momo-jun commented on code in PR #15239:
URL: https://github.com/apache/pulsar/pull/15239#discussion_r860551436
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site2/docs/tiered-storage-filesystem.md:
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@@ -520,4 +520,66 @@ Execute the following commands in the repository where you
download Pulsar tarba
And the **Capacity Used** is changed from 4 KB to 116.46 KB.
- 
\ No newline at end of file
+ 
+
+## Read offloaded data from fileSystem
+
+This section provides detailed instructions about how to read data out as
Ledger Entry in the file system.
+
+* The data was offloaded as `MapFile` to the following path:
+ ```properties
+ path = storageBasePath + "/" + managedLedgerName + "/" + ledgerId + "-" +
uuid.toString();
+ ```
+ 1. storageBasePath is the value of `hadoop.tmp.dir`, configured in
`broker.conf` or `filesystem_offload_core_site.xml`
+ 2. managedLedgerName is the name of the persistentTopic manager Ledger
+ ```shell
+ managedLedgerName of persistent://public/default/topics-name is
public/default/persistent/topics-name.
+ ```
+ Can use the following method to get the managedLedgerName:
+ ```shell
+ String managedLedgerName =
TopicName.get("persistent://public/default/topics-name").getPersistenceNamingEncoding();
+ ```
+
+* Create a reader to read `MapFile` according to the above path and the
`configuration` of the file system
+ ```shell
+ MapFile.Reader reader = new MapFile.Reader(new Path(dataFilePath),
configuration);
+ ```
+* Read data as `LedgerEntry` from FileSystem.
+ ```java
+ LongWritable key = new LongWritable();
+ BytesWritable value = new BytesWritable();
+ key.set(nextExpectedId - 1);
+ reader.seek(key);
+ reader.next(key, value);
+ int length = value.getLength();
+ long entryId = key.get();
+ ByteBuf buf = PooledByteBufAllocator.DEFAULT.buffer(length, length);
+ buf.writeBytes(value.copyBytes());
+ LedgerEntryImpl ledgerEntry = LedgerEntryImpl.create(ledgerId, entryId,
length, buf);
+ ```
+* Deserialize the `ledgerEntry` to `Message`.
Review Comment:
```suggestion
3. Deserialize the `ledgerEntry` to `Message`.
```
Which one is correct? `LedgerEntry` or `ledgerEntry`? The difference between
step2 and step3 may make users confused.
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