Compress the state response to reduce the state sync data transfer

[liuchengxu]

Is there an existing issue?

• I have searched the existing issues

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• This is not a support question.

Motivation

The state syncing could download several GiB of data if the state size of the chain is huge, which is not uncommon nowadays.
This poses a significant challenge for nodes with slow network connections. Additionally, since state sync currently lacks a persistence feature (#4), any network disruption forces the node to re-download the entire state, which is annoying.

Request

Reduce the state syncing download size.

Solution

Compress the state response before sending it to the node and uncompress the state response on the receiver side.

diff --git a/substrate/client/network/sync/Cargo.toml b/substrate/client/network/sync/Cargo.toml
index 17e3e2119d..047fffa31f 100644
--- a/substrate/client/network/sync/Cargo.toml
+++ b/substrate/client/network/sync/Cargo.toml
@@ -48,6 +48,7 @@ sp-consensus = { workspace = true, default-features = true }
 sp-core = { workspace = true, default-features = true }
 sp-consensus-grandpa = { workspace = true, default-features = true }
 sp-runtime = { workspace = true, default-features = true }
+zstd = { workspace = true }

 [dev-dependencies]
 mockall = { workspace = true }
diff --git a/substrate/client/network/sync/src/engine.rs b/substrate/client/network/sync/src/engine.rs
index bb6e7a98a8..3915d3845e 100644
--- a/substrate/client/network/sync/src/engine.rs
+++ b/substrate/client/network/sync/src/engine.rs
@@ -1204,7 +1204,10 @@ where
        }

        fn decode_state_response(response: &[u8]) -> Result<OpaqueStateResponse, String> {
-               let response = StateResponse::decode(response)
+               let response = zstd::stream::decode_all(response).expect("Failed to uncompress state response");
+               let response = StateResponse::decode(response.as_slice())
                        .map_err(|error| format!("Failed to decode state response: {error}"))?;

                Ok(OpaqueStateResponse(Box::new(response)))
diff --git a/substrate/client/network/sync/src/state_request_handler.rs b/substrate/client/network/sync/src/state_request_handler.rs
index 0e713626ec..bb07bdd9bc 100644
--- a/substrate/client/network/sync/src/state_request_handler.rs
+++ b/substrate/client/network/sync/src/state_request_handler.rs
@@ -264,7 +272,15 @@ where

                        let mut data = Vec::with_capacity(response.encoded_len());
                        response.encode(&mut data)?;
-                       Ok(data)
+                       let compressed_data = zstd::stream::encode_all(data.as_slice(), 0).expect("Failed to compress state response");
+                       Ok(compressed_data)
                } else {
                        Err(())
                };

This is a low-hanging fruit that can reduce the state sync data significantly as demonstrated by my local experiments. I conducted state sync tests at various block heights (before height 300000) using both the fast and fast-unsafe modes for subcoin, the Uncompressed Total State Sync Data is calculated as sum(data.len()), the Compressed Total State Sync Data is calculated as sum(compressed_data.len()). The results are promising, indicating that several GiB of state sync data can be saved, especially when dealing with large chain states. The final state size of subcoin may be 12+GiB, this optimization will greatly help the state sync of subcoin.

--sync	Uncompressed Total State Sync Data (bytes)	Compressed Total State Sync Data (bytes)	Compressed/Uncompressed
fast-unsafe	149,517,161	50,284,623	0.34
fast-unsafe	205,400,559	70,742,393	0.34
fast-unsafe	597,683,313	202,993,329	0.34
fast-unsafe	1,239,830,694	480,632,754	0.39
fast-unsafe	2,182,810,408	841,870,855	0.39
fast	820,180,264	338,889,711	0.41
fast	1,486,307,891	631,430,018	0.42

We can make this configurable if necessary.

Are you willing to help with this request?

Yes!

[burdges]

It's odd this data is compressible, well maybe our underlying formats needs some redesign there.

We've a few places where it'd be nice if we used libtorrent or some rust rewrite.

[liuchengxu]

I'm not surprised that the data is compressible as the state response is produced by reading the state sequentially, which means many entries share the same storage prefix.

[bkchr]

I think the idea is good, if it works like you say. Would be really nice if you can provide some numbers for Polkadot as well.

This would require a RFC, because we need to change the networking messages.

[liuchengxu]

I only ran the node for a while and didn't sync to the recent blocks, here are the numbers for Polkadot:

--sync	Block Height	Compressed Data Size	Uncompressed Data Size	Compressed/ Uncompressed
warp	442,867	2,029,088	6,601,871	0.31
warp	850,945	3,448,748	12,578,916	0.27
warp	1,191,454	5,997,874	21,447,825	0.28
warp	1,508,438	9,133,749	28,606,326	0.32
warp	1,857,024	17,016,693	51,222,672	0.33

UPD:
The result of warp sync to the recent blocks of Polkadot: Compressed/Uncompressed = 351179285 / 528800495 = 0.66, we can save 169 MiB out of 504MiB if compressing the state response message.

2024-08-13 22:25:09 ⚙️  State sync, Downloading state, 84%, 676.58 Mib (7 peers), best: #0 (0x91b1…90c3), finalized #0 (0x91b1…90c3), ⬇ 745.1kiB/s ⬆ 2.4kiB/s
2024-08-13 22:25:09 ================ total_bytes_compressed: 349292448, total_bytes_uncompressed: 526850326
2024-08-13 22:25:12 ================ total_bytes_compressed: 351179285, total_bytes_uncompressed: 528800495
2024-08-13 22:25:14 ⚙️  State sync, Importing state, 84%, 680.50 Mib (8 peers), best: #0 (0x91b1…90c3), finalized #0 (0x91b1…90c3), ⬇ 461.8kiB/s ⬆ 1.3kiB/s
2024-08-13 22:25:19 ⚙️  State sync, Importing state, 84%, 680.50 Mib (8 peers), best: #0 (0x91b1…90c3), finalized #0 (0x91b1…90c3), ⬇ 69.9kiB/s ⬆ 5.1kiB/s
2024-08-13 22:25:24 ⚙️  State sync, Importing state, 84%, 680.50 Mib (8 peers), best: #0 (0x91b1…90c3), finalized #0 (0x91b1…90c3), ⬇ 63.6kiB/s ⬆ 4.6kiB/s
2024-08-13 22:25:29 ⚙️  State sync, Importing state, 84%, 680.50 Mib (8 peers), best: #0 (0x91b1…90c3), finalized #0 (0x91b1…90c3), ⬇ 57.3kiB/s ⬆ 1.9kiB/s
2024-08-13 22:25:34 ⚙️  State sync, Importing state, 84%, 680.50 Mib (9 peers), best: #0 (0x91b1…90c3), finalized #0 (0x91b1…90c3), ⬇ 55.5kiB/s ⬆ 1.1kiB/s
2024-08-13 22:25:39 ⚙️  State sync, Importing state, 84%, 680.50 Mib (9 peers), best: #0 (0x91b1…90c3), finalized #0 (0x91b1…90c3), ⬇ 59.8kiB/s ⬆ 3.3kiB/s
2024-08-13 22:25:44 ⚙️  State sync, Importing state, 84%, 680.50 Mib (9 peers), best: #0 (0x91b1…90c3), finalized #0 (0x91b1…90c3), ⬇ 89.0kiB/s ⬆ 5.7kiB/s
2024-08-13 22:25:49 ⚙️  State sync, Importing state, 84%, 680.50 Mib (9 peers), best: #0 (0x91b1…90c3), finalized #0 (0x91b1…90c3), ⬇ 59.6kiB/s ⬆ 0.7kiB/s
2024-08-13 22:25:54 ⚙️  State sync, Importing state, 84%, 680.50 Mib (9 peers), best: #0 (0x91b1…90c3), finalized #0 (0x91b1…90c3), ⬇ 26.9kiB/s ⬆ 1.2kiB/s
2024-08-13 22:25:59 ⚙️  State sync, Importing state, 84%, 680.50 Mib (9 peers), best: #0 (0x91b1…90c3), finalized #0 (0x91b1…90c3), ⬇ 46.1kiB/s ⬆ 2.3kiB/s
2024-08-13 22:26:01 State sync is complete, continuing with block sync.

total_bytes_compressed is calculated as sum(compressed_size), total_bytes_uncompressed is calculated as sum(uncompressed_size).

diff --git a/substrate/client/network/sync/src/engine.rs b/substrate/client/network/sync/src/engine.rs
index ee7576c22f1..a7ab4a92385 100644
--- a/substrate/client/network/sync/src/engine.rs
+++ b/substrate/client/network/sync/src/engine.rs
@@ -1207,11 +1212,14 @@ where
                Ok(request.encode_to_vec())
        }

-       fn decode_state_response(response: &[u8]) -> Result<OpaqueStateResponse, String> {
+       fn decode_state_response(response: &[u8]) -> Result<(usize, usize, OpaqueStateResponse), String> {
+               let compressed_data = zstd::stream::encode_all(response, 0).expect("Failed to compress state response");
+               let compressed_size = compressed_data.len();
+               let uncompressed_size = response.len();
                let response = StateResponse::decode(response)
                        .map_err(|error| format!("Failed to decode state response: {error}"))?;

-               Ok(OpaqueStateResponse(Box::new(response)))
+               Ok((compressed_size, uncompressed_size, OpaqueStateResponse(Box::new(response))))
        }

[liuchengxu]

Opened RFC polkadot-fellows/RFCs#112