 wherein said snapshot is for a file spanning multiple blocks and said subset is not part of said file.

12. The computer-readable storage medium of claim 8 further comprising updating a bitmap of said volume, wherein said designating comprises setting bits in said bitmap that correspond to blocks in said subset.

13. The computer-readable storage medium of claim 12 wherein said method further comprises updating said bitmap each time a block of data in said volume is successfully read before it is copy-on-written.

14. A computer-readable storage medium having computer-executable components comprising:

a first application operable for writing data to a volume comprising a plurality of blocks of data;

a snapshot driver operable for creating a snapshot for said volume; and

a second application operable for reading data from said volume and from said snapshot, wherein said second application designates a subset of said blocks of data as blocks not of interest, wherein further a copy-on-write of a block of data in said subset is not performed if said first application initiates a write to said block of data.

15. The computer-readable storage medium of claim 14 wherein said subset comprises a block of data that has been successfully read.

16. The computer-readable storage medium of claim 14 wherein said snapshot is for a portion of said volume and said subset is not in said portion.

17. The computer-readable storage medium of claim 14 wherein said snapshot is for a file that spans a portion of said volume and said subset is not in said portion.

18. The computer-readable storage medium of claim 14 wherein a first bitmap of said volume is used to track which of said blocks is in said subset.

19. The computer-readable storage medium of claim 18 wherein a second bitmap of said volume is used to track which of said blocks have been copy-on-written to said snapshot.

20. The computer-readable storage medium of claim 18 wherein said first bitmap is updated each time a block of data in said volume is successfully read before it is copy-on-written to said snapshot.