[ https://issues.apache.org/jira/browse/HDFS-7285?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14279422#comment-14279422 ]
Tsz Wo Nicholas Sze commented on HDFS-7285: ------------------------------------------- Thanks for posting the design doc. It looks really nice! Some comments/questions: - Is a BlockGroup only used by one file, i.e. it cannot be shared by servel files? - Suppose the answer to the above question is yes. Then, how to encode small files? - HDFS-3107 "HDFS truncate" is now committed. We should revisit it. (EC with truncation is still a non-goal in the design doc.) - "Due to the complexity we will not support hflush or hsync for EC files at this phase." Then what will happen if users call hflush/hsync? > 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: HDFSErasureCodingDesign-20141028.pdf, > HDFSErasureCodingDesign-20141217.pdf, fsimage-analysis-20150105.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. -- This message was sent by Atlassian JIRA (v6.3.4#6332)