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From "Walter Su (JIRA)" <j...@apache.org>
Subject [jira] [Updated] (HDFS-8501) Erasure Coding: BlockInfoStriped.removeStorage(..) move storage with same blockIndex to front slot
Date Sat, 29 Aug 2015 01:00:48 GMT

     [ https://issues.apache.org/jira/browse/HDFS-8501?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
]

Walter Su updated HDFS-8501:
----------------------------
    Summary: Erasure Coding: BlockInfoStriped.removeStorage(..) move storage with same blockIndex
to front slot  (was: Erasure Coding: Improve memory efficiency of BlockInfoStriped)

> Erasure Coding: BlockInfoStriped.removeStorage(..) move storage with same blockIndex
to front slot
> --------------------------------------------------------------------------------------------------
>
>                 Key: HDFS-8501
>                 URL: https://issues.apache.org/jira/browse/HDFS-8501
>             Project: Hadoop HDFS
>          Issue Type: Sub-task
>            Reporter: Walter Su
>            Assignee: Walter Su
>         Attachments: HDFS-8501-HDFS-7285.01.patch
>
>
> Erasure Coding: Improve memory efficiency of BlockInfoStriped
> Assume we have a BlockInfoStriped:
> {noformat}
> triplets[] = {s0, s1, s2, s3}
> indices[] = {0, 1, 2, 3}
> {noformat}
> {{s0}} means {{storage_0}}
> When we run balancer/mover to re-locate replica on s2, firstly it becomes:
> {noformat}
> triplets[] = {s0, s1, s2, s3, s4}
> indices[] = {0, 1, 2, 3, 2}
> {noformat}
> Then the replica on s2 is removed, finally it becomes:
> {noformat}
> triplets[] = {s0, s1, null, s3, s4}
> indices[] = {0, 1, -1, 3, 2}
> {noformat}
> The worst case is:
> {noformat}
> triplets[] = {null, null, null, null, s4, s5, s6, s7}
> indices[] = {-1, -1, -1, -1, 0, 1, 2, 3}
> {noformat}
> We should learn from {{BlockInfoContiguous.removeStorage(..)}}. When a storage is removed,
we move the last item front.
> With the improvement, the worst case become:
> {noformat}
> triplets[] = {s4, s5, s6, s7, null}
> indices[] = {0, 1, 2, 3, -1}
> {noformat}
> We have an empty slot.
> Notes:
> Assume we copy 4 storage first, then delete 4. Even with the improvement, the worst case
could be:
> {noformat}
> triplets[] = {s4, s5, s6, s7, null, null, null, null}
> indices[] = {0, 1, 2, 3, -1, -1, -1, -1}
> {noformat}
> But the Balancer uses {{delHint}}. So when add one will always delete one. So this case
won't happen for striped and contiguous blocks.
> *idx_i must be moved to slot_i.* So slot_i will have idx_i. So we can do further improvement
in HDFS-8032.



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