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From "Walter Su (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-7285) Erasure Coding Support inside HDFS
Date Thu, 20 Aug 2015 13:40:46 GMT

    [ https://issues.apache.org/jira/browse/HDFS-7285?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14704910#comment-14704910

Walter Su commented on HDFS-7285:

Comments on HDFS-7285-merge branch.
1. Unused import.
+import com.google.common.base.Preconditions; //DFSInputStream.java
+import org.apache.commons.logging.Log;  //DFSOutputStream.java
+import org.apache.commons.logging.LogFactory;

3. Please replace it with {{stat.getErasureCodingPolicy()==null}}
+    if(stat.getReplication() == 0) {  //DFSOutputStream.java
+      throw new IOException("Not support appending to a striping layout file yet.");
+    }

4. redundant space character.
-public abstract class BlockInfo extends Block  //BlockInfo.java
+public abstract class  BlockInfo extends Block

5. indent
-  private LocatedBlock createLocatedBlock(final BlockInfo blk, final long pos) {  //BlockManager.java
+  private LocatedBlock createLocatedBlock(final BlockInfo blk, final long pos
+  ) throws IOException {

6. truncation is file level. Maybe assert !file.isStriped() is less confusing? And also {{createNewBlock(false)}}
can be less confusing.
+     BlockInfo oldBlock = file.getLastBlock();  //FSDirTruncateOp.java
++    assert !oldBlock.isStriped();
++          fsn.createNewBlock(file.isStriped()) : new Block(

7. unnecessary change
-    fsn.getBlockManager().setBlockToken(lBlk, //FSDirWriteFileOp.java
-        BlockTokenIdentifier.AccessMode.WRITE);
+    fsn.getFSDirectory().getBlockManager().
+        setBlockToken(lBlk, BlockTokenIdentifier.AccessMode.WRITE);
8. unnecessary change. (wiki says using [Sun's conventions|http://www.oracle.com/technetwork/java/javase/documentation/codeconventions-136091.html],
which says "Break before an operator")
 -          if (blockInfo.getBlockCollection().getStoragePolicyID()  //FSNamesystem.java
 -              == lpPolicy.getId()) {  // feature branch
 +          BlockInfo blockInfo = getStoredBlock(b);
-           if (blockInfo.getBlockCollection().getStoragePolicyID() == lpPolicy.getId()) {
 // trunk
++          if (blockInfo.getBlockCollection().getStoragePolicyID() ==
++              lpPolicy.getId()) {
Thanks [~zhz] for the great work!

> Erasure Coding Support inside HDFS
> ----------------------------------
>                 Key: HDFS-7285
>                 URL: https://issues.apache.org/jira/browse/HDFS-7285
>             Project: Hadoop HDFS
>          Issue Type: New Feature
>            Reporter: Weihua Jiang
>            Assignee: Zhe Zhang
>         Attachments: Consolidated-20150707.patch, Consolidated-20150806.patch, Consolidated-20150810.patch,
ECAnalyzer.py, ECParser.py, HDFS-7285-initial-PoC.patch, HDFS-7285-merge-consolidated-01.patch,
HDFS-7285-merge-consolidated-trunk-01.patch, HDFS-7285-merge-consolidated.trunk.03.patch,
HDFS-7285-merge-consolidated.trunk.04.patch, HDFS-EC-Merge-PoC-20150624.patch, HDFS-EC-merge-consolidated-01.patch,
HDFS-bistriped.patch, HDFSErasureCodingDesign-20141028.pdf, HDFSErasureCodingDesign-20141217.pdf,
HDFSErasureCodingDesign-20150204.pdf, HDFSErasureCodingDesign-20150206.pdf, HDFSErasureCodingPhaseITestPlan.pdf,
> Erasure Coding (EC) can greatly reduce the storage overhead without sacrifice of data
reliability, comparing to the existing HDFS 3-replica approach. For example, if we use a 10+4
Reed Solomon coding, we can allow loss of 4 blocks, with storage overhead only being 40%.
This makes EC a quite attractive alternative for big data storage, particularly for cold data.

> Facebook had a related open source project called HDFS-RAID. It used to be one of the
contribute packages in HDFS but had been removed since Hadoop 2.0 for maintain reason. The
drawbacks are: 1) it is on top of HDFS and depends on MapReduce to do encoding and decoding
tasks; 2) it can only be used for cold files that are intended not to be appended anymore;
3) the pure Java EC coding implementation is extremely slow in practical use. Due to these,
it might not be a good idea to just bring HDFS-RAID back.
> We (Intel and Cloudera) are working on a design to build EC into HDFS that gets rid of
any external dependencies, makes it self-contained and independently maintained. This design
lays the EC feature on the storage type support and considers compatible with existing HDFS
features like caching, snapshot, encryption, high availability and etc. This design will also
support different EC coding schemes, implementations and policies for different deployment
scenarios. By utilizing advanced libraries (e.g. Intel ISA-L library), an implementation can
greatly improve the performance of EC encoding/decoding and makes the EC solution even more
attractive. We will post the design document soon. 

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