cbbackupmgr: Investigate ways to reduce the size of merged backups (merged backup 40% bigger than size of incremental backups merge)

Build : 5.0.0-3088

Size of merged backup is ~40% more than the total size of incremental backups.

Merging two incremental backups of 17GB each resulted into a backup of 47.04 GB. The cluster is a 2 node cluster with 2 buckets with constant KV ops consisting of creates, updates and deletes (65% creates, 25% updates, 10% deletes). The buckets had 28,934,766 & 28,351,132 items respectively after the 2nd incremental backup was taken.

root@choco:/tmp/backup/logs# /opt/couchbase/bin/cbbackupmgr list -a /tmp/backup/ -r backup
Size Items Name
33.16GB - + backup
16.21GB - + 2017-06-15T16_19_41.031911829+05_30
6.99GB - + default
368B 0 bucket-config.json
6.99GB 3663609 + data
6.99GB 3663609 shard_0.fdb
2B 0 full-text.json
4B 0 gsi.json
2B 0 views.json
9.22GB - + other
366B 0 bucket-config.json
9.22GB 5563562 + data
9.22GB 5563562 shard_0.fdb
2B 0 full-text.json
4B 0 gsi.json
2B 0 views.json
16.95GB - + 2017-06-15T16_27_12.019202441+05_30
8.55GB - + default
368B 0 bucket-config.json
8.55GB 5627251 + data
8.55GB 5627251 shard_0.fdb
2B 0 full-text.json
4B 0 gsi.json
2B 0 views.json
8.40GB - + other
366B 0 bucket-config.json
8.40GB 5866006 + data
8.40GB 5866006 shard_0.fdb
2B 0 full-text.json
4B 0 gsi.json
2B 0 views.json
root@choco:/tmp/backup/logs# /opt/couchbase/bin/cbbackupmgr merge -a /tmp/backup/ -r backup --start 1 --end 2
Merge completed successfully
root@choco:/tmp/backup/logs# /opt/couchbase/bin/cbbackupmgr list -a /tmp/backup/ -r backup
Size Items Name
47.04GB - + backup
47.04GB - + 2017-06-15T16_27_12.019202441+05_30
20.32GB - + default
368B 0 bucket-config.json
20.32GB 9279587 + data
20.32GB 9279587 shard_0.fdb
2B 0 full-text.json
2B 0 gsi.json
2B 0 views.json
26.72GB - + other
366B 0 bucket-config.json
26.72GB 11424018 + data
26.72GB 11424018 shard_0.fdb
2B 0 full-text.json
2B 0 gsi.json
2B 0 views.json