For consistency cbbackupmgr is forced to open all vbucket streams at once. This a has a performance impact because the streams are interlaced, which causes cbbackupmgr to jump between different vbucket files.
For supporting backup to S3 it is too expensive to hold all 1024 vbuckets open at once, because to be cost efficient the upload chunks have to be large.
A possible solution is to open and stream one vbucket at a time, however this causes consistency problems.
It is suggested that a DCP control message cold be used to state that it will open the file handle to all vbuckets, which gives consistency, however stream only one vbucket at a time
|Restore BackfillManager "backfill_order" stat