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A High Availability story for HDFS
Dhruba Borthakur & Dmytro Molkov
firstname.lastname@example.org & email@example.com
Presented at The Hadoop User Group Meeting,
Sept 29, 2010
How infrequently does the NameNode (NN) stop?
Hadoop Software Bugs
– Two directories in fs.name.dir, but when a write to first
directory failed, the NN ignored the second one (once)
– Upgrade from 0.17 to 0.18 caused data corruption
– Fsimage partition ran out of space (once)
– Network Load Anomalies (about 10 times)
– Deploy new patches (once every month)
What does the SecondaryNameNode do?
Periodically merges Transaction logs
Requires the same amount of memory as NN
Why is it separate from NN?
– Avoids fine-grain locking of NN data structures
– Avoids implementing copy-on-write for NN data
Renamed as CheckpointNode (CN) in 0.21 release.
Shortcomings of the SecondaryNameNode?
Does not have a copy of the latest transaction log
Periodic and is not continuous
– Configured to run every hour
If the NN dies, the SecondaryNameNode does not take
over the responsibilities of the NN
NN streams transaction log to BackupNode
BackupNode applies log to in-memory and disk image
BN always commit to disk before success to NN
If BN restarts, it has to catch up with NN
Available in HDFS 0.21 release
Limitations of BackupNode (BN)
Maximum of one BackupNode per NN
– Support only two-machine failure
NN does not forward block reports to BackupNode
Time to restart from 12 GB image, 70M files + 100 M
– 3 – 5 minutes to read the image from disk
– 20 min to process block reports
– BN will still take 25+ minutes to failover!
Overlapping Clusters for HA
“Always available for write” model
Two logical clusters each with their own NN
Each physical machine runs two instances of DataNode
Two DataNode instances share the same physical storage
Application has logic to failover writes from one HDFS
cluster to another
More details at http://hadoopblog.blogspot.com/2009/06/hdfs-
HDFS can store transaction logs in Zookeeper/Bookeeper
Transaction log need not be stored in NFS filer
A new NN will still have to process block reports
– Not good for HA yet, because NN failover will take 30 minutes
Our use case for High Availability
Failover should occur in less than a minute
Failovers are needed only for new software upgrades
DataNodes send block location
information to only one
NameNode needs block locations
in memory to serve clients
The in-memory metadata for 100
million files could be 60 GB,
message “yes, I
have blockid 123”
block location from
Introduction for AvatarNode
– Coordinated via zookeeper
– Failover in few seconds
– Wrapper over NameNode
– Writes transaction log to filer
– Reads transactions from filer
– Latest metadata in memory
block location from
Primary or Standby
ZooKeeper integration for Clients
– Connects to ZooKeeper to figure out who the Primary node is.
There is a znode in ZooKeeper that has the current address of the
primary. Clients read it on creation and during failover.
– Is aware of the failover state and pauses until it is over. If the
znode is empty the cluster is failing over to the new Primary, just
wait for that to finish.
– Handles failures in calls to the NameNode. If the call failed with
network exception – checks for the failover in progress and retries
the call after the new Primary is up.
Four steps to failover
Wipe ZooKeeper entry. Clients will know the failover is in
progress. (0 seconds)
Stop the primary namenode. Last bits of data will be
flushed to Transaction Log and it will die. (Seconds)
Switch Standby to Primary. It will consume the rest of the
Transaction log and get out of safemode ready to serve
Update the entry in ZooKeeper. All the clients waiting for
failover will pick up the new connection. (0 seconds)
After: Start the first node in the Standby Mode (Takes a
while, but the cluster is up and running)
Why add ZooKeeper to the mix
Provides a clean way to execute failovers in the application
A centralized control of all the clients. Gives us the ability
to Pause clients until the failover is done.
A good stepping stone for future improvements needed to
perform automatic failover:
– Nodes voting on who will be the primary
– DataNodes knowing who has the authority to delete blocks
ZooKeeper is an option
Can be implemented using IP failover based on the existing
The clients will not know if the failover is done and it is
safe to make the call again.
IP failover works well in tightly coupled system (both nodes
in one rack) so the Single Point of Failure is still there (rack
There is no need to run a dedicated ZooKeeper cluster.
It is not all about failover
The Standby node has a lot of CPU that is not used
The Standby node has a full (but delayed a bit) picture of
Blocks and the Namesystem.
Send all reads that can deal with stale data to Standby.
Have pluggable services run as a part of Standby node and
use the metadata of the filesystem directly from the
shared memory instead of querying the namenode all the
My Hadoop Blog: