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https://issues.apache.org/jira/browse/HDFS-1359?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12903495#action_12903495
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Suresh Srinivas commented on HDFS-1359:
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Some relevant sections from the design document attached to HDFS-1052. For more
details see the design document:
# A Block Pool is an independent set of blocks that belongs to a single
entity (e.g. namespace). A block pool is managed independently of other pools,
allowing owning entities to generate Block IDs for new blocks without the need
for co-ordination with other entities.
# Block Pool ID: A block pool ID identifies a block pool. It is a unique ID
across all clusters. When a new namespace is created (as part of the format
operation of a NN) a unique id is generated and stored persistently by the
namenodes. Using a unique BlockPoolID simplifies the creation, as it is done on
the fly on the namenodes instead of an error prone alternative of an
administrator configuring it. Namenodes persist the BlockPoolID information on
the disk and reuse it in subsequent restarts.
# Why is BlockPoolID globally unique?
#* Rather than assure the uniqueness of block pool ID via admin configuration,
a unique id can be generated programmatically.
#* Clearly it has to be unique within a cluster. It is equally easy to make
them globally unique or cluster unique. Only cost is # of bits.
#* If BPID is not unique, due to the possibility of reusing a deleted BPID, two
instances of block pools might conflict. An associated birthmark is needed to
distinguish the two instances.
#* Allows cluster merges, where block pools from the clusters can be retained.
> HDFS federation: Add BlockPoolID to block
> -----------------------------------------
>
> Key: HDFS-1359
> URL: https://issues.apache.org/jira/browse/HDFS-1359
> Project: Hadoop HDFS
> Issue Type: Sub-task
> Reporter: Suresh Srinivas
>
> This is a subtask to HDFS-1052 to add BlockPoolID to Block.
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