Hi, The following series of patches implements in btrfs an ioctl to do offline deduplication of file extents.
To be clear, "offline" in this sense means that the file system is mounted and running, but the dedupe is not done during file writes, but after the fact when some userspace software initiates a dedupe. The primary patch is loosely based off of one sent by Josef Bacik back in January, 2011. http://permalink.gmane.org/gmane.comp.file-systems.btrfs/8508 I've made significant updates and changes from the original. In particular the structure passed is more fleshed out, this series has a high degree of code sharing between itself and the clone code, and the locking has been updated. The ioctl accepts a struct: struct btrfs_ioctl_same_args { __u64 logical_offset; /* in - start of extent in source */ __u64 length; /* in - length of extent */ __u16 total_files; /* in - total elements in info array */ __u16 files_deduped; /* out - number of files that got deduped */ __u32 reserved; struct btrfs_ioctl_same_extent_info info[0]; }; Userspace puts each duplicate extent (other than the source) in an item in the info array. As there can be multiple dedupes in one operation, each info item has it's own status and 'bytes_deduped' member. This provides a number of benefits: - We don't have to fail the entire ioctl because one of the dedupes failed. - Userspace will always know how much progress was made on a file as we always return the number of bytes deduped. #define BTRFS_SAME_DATA_DIFFERS 1 /* For extent-same ioctl */ struct btrfs_ioctl_same_extent_info { __s64 fd; /* in - destination file */ __u64 logical_offset; /* in - start of extent in destination */ __u64 bytes_deduped; /* out - total # of bytes we were able * to dedupe from this file */ /* status of this dedupe operation: * 0 if dedup succeeds * < 0 for error * == BTRFS_SAME_DATA_DIFFERS if data differs */ __s32 status; /* out - see above description */ __u32 reserved; }; The kernel patches are based off Linux v3.8. The ioctl has been tested in the most basic sense, and should not be trusted to keep your data safe. There are bugs. A git tree for the kernel changes can be found at: https://github.com/markfasheh/btrfs-extent-same I have a userspace project, duperemove available at: https://github.com/markfasheh/duperemove Hopefully this can serve as an example of one possible usage of the ioctl. duperemove takes a list of files as argument and will search them for duplicated extents. My most recent changes have been to integrate it with btrfs_extent_same so that the '-D' switch will have it fire off dedupe requests once processing of data is complete. Integration with extent_same has *not* been tested yet so don't expect that to work flawlessly. Within the duperemove repo is a file, btrfs-extent-same.c that acts as a test wrapper around the ioctl. It can be compiled completely seperately from the rest of the project via "make btrfs-extent-same". This makes direct testing of the ioctl more convenient. Code review is very much appreciated. Thanks, --Mark -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
