opt(snapshot): use full table copy when streaming the entire data (#7870

)

When streaming the entire data in the snapshot (snap.SinceTs=0), we can do
an entire table copy instead of iterating over the KVs. This brings about
3x performance improvement, as well as the CPU of the sender, is idle.
Refer dgraph-io/badger#1700 for more details.

go.mod

@@ -10,7 +10,7 @@ require (
 	github.com/Masterminds/semver/v3 v3.1.0
 	github.com/Shopify/sarama v1.27.2
 	github.com/blevesearch/bleve v1.0.13
-	github.com/dgraph-io/badger/v4 v4.0.2-0.20230307194143-24ef3aab9eda
+	github.com/dgraph-io/badger/v4 v4.0.2-0.20230307194918-e55f05f923e1
 	github.com/dgraph-io/dgo/v210 v210.0.0-20210407152819-261d1c2a6987
 	github.com/dgraph-io/gqlgen v0.13.2
 	github.com/dgraph-io/gqlparser/v2 v2.2.1

go.sum

@@ -148,8 +148,8 @@ github.com/davecgh/go-spew v1.1.0/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSs
 github.com/davecgh/go-spew v1.1.1 h1:vj9j/u1bqnvCEfJOwUhtlOARqs3+rkHYY13jYWTU97c=
 github.com/davecgh/go-spew v1.1.1/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
 github.com/dgraph-io/badger v1.6.0/go.mod h1:zwt7syl517jmP8s94KqSxTlM6IMsdhYy6psNgSztDR4=
-github.com/dgraph-io/badger/v4 v4.0.2-0.20230307194143-24ef3aab9eda h1:YtklLh8nmh/Y5TJPjJlcg3Ev1HMjasUAn4ty213CI4Y=
-github.com/dgraph-io/badger/v4 v4.0.2-0.20230307194143-24ef3aab9eda/go.mod h1:Ghj1IjxpaEbozo6F4jV3QDYyah5uzYJWDcjZ82dBDdg=
+github.com/dgraph-io/badger/v4 v4.0.2-0.20230307194918-e55f05f923e1 h1:0ApclJzjWzS//p8DHlNrOObn9M9g5HeD30qNgm7/BJg=
+github.com/dgraph-io/badger/v4 v4.0.2-0.20230307194918-e55f05f923e1/go.mod h1:Ghj1IjxpaEbozo6F4jV3QDYyah5uzYJWDcjZ82dBDdg=
 github.com/dgraph-io/dgo/v210 v210.0.0-20210407152819-261d1c2a6987 h1:5aN6H88a2q3HkO8vSZxDlgjEpJf4Fz8rfy+/Wzx2uAc=
 github.com/dgraph-io/dgo/v210 v210.0.0-20210407152819-261d1c2a6987/go.mod h1:dCzdThGGTPYOAuNtrM6BiXj/86voHn7ZzkPL6noXR3s=
 github.com/dgraph-io/gqlgen v0.13.2 h1:TNhndk+eHKj5qE7BenKKSYdSIdOGhLqxR1rCiMso9KM=

worker/snapshot.go

@@ -210,6 +210,11 @@ func doStreamSnapshot(snap *pb.Snapshot, out pb.Worker_StreamSnapshotServer) err
 	// Use the default implementation. We no longer try to generate a rolled up posting list here.
 	// Instead, we just stream out all the versions as they are.
 	stream.KeyToList = nil
+	stream.SinceTs = snap.SinceTs
+	if snap.SinceTs == 0 {
+		// Do full table copy when streaming the entire data.
+		stream.FullCopy = true
+	}
 	stream.Send = func(buf *z.Buffer) error {
 		kvs := &pb.KVS{Data: buf.Bytes()}
 		return out.Send(kvs)