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https://issues.apache.org/jira/browse/HDFS-2601?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13160238#comment-13160238
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Karthik Ranganathan commented on HDFS-2601:
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@Andrew - just trying to understand a couple of points in more detail.
<< HDFS is stable, but not for this use case, yet, and we'd need a lot of work
to get it there. >>
Are you talking about the fact that there would be bugs in HDFS as is so those
need to be fixed, or the code complexity of implementing this approach? If the
former, can you elaborate? If the latter, then we can go ahead with evaluating
this approach and if we like it, then do an evaluation of the code complexity
and make a decision depending on what we find.
<< And we still end up with a stack that, like NFS, is a gazillion lines of
code, but handles a lot of different stuff. Most of which we don't need for the
image/edits application. >>
I was thinking the opposite - that the HDFS write pipeline and recovery logic
is battle-tested (in many respects - architecture, failure mode, code, actual
usage) at such a large scale that it makes it rather attractive for such a
critical piece of data as opposed to writing something new (however simple) and
having to prove it works from the scratch?
> Proposal to store edits and checkpoints inside HDFS itself for namenode HA
> --------------------------------------------------------------------------
>
> Key: HDFS-2601
> URL: https://issues.apache.org/jira/browse/HDFS-2601
> Project: Hadoop HDFS
> Issue Type: New Feature
> Components: name-node
> Reporter: Karthik Ranganathan
>
> Would have liked to make this a "brainstorming" JIRA but couldn't find the
> option for some reason.
> I have talked to a quite a few people about this proposal at Facebook
> internally (HDFS folks like Hairong and Dhruba, as well as HBase folks
> interested in this feature), and wanted to broaden the audience.
> At the core of the HA feature, we need 2 things:
> A. the secondary NN (or avatar stand-by or whatever we call it) needs to read
> all the fsedits and fsimage data written by the primary NN
> B. Once the stand-by has taken over, the old NN should not be allowed to make
> any edits
> The basic idea is as follows (there are some variants, we can hone in on the
> details if we like the general approach):
> 1. The write path for fsedits and fsimage:
> 1.1 The NN uses a dfs client to write fsedits and fsimage. These will be
> regular hdfs files written using the write pipeline.
> 1.2 Let us say the fsimage and fsedits files are written to a well-known
> location in the local HDFS itself (say /.META or some such location)
> 1.3 The create files and add blocks to files in this path are not written to
> fsimage or fsedits. The location of the blocks for the files in this location
> are known to all namenodes - primary and standby - somehow (some
> possibilities here - write these block ids to zk or use reserved block ids or
> write some meta-data into the blocks itself and store the blocks in a well
> known location on all the datanodes)
> 1.4 If the replication factor on the write pipeline decreases, we close the
> block immediately and allow NN to re-replicate to bring up the replication
> factor. We continue writing to a new block
> 2. The read path on a NN failure
> 2.1 Since the new NN "knows" the location of the blocks for the fsedits and
> fsimage (again the same possibilities as mentioned above), there is nothing
> to do to determine this
> 2.2 It can read the files it needs using the HDFS client itself
> 3. Fencing - if a NN is unresponsive, a new NN takes over, old NN should not
> be allowed to perform any action
> 3.1 Use HDFS lease recovery for the fsedits and fsimage files - the new NN
> will close all these files baing written to by the old NN (and hence all the
> blocks)
> 3.2 The new NN (avatar NN) will write its address into ZK to let everyone
> know its the master
> 3.3 The new NN now gets the lease for these files and starts writing into the
> fsedits and fsimage
> 3.4 The old NN cannot write into the file as the block it was writing to was
> closed and it does not have the lease. If it needs to re-open these files, it
> needs to check zk to see it is indeed the current master, if not it should
> exit.
> 4. Misc considerations:
> 4.1 If needed, we can specify favored nodes to place the blocks for this data
> in specific set of nodes (say we want to use a different set of RAIDed nodes,
> etc).
> 4.2 Since we wont record the entries for /.META in fsedits and fsimage, a
> "hadoop dfs -ls /" command wont show the files. This is probably ok, and can
> be fixed if not.
> 4.3 If we have 256MB block sizes, then 20GB fsimage file would need 80 block
> ids - the NN would need only these 80 block ids to read all the fsedits data.
> The fsimage data is even lesser. This is very tractable using a variety of
> the techniques (the possibilities mentioned above).
> The advantage is that we are re-using the existing HDFS client (with some
> enhancements of course), and making the solution self-sufficient on the
> existing HDFS. Also, the operational complexity is greatly reduced.
> Thoughts?
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