为提高效率，提问时请提供以下信息，问题描述清晰可优先响应。
- 【TiDB 版本】：TiDB-v3.1.0-beta.1
- 【问题描述】：使用br进行备份测试的时候 发现备份完之后 备份的目录为空 并且日志有报错信息
[2020/03/09 17:07:25.162 +08:00] [ERROR] [client.go:740] [“failed in fast checksum”] [database=sbtest] [table=sbtest9] [“origin tidb crc64”=685805977730786940] [“calculated crc64”
=0] [“origin tidb total kvs”=2000080] [“calculated total kvs”=0] [“origin tidb total bytes”=250010000] [“calculated total bytes”=0] [stack=“github.com/pingcap/log.Error\
\t/home/j
enkins/agent/workspace/release_tidb_3.1/go/pkg/mod/github.com/pingcap/log@v0.0.0-20190715063458-479153f07ebd/global.go:42\ngithub.com/pingcap/br/pkg/backup.(*Client).FastChecksum
n\t/home/jenkins/agent/workspace/release_tidb_3.1/go/src/github.com/pingcap/br/pkg/backup/client.go:740\ngithub.com/pingcap/br/cmd.runBackup\
\t/home/jenkins/agent/workspace/relea
se_tidb_3.1/go/src/github.com/pingcap/br/cmd/backup.go:140\ngithub.com/pingcap/br/cmd.newDbBackupCommand.func1\
\t/home/jenkins/agent/workspace/release_tidb_3.1/go/src/github.com/
pingcap/br/cmd/backup.go:215\ngithub.com/spf13/cobra.(*Command).execute\
\t/home/jenkins/agent/workspace/release_tidb_3.1/go/pkg/mod/github.com/spf13/cobra@v0.0.5/command.go:826\
github.com/spf13/cobra.(*Command).ExecuteC\
\t/home/jenkins/agent/workspace/release_tidb_3.1/go/pkg/mod/github.com/spf13/cobra@v0.0.5/command.go:914\ngithub.com/spf13/cobra.(*Comm
and).Execute\
\t/home/jenkins/agent/workspace/release_tidb_3.1/go/pkg/mod/github.com/spf13/cobra@v0.0.5/command.go:864\
main.main\
\t/home/jenkins/agent/workspace/release_tidb_3.1
/go/src/github.com/pingcap/br/main.go:56\
runtime.main\
\t/usr/local/go/src/runtime/proc.go:203”]
[2020/03/09 17:07:25.163 +08:00] [ERROR] [backup.go:145] [“backup FastChecksum failed!”] [stack=“github.com/pingcap/log.Error\
\t/home/jenkins/agent/workspace/release_tidb_3.1/go/
pkg/mod/github.com/pingcap/log@v0.0.0-20190715063458-479153f07ebd/global.go:42\ngithub.com/pingcap/br/cmd.runBackup\
\t/home/jenkins/agent/workspace/release_tidb_3.1/go/src/github
.com/pingcap/br/cmd/backup.go:145\ngithub.com/pingcap/br/cmd.newDbBackupCommand.func1\
\t/home/jenkins/agent/workspace/release_tidb_3.1/go/src/github.com/pingcap/br/cmd/backup.go:
215\ngithub.com/spf13/cobra.(Command).execute\
\t/home/jenkins/agent/workspace/release_tidb_3.1/go/pkg/mod/github.com/spf13/cobra@v0.0.5/command.go:826\ngithub.com/spf13/cobra.(
Command).ExecuteC\
\t/home/jenkins/agent/workspace/release_tidb_3.1/go/pkg/mod/github.com/spf13/cobra@v0.0.5/command.go:914\ngithub.com/spf13/cobra.(*Command).Execute\
\t/home/jen
kins/agent/workspace/release_tidb_3.1/go/pkg/mod/github.com/spf13/cobra@v0.0.5/command.go:864\
main.main\
\t/home/jenkins/agent/workspace/release_tidb_3.1/go/src/github.com/pingca
p/br/main.go:56\
runtime.main\
\t/usr/local/go/src/runtime/proc.go:203”]
