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From "Yoram Arnon (JIRA)" <j...@apache.org>
Subject [jira] Created: (HADOOP-90) DFS is succeptible to data loss in case of name node failure
Date Fri, 17 Mar 2006 02:51:15 GMT
DFS is succeptible to data loss in case of name node failure
------------------------------------------------------------

         Key: HADOOP-90
         URL: http://issues.apache.org/jira/browse/HADOOP-90
     Project: Hadoop
        Type: Bug
  Components: dfs  
    Versions: 0.1    
    Reporter: Yoram Arnon


Currently, DFS name node stores its log and state in local files.
This has the disadvantage that a hardware failure of the name node causes a total data loss.

Several approaches may be used to address this flaw:
1. replicate the name server state files using copy or rsync once in a while, either manually
or using a cron job.
2. set up secondary name servers and a protocol whereby the primary updates the secondaries.
In case of failure, a secondary can take over.
3. store the state files as distributed, replicated files in the DFS itself. The difficulty
is that it becomes a bootstrap problem, where the name node needs some information, typically
stored in its state files, in order to read those same state files.

solution 1 is fine for non critical systems, but for systems that need to guarantee no data
loss it's insufficient.
Solutions 2 and 3 both seem valid; 3 seems more elegant in that it doesn't require an extra
protocol, it leverages the DFS and allows any level of replication for robustness. Below is
a proposition for  solution 3.

1.	The name node, when it starts up, needs some basic information. That information is not
large and can easily be stored in a single block of DFS. We hard code the block location,
using block id 0. Block 0 will contain the list of blocks that contain the name node metadata
- not the metadata itself (file names, servers, blocks etc), just the list of blocks that
contain it. With a block identified by 8 bytes, and 32 MB blocks, we can fit 256K block id's
in block 0. 256K blocks of 32MB each can hold 8TB of metadata, which can map a large enough
file system, so a single block of block_ids is sufficient.
2.	The name node writes his state basically the same way as now: log file plus occasional
full state. DFS needs to change to commit changes to open files while allowing continued writing
to them, or else the log file wouldn't be valid on name server failure, before the file is
closed. 
3.	The name node will use double buffering for its state, using blocks 0 and 1. Starting with
block 0, it writes its state, then a log of changes. When it's time to write a new state it
writes it to node 1. The state includes a generation number, a single byte starting at 0,
to enable the name server to identify the valid state. A CRC is written at the end of the
block to mark its validity and completeness. The log file is identified by the same generation
number as the state it relates to. 
4.	The log file will be limited to a single block as well. When that block fills up a new
state is written. 32MB of transaction logs should suffice. If not, we could set aside a set
of blocks, and set aside a few locations in the super-block (block 0/1) to store that set
of block ids.
5.	The super-block, the log and the metadata blocks may be exposed as read only files in reserved
files in the DFS: /.metadata/* or something.
6.	When a name nodes starts, it waits for data nodes to connect to it to report their blocks.
It waits until it gets a report about blocks 0 and 1, from which it can continue to read its
entire state. After that it continues normally.


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