RFC dm-dedup: device-mapper deduplication target

This is a second request for comments for dm-dedup.

Updates compared to the first submission:

- code is updated to kernel 3.16
- construction parameters are now positional (as in other targets)
- documentation is extended and brought to the same format as in other targets

Dm-dedup is a device-mapper deduplication target. Every write coming to the dm-dedup instance is deduplicated against previously written data. For datasets that contain many duplicates scattered across the disk (e.g., collections of virtual machine disk images and backups) deduplication provides a significant amount of space savings.

To quickly identify duplicates, dm-dedup maintains an index of hashes for all written blocks. A block is a user-configurable unit of deduplication with a recommended block size of 4KB. dm-dedup's index, along with other deduplication metadata, resides on a separate block device, which we refer to as a metadata device. Although the metadata device can be on any block device, e.g., an HDD or its own partition, for higher performance we recommend to use SSD devices to store metadata.

Dm-dedup is designed to support pluggable metadata backends. A metadata backend is responsible for storing metadata: LBN-to-PBN and HASH-to-PBN mappings, allocation maps, and reference counters. (LBN: Logical Block Number, PBN: Physical Block Number). Currently we implemented "cowbtree" and "inram" backends. The cowbtree uses device-mapper persistent API to store metadata. The inram backend stores all metadata in RAM as a hash table.

Detailed design is described here:

http://www.fsl.cs.sunysb.edu/docs/ols-dmdedup/dmdedup-ols14.pdf

Our preliminary experiments on real traces demonstrate that Dmdedup can even exceed the performance of a disk drive running ext4. The reasons are that (1) deduplication reduces I/O traffic to the data device, and (2) Dmdedup effectively sequentializes random writes to the data device.

Dmdedup is developed by a joint group of researchers from Stony Brook University, Harvey Mudd College, and EMC. See the documentation patch for more details.

Vasily Tarasov (10):
dm-dedup: main data structures
dm-dedup: core deduplication logic
dm-dedup: hash computation
dm-dedup: implementation of the read-on-write procedure
dm-dedup: COW B-tree backend
dm-dedup: inram backend
dm-dedup: Makefile changes
dm-dedup: Kconfig changes
dm-dedup: status function
dm-dedup: documentation

 Documentation/device-mapper/dedup.txt |  205 +++++++
 drivers/md/Kconfig                    |    8 +
 drivers/md/Makefile                   |    2 +
 drivers/md/dm-dedup-backend.h         |  114 ++++
 drivers/md/dm-dedup-cbt.c             |  755 ++++++++++++++++++++++++++
 drivers/md/dm-dedup-cbt.h             |   44 ++
 drivers/md/dm-dedup-hash.c            |  145 +++++
 drivers/md/dm-dedup-hash.h            |   30 +
 drivers/md/dm-dedup-kvstore.h         |   51 ++
 drivers/md/dm-dedup-ram.c             |  580 ++++++++++++++++++++
 drivers/md/dm-dedup-ram.h             |   43 ++
 drivers/md/dm-dedup-rw.c              |  248 +++++++++
 drivers/md/dm-dedup-rw.h              |   19 +
 drivers/md/dm-dedup-target.c          |  946 +++++++++++++++++++++++++++++++++
 drivers/md/dm-dedup-target.h          |  100 ++++
 15 files changed, 3290 insertions(+), 0 deletions(-)
 create mode 100644 Documentation/device-mapper/dedup.txt
 create mode 100644 drivers/md/dm-dedup-backend.h
 create mode 100644 drivers/md/dm-dedup-cbt.c
 create mode 100644 drivers/md/dm-dedup-cbt.h
 create mode 100644 drivers/md/dm-dedup-hash.c
 create mode 100644 drivers/md/dm-dedup-hash.h
 create mode 100644 drivers/md/dm-dedup-kvstore.h
 create mode 100644 drivers/md/dm-dedup-ram.c
 create mode 100644 drivers/md/dm-dedup-ram.h
 create mode 100644 drivers/md/dm-dedup-rw.c
 create mode 100644 drivers/md/dm-dedup-rw.h
 create mode 100644 drivers/md/dm-dedup-target.c
 create mode 100644 drivers/md/dm-dedup-target.h