HDFS

Handling failures in Hadoop [フレーム]

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HDFS is designed to be a highly scalable storage system and sites at Facebook and Yahoo have 20PB size file systems in production deployments. The HDFS NameNode is the master of the Hadoop Distributed File System (HDFS). It maintains the critical data structures of the entire file system. Most of HDFS design has focussed on scalability of the system, i.e. the ability to support a large number of slave nodes in the cluster and an even larger number of files and blocks. However, a 20PB size cluster with 30K simultaneous clients requesting service from a single NameNode means that the NameNode has to run on a high-end non-commodity machine. There has been some efforts to scale the NameNode horizontally, i.e. allow the NameNode to run on multiple machines. I will defer analyzing those horizontal-scalability-efforts for a future blog post, instead let's discuss ways and means to make our singleton NameNode support an even greater load. [フレーム]

I have encountered plenty of questions about the single point of failure for the HDFS NameNode. The most common concern being that if the NameNode dies, then the whole cluster is unavailable. This means that HDFS is unsuitable for applications that need a high degree of uptime. This is not a problem when you run map-reduce jobs on HDFS, especially because a map-reduce system is a batch system and the uptime requirements of a batch system is typically not that stringent.

In recent times, I am starting to see lots of distributed applications that are using HDFS as a general purpose storage system. These applications range from multimedia servers that store mails, photos, videos, etc to systems that store updates from thousands of distributed sensors. These applications prefer HDFS because they can store a large volume of data. These applications do not use map-reduce to mine any of the information stored in HDFS. However, these applications do need consistency and availability of data. The Brewer's CAP Theorem states that most distributed systems need some tradeoffs among Consistency, Availability and Partition tolerance. HDFS's does an excellent job of providing Consistency of data at all times. Traditionally, it did not address Availability and Partition tolerance earlier. That could change to a certain extent with HDFS 0.21 release. HDFS 0.21 has a new entity called the BackupNode that receives real-time updates about transactions from the NameNode. This is the first step in making making HDFS highly available.

Here is a slide-deck that I wanted to present at ApacheCon 2009 about the current state of affairs regarding High Availabilty with HDFS. [フレーム]

I was invited to present a talk about Hadoop File System Architecture at Microsoft Research at Seattle. This is a research group and is focussed on long-term research, so it is no surprise that they are interested in knowing how a growing company like Facebook is using Hadoop to its advantage.

I met a few folks who chatted with me about how Microsoft SQL Server is being modified to handle large scala databases. These folks heartily agreed with a comment I made in my presentation that Dr. Dewitt and Dr. Stonebraker is missing the point when they are comparing performance numbers between Hadoop with traditional Database systems.... rather than comparing the scalability and fault-tolerance of these systems. I had learned some of the fundamentals of Database systems from Professor Dewitt during my graduate studies at Uiversity of Wisconsin Madison, but Dr Dewitt is a Microsoft employee now!

The fact that Facebook uses the SQL interface of Hive layered over Hadoop makes it even more interesting to Microsoft. They wanted to know the performance difference between Hive and PIG and would like to compare them to their distributed-SQL-Server software.

Here are the slides I used for my presentation. [フレーム]

I presented a set of slides that describes the Hadoop development at Facebook at the HadoopWorld conference in New York today. It was well received by more than 100 people. I have presented at many-a-conferences in the west coast but this is the first time I have presented at a conference in New York... there are more hadoop users here versus mostly hadoop developers in the west coast. There were plenty of questions, especially about Hadoop-Archive and Realtime-Hadoop. There were people asking me questions about HDFS Symbolic links and HDFS-scribe copier.

Earlier, I visited the university of Notre Dame to conduct a department seminar and present a guest lecture for the graduate students at the Department of Computer Science. There is plenty of interesting research being led by Prof Douglas Thain. One interesting research idea that came up was to place HDFS block replicas by analyzing HDFS access patterns. It is possible that we can provide HDFS datanode/namenode logs to researchers who can analyze these logs to come up with better algorithms for HDFS block replica placement. [フレーム]

Most Hadoop administrators set the default replication factor for their files to be three. The main assumption here is that if you keep three copies of the data, your data is safe. I have observed this to be true in the big clusters that we manage and operate. In actuality, administrators are managing two failure aspects: data corruption and data availability.

If all the datanodes on which the replicas of a block exist catch fire at the same time, then that data is lost and cannot be recovered. Or if an administrative error causes all the existing replicas of a block to be deleted, then it is a catastrophic failure. This is data corruption. On the other hand, if a rack switch goes down for sometime, the datanodes on that rack are in-accessible during that time. When that faulty rack switch is fixed, the data on the rack rejoins the HDFS cluster and life goes on as usual. This is a data avilability issue; in this case data was not corrupted or lost, it was just unavailable for some time. HDFS keeps three copies of a block on three different datanodes to protect against true data corruption. HDFS also tries to distribute these three replicas on more than one rack to protect against data availability issues. The fact that HDFS actively monitors any failed datanode(s) and upon failure detection immediately schedules re-replication of blocks (if needed) implies that three copies of data on three different nodes is sufficient to avoid corrupted files.

HDFS uses a simple but highly effective policy to allocate replicas for a block. If a process that is running on any of the HDFS cluster nodes open a file for writing a block, then one replica of that block is allocated on the same machine on which the client is running. The second replica is allocated on a randomly chosen rack that is different from the rack on which the first replica was allocated. The third replica is allocated on a randomly chosen machine on the same remote rack that was chosen in the earlier step. This means that a block is present on two unique racks. One point to note is that there is no relationship between replicas of different blocks of the same file as far as their location is concerned. Each block is allocated independently.

The above algorithm is great for availability and scalability. However, there are scenarios where co-locating many block of the same file on the same set of datanode(s) or rack(s) is beneficial for performance reasons. For example, if many blocks of the same file are present on the same datanode(s), a single mapper instance could process all these blocks using the CombineFileInputFormat. Similarly, if a dataset contains many small files that are co-located on the same datanode(s) or rack(s), one can use CombineFileInputFormat to process all these file together by using fewer mapper instances via CombineFileInputFormat. If an application always uses one dataset with another dataset (think Hive or Pig join), then co-locating these two datasets on the same set of datanodes is beneficial.

Another reason when one might want to allocate replicas using a different policy is to ensure that replicas and their parity blocks truly reside in different failure domains. The erasure code work in HDFS could effectively bring down the physical replication factor of a file to about 1.5 (while keeping the logical replication factor at 3) if it can place replicas of all blocks in a stripe more intelligently.

Yet another reason, however exotic, is to allow HDFS to place replicas based on the HeatMap of your cluster. If one of of the node in the cluster is at a higher temperature than that of another, then it might be better to prefer the cooler node while allocating a new replica. If you want to experiment with HDFS across two data centers, you might want to try out new policies for replica placement.

Well, now you can finally get your hands wet! HDFS-385 is part of the Hadoop trunk and will be part of the next major HDFS 0.21 release. This feature provides a way for the adventurous developer to write Java code that specifies how HDFS should allocate replicas of blocks of a file. The API is experimental in nature, and could change in the near future if we discover any in-efficiencies in it. Please let the Hadoop community know if you need any changes in this API or if you come across novel uses of this API. [フレーム]

The Hadoop Distributed File System has been great in providing a cloud-type file system. It is robust (when administered correctly :-)) and highly scalable. However, one of the main drawbacks of HDFS is that each piece of data is replicated in three places. This is acceptable because disk storage is cheap and is becoming cheaper by the day; this isn't a problem if you have a relatively small to medium size cluster. The price difference (in absolute terms) is not much whether you use 15 disks or whether you use 10 disks. If we consider the cost of 1ドル per GByte, the price difference between fifteen 1 TB disk and ten 1 TB disk is only 5ドルK. But when the total size of your cluster is 10 PBytes, then the costs savings in storing the data in two places versus three is a huge ten million dollars!

The reason HDFS stores disk blocks in triplicate is because it uses commodity hardware and there is non-negligible probability of a disk failure. It has been observed that a replication factor of 3 and the fact the HDFS aggressively detects failures and immediately replicates failed -block-replicas is sufficient to never lose any data in practice. The challenge now is to achieve an effective replication factor of 3 while keeping the real physical replication factor at close to 2! How best to do it than by using Erasure Codes.

I heard about this idea called DiskReduce from the folks at CMU. The CMU PDL Labs has been a powerhouse of research in file systems and it is no surprise that they proposed a elegant way of implementing erasure codes in HDFS. I borrowed heavily from their idea in my implementation of Erasure Codes in HDFS described in HDFS-503. One of the main motivation of my design is to keep the HDFS Erasure Coding as a software layer above HDFS rather than inter-twining it inside of HDFS code. The HDFS code is complex by itself and it is really nice to not have to make it more complex and heavyweight.

Distributed Raid File System consists of two main software components. The first component is the RaidNode, a daemon that creates parity files from specified HDFS files. The second component "raidfs" is a software that is layered over a HDFS client and it intercepts all calls that an application makes to the HDFS client. If the HDFS client encounters corrupted data while reading a file, the raidfs client detects it; it uses the relevant parity blocks to recover the corrupted data (if possible) and returns the data to the application. The application is completely transparent to the fact that parity data was used to satisfy it's read request. The Distributed Raid File System can be configured in such a way that a set of data blocks of a file are combined together to form one or more parity blocks. This allows one to reduce the replication factor of a HDFS file from 3 to 2 while keeping the failure probabilty relatively same as before.

I have seen that using a stripe size of 10 blocks decreases the physical replication factor of a file to 2.2 while keeping the effective replication factor of a file at 3. This typically results in saving 25% to 30% of storage space in a HDFS cluster.

One of the shortcoming of this implementation is that we need a parity file for every file in HDFS. This potentially increases the number of files in the NameNode. To alleviate this problem, I will enhance this implementation (in future) to use the Hadoop Archive feature to archive all the parity files together in larger containers so that the NameNode does not have to support additional files when the HDFS Erasure Coding is switched on. This works reasonably well because it is a very very rare case that the parity files are ever used to satisfy a read request.

I am hoping that this feature becomes part of Hadoop 0.21 release scheduled for September 2009! [フレーム]

It is finally here: you can configure the open source log-aggregator, scribe, to log data directly into the Hadoop distributed file system. [フレーム]

For quite a while, I have been thinking on blogging about Hadoop in general and Hadoop distributed file system (HDFS) in particular. Why, you may ask?

Firstly, I have been contacted by students from as far as Bangladesh and Fiji asking me questions about HDFS via email. This made me think that disseminating internal details about HDFS to the whole wide world would really benefit a lot of people. I like to interact with these budding engineers; and their questions, though elementary in nature, sometimes makes me really ruminate on why we adopted a particular design and not another. I will sprinkle a few of these examples next week.

Secondly, I visited a few Universities last month, among them Carnegie Mellon University and my alma-mater Univ of Wisconsin. On my flight, I was getting bored to death, because I really did not like the movie that was playing and I did not carry any material to read. (Usually I like to read and re-read Sherlock Holmes over and over again.) But like they say, " an idle mind is the devil's workhop".... I started to jot down some exotic design ideas about HDFS.... And lo behold, I have a list of ideas that I would like to share! I will post them next week as well. [フレーム]