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https://issues.apache.org/jira/browse/HDFS-7339?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14289901#comment-14289901
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Zhe Zhang commented on HDFS-7339:
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bq. First a quick comment about the current SequentialBlockGroupIdGenerator and
SequentialBlockIdGenerator. The current patch tries to use a flag to
distinguish contiguous and stripped blocks. However, since there may still be
conflicts coming from historical randomly assigned block ID, for blocks in
block reports, we still to check two places to determine if this is a
contiguous block or a stripped block.
If a block's ID has the 'striped' flag bit, we always _attempt_ to look up the
block group map first. Without rolling upgrade we only need this one lookup.
And yes, we do need to check two places in the worst case. Given that HDFS-4645
will be over 2 years old by the time erasure coding is released, I guess this
won't happen a lot?
> Allocating and persisting block groups in NameNode
> --------------------------------------------------
>
> Key: HDFS-7339
> URL: https://issues.apache.org/jira/browse/HDFS-7339
> Project: Hadoop HDFS
> Issue Type: Sub-task
> Reporter: Zhe Zhang
> Assignee: Zhe Zhang
> Attachments: HDFS-7339-001.patch, HDFS-7339-002.patch,
> HDFS-7339-003.patch, HDFS-7339-004.patch, HDFS-7339-005.patch,
> HDFS-7339-006.patch, Meta-striping.jpg, NN-stripping.jpg
>
>
> All erasure codec operations center around the concept of _block group_; they
> are formed in initial encoding and looked up in recoveries and conversions. A
> lightweight class {{BlockGroup}} is created to record the original and parity
> blocks in a coding group, as well as a pointer to the codec schema (pluggable
> codec schemas will be supported in HDFS-7337). With the striping layout, the
> HDFS client needs to operate on all blocks in a {{BlockGroup}} concurrently.
> Therefore we propose to extend a file’s inode to switch between _contiguous_
> and _striping_ modes, with the current mode recorded in a binary flag. An
> array of BlockGroups (or BlockGroup IDs) is added, which remains empty for
> “traditional” HDFS files with contiguous block layout.
> The NameNode creates and maintains {{BlockGroup}} instances through the new
> {{ECManager}} component; the attached figure has an illustration of the
> architecture. As a simple example, when a {_Striping+EC_} file is created and
> written to, it will serve requests from the client to allocate new
> {{BlockGroups}} and store them under the {{INodeFile}}. In the current phase,
> {{BlockGroups}} are allocated both in initial online encoding and in the
> conversion from replication to EC. {{ECManager}} also facilitates the lookup
> of {{BlockGroup}} information for block recovery work.
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