diff --git a/models/actions/task.go b/models/actions/task.go
index d7590407e7..e671a06079 100644
--- a/models/actions/task.go
+++ b/models/actions/task.go
@@ -50,7 +50,7 @@ type ActionTask struct {
 	LogInStorage bool        // read log from database or from storage
 	LogLength    int64       // lines count
 	LogSize      int64       // blob size
-	LogIndexes   *LogIndexes `xorm:"BLOB"` // line number to offset
+	LogIndexes   *LogIndexes `xorm:"LONGBLOB"` // line number to offset
 	LogExpired   bool        // files that are too old will be deleted
 
 	Created timeutil.TimeStamp `xorm:"created"`
diff --git a/models/actions/utils.go b/models/actions/utils.go
index c89628a6c8..5498742443 100644
--- a/models/actions/utils.go
+++ b/models/actions/utils.go
@@ -29,6 +29,20 @@ func generateSaltedToken() (string, string, string, string, error) {
 	return token, salt, hash, token[:8], nil
 }
 
+/*
+LogIndexes is the index for mapping log line number to buffer offset.
+Because it uses varint encoding, it is impossible to predict its size.
+But we can make a simple estimate with an assumption that each log line has 200 byte, then:
+| lines     | file size           | index size         |
+|-----------|---------------------|--------------------|
+| 100       | 20 KiB(20000)       | 258 B(258)         |
+| 1000      | 195 KiB(200000)     | 2.9 KiB(2958)      |
+| 10000     | 1.9 MiB(2000000)    | 34 KiB(34715)      |
+| 100000    | 19 MiB(20000000)    | 386 KiB(394715)    |
+| 1000000   | 191 MiB(200000000)  | 4.1 MiB(4323626)   |
+| 10000000  | 1.9 GiB(2000000000) | 47 MiB(49323626)   |
+| 100000000 | 19 GiB(20000000000) | 490 MiB(513424280) |
+*/
 type LogIndexes []int64
 
 func (indexes *LogIndexes) FromDB(b []byte) error {
diff --git a/models/migrations/v-dev.go b/models/migrations/v-dev.go
index 26065238c0..408e59df0f 100644
--- a/models/migrations/v-dev.go
+++ b/models/migrations/v-dev.go
@@ -121,7 +121,7 @@ func addActionsTables(x *xorm.Engine) error {
 		LogInStorage bool     // read log from database or from storage
 		LogLength    int64    // lines count
 		LogSize      int64    // blob size
-		LogIndexes   *[]int64 `xorm:"BLOB"` // line number to offset
+		LogIndexes   *[]int64 `xorm:"LONGBLOB"` // line number to offset
 		LogExpired   bool     // files that are too old will be deleted
 
 		Created timeutil.TimeStamp `xorm:"created"`