Add documentation for export (#308)

Signed-off-by: Peng Guanwen <pg999w@outlook.com>

content/docs/dev/reference/clients/java.md

@@ -11,4 +11,31 @@ This document, like our Java API, is still a work in progress. In the meantime,
 
 {{< warning >}}
 You should not use the Java client for production use until it is released.
-{{< /warning >}}
+{{< /warning >}}
+
+## Parse SST file
+
+```java
+import java.io.File;
+import java.util.Iterator;
+import java.util.List;
+
+import org.tikv.br.BackupDecoder;
+import org.tikv.br.BackupMetaDecoder;
+import org.tikv.br.SSTDecoder;
+import org.tikv.common.util.Pair;
+
+BackupMetaDecoder metaDecoder = BackupMetaDecoder.parse("/path/to/sst/backupmeta");
+BackupDecoder sstBackup = new BackupDecoder(metaDecoder.getBackupMeta());
+File folder = new File("/path/to/sst");
+File[] listOfFiles = folder.listFiles();
+
+for (File file : listOfFiles) {
+    if (file.getName().endsWith("sst")) {
+        SSTDecoder decoder = sstBackup.decodeSST(file.getAbsolutePath());
+        Iterator<Pair<ByteString, ByteString>> iter = decoder.getIterator();
+        // Do something with iter
+    }
+}
+```
+

content/docs/dev/tasks/export/introduction.md

@@ -0,0 +1,63 @@
+---
+title: Export
+description: Export data to SST
+menu:
+    "dev":
+        parent: Tasks
+---
+
+This document describes how to use [BR Raw KV backup](br-rawkv) capability to export KV data to remote storages as SST files.
+
+[br-rawkv]: https://docs.pingcap.com/tidb/stable/use-br-command-line-tool#restore-raw-kv-experimental-feature
+
+# Basic Usage
+
+```bash
+br backup raw --pd ⟨pd address⟩ \
+  -s ⟨storage url⟩ \
+  --start ⟨start key⟩ \
+  --end ⟨end key⟩ \
+  --format ⟨key format⟩ \
+  --ratelimit ⟨in MiB/s⟩
+```
+
+This will export all KV data in the range `[start key, end key)` to the specified storage in the format of SST.
+
+## Supported storage
+
+The storage URL support following schemes:
+
+| Service                                     | Scheme  | Example                             |
+|---------------------------------------------|---------|-------------------------------------|
+| Local filesystem, distributed on every node | local   | `local:///path/to/dest/`            |
+| Hadoop HDFS and other compatible services   | hdfs    | `hdfs:///prefix/of/dest/`           |
+| Amazon S3 and other compatible services     | s3      | `s3://bucket-name/prefix/of/dest/`  |
+| GCS                                         | gcs, gs | `gcs://bucket-name/prefix/of/dest/` |
+| Write to nowhere（for benchmark only）      | noop    | `noop://`                           |
+
+S3 and GCS can be configured using URL and command line parameters, see the BR documentation [External Storage](br-external-storage) for more information.
+
+[br-external-storage]: https://docs.pingcap.com/tidb/stable/backup-and-restore-storages
+
+### HDFS configuration
+
+To use HDFS storage, [Apache Hadoop](hdfs-client) or compatible client should be installed and currectly configured on all BR and TiKV machines. The `bin/hdfs` binary in hadoop installation will be used by BR and TiKV.
+
+Various configuration should be provided for HDFS storage to work, see the following table.
+
+| Component | Configuration                         | Environment variable | Configuration file item  |
+|-----------|---------------------------------------|----------------------|--------------------------|
+| BR        | hadoop installation directory         | HADOOP_HOME          | (None)                   |
+| TiKV      | hadoop installation directory         | HADOOP_HOME          | backup.hadoop.home       |
+| TiKV      | linux user to use when calling hadoop | HADOOP_LINUX_USER    | backup.hadoop.linux-user |
+
+For TiKV, the configuration file have higher priority than environment variables.
+
+[hdfs-client]: https://hadoop.apache.org/docs/stable/
+
+### Parse SST file
+
+#### Java Client
+
+The exported SST file can be parsed using [Java Client](../../../reference/clients/java).
+