diff --git a/src/backup/sync.js b/src/backup/sync.js
index fae2321..a4e214b 100644
--- a/src/backup/sync.js
+++ b/src/backup/sync.js
@@ -7,7 +7,7 @@ const TYPE_ = require('./message_types.json');
 const { _list, packet_, _nextNode } = require("./values");
 const { SYNC_WAIT_TIME } = require('../_constants')['backup'];
 
-const SESSION_GROUP_CONCAT_MAX_LENGTH = 100000 //MySQL default max value is 1024, which is too low for block grouping
+//const SESSION_GROUP_CONCAT_MAX_LENGTH = 100000 //MySQL default max value is 1024, which is too low for block grouping
 
 const col_aggregate = (function () {
     let ds = require("../data_structure.json");
@@ -28,7 +28,7 @@ const _x = {
         return `${H_struct.VECTOR_CLOCK} > '${lower_vc}' AND ${H_struct.VECTOR_CLOCK} < '${upper_vc}'`;
     },
     get hash_algo_sql() {
-        return `MD5(GROUP_CONCAT(${col_aggregate}))`;
+        return `SUM(CRC32(MD5(${col_aggregate})))`;
     },
     t_name(node_i) {
         return '_' + node_i;
@@ -113,13 +113,13 @@ const queueSync = {
     }
 };
 
-function setSessionVar() {
+/*function setSessionVar() {
     return new Promise((resolve, reject) => {
         Database.query(`SET SESSION group_concat_max_len = ${SESSION_GROUP_CONCAT_MAX_LENGTH}`)
             .then(result => resolve(result))
             .catch(error => reject(error))
     })
-}
+}*/
 
 //R: request hashes for node_i
 function requestDataSync(node_i, ws) {
@@ -131,27 +131,27 @@ function sendBlockHashes(node_i, ws) {
     if (!_list.stored.includes(node_i))
         return console.debug(`Block hash is requested for ${node_i}, but its not in stored list`);
     let t_name = _x.t_name(node_i);
-    setSessionVar().then(_ => {
-        let statement = `SELECT ${_x.block_calc_sql} AS block_n, ${_x.hash_algo_sql} as hash`
-            + ` FROM ${t_name} GROUP BY block_n ORDER BY block_n`;
-        let last_block;
-        Database.query_stream(statement, r => {
-            ws.send(packet_.construct({
-                type_: TYPE_.RES_HASH,
-                node_i: node_i,
-                block_n: r.block_n,
-                hash: r.hash
-            }));
-            last_block = r.block_n;
-        }).then(result => {
-            console.debug(`Sent ${result} block hashes`);
-            ws.send(packet_.construct({
-                type_: TYPE_.VAL_LAST_BLK,
-                node_i: node_i,
-                block_n: last_block,
-            }));
-        }).catch(error => console.error(error))
-    }).catch(error => console.error(error));
+    //setSessionVar().then(_ => {
+    let statement = `SELECT ${_x.block_calc_sql} AS block_n, ${_x.hash_algo_sql} as hash`
+        + ` FROM ${t_name} GROUP BY block_n ORDER BY block_n`;
+    let last_block;
+    Database.query_stream(statement, r => {
+        ws.send(packet_.construct({
+            type_: TYPE_.RES_HASH,
+            node_i: node_i,
+            block_n: r.block_n,
+            hash: r.hash
+        }));
+        last_block = r.block_n;
+    }).then(result => {
+        console.debug(`Sent ${result} block hashes`);
+        ws.send(packet_.construct({
+            type_: TYPE_.VAL_LAST_BLK,
+            node_i: node_i,
+            block_n: last_block,
+        }));
+    }).catch(error => console.error(error))
+    //}).catch(error => console.error(error));
 }
 
 //R: check hash and request data if hash mismatch
@@ -165,20 +165,20 @@ function checkBlockHash(node_i, block_n, hash) {
 function verifyBlockHash(node_i, block_n, hash) {
     return new Promise((resolve, reject) => {
         let t_name = _x.t_name(node_i);
-        setSessionVar().then(_ => {
-            let statement = `SELECT ${_x.hash_algo_sql} as hash`
-                + ` FROM ${t_name} WHERE ${_x.block_calc_sql}  = ?`;
-            Database.query(statement, [block_n]).then(result => {
-                if (!result.length || result[0].hash != hash) { //Hash mismatch
-                    //ReSync Block
-                    let clear_statement = `DELETE FROM ${t_name} WHERE ${_x.block_range_sql(block_n)}`;
-                    Database.query(clear_statement)
-                        .then(_ => resolve(false))
-                        .catch(error => reject(error))
-                } else //Hash is verified
-                    resolve(true);
-            }).catch(error => reject(error))
+        //setSessionVar().then(_ => {
+        let statement = `SELECT ${_x.hash_algo_sql} as hash`
+            + ` FROM ${t_name} WHERE ${_x.block_calc_sql}  = ?`;
+        Database.query(statement, [block_n]).then(result => {
+            if (!result.length || result[0].hash != hash) { //Hash mismatch
+                //ReSync Block
+                let clear_statement = `DELETE FROM ${t_name} WHERE ${_x.block_range_sql(block_n)}`;
+                Database.query(clear_statement)
+                    .then(_ => resolve(false))
+                    .catch(error => reject(error))
+            } else //Hash is verified
+                resolve(true);
         }).catch(error => reject(error))
+        //}).catch(error => reject(error))
     })
 }