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https://issues.apache.org/jira/browse/HBASE-5504?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13257523#comment-13257523
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Eric Newton commented on HBASE-5504:
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We implemented delayed file cleanup because that is what was described in the
BigTable paper. A tablet will have files in multiple directories, including
directories under different tables, as is the case just after a table copy.
The file information is stored in the metadata table. Splits, bulk import and
table copying all create references to files, which remain shared. Data will
exist in the file, but outside the tablet's range, which is why we chop them
during merge. The tablet server keeps the list up-to-date in the metadata
table as files are bulk loaded and compacted. We also keep the list of files
that are candidates for deletion in the metadata table, so we aren't asking the
NameNode for information about files during GC. Bulk import, compaction, and
gc must all be coordinated such that the file isn't inadvertently deleted while
still being imported into other tablets.
> Online Merge
> ------------
>
> Key: HBASE-5504
> URL: https://issues.apache.org/jira/browse/HBASE-5504
> Project: HBase
> Issue Type: Brainstorming
> Components: client, master, shell, zookeeper
> Affects Versions: 0.94.0
> Reporter: Mubarak Seyed
> Fix For: 0.96.0
>
>
> As discussed, please refer the discussion at
> [HBASE-4991|https://issues.apache.org/jira/browse/HBASE-4991]
> Design suggestion from Stack:
> {quote}
> I suggest a design below. It has some prerequisites, some general function
> that this feature could use (and others). The prereqs if you think them good,
> could be done outside of this JIRA.
> Here's a suggested rough outline of how I think this feature should run. The
> feature I'm describing below is merge and deleteRegion for I see them as in
> essence the same thing.
> (C) Client, (M) Master, RS (Region server), ZK (ZooKeeper)
> 1. Client calls merge or deleteRegion API. API is a range of rows. (C)
> 2. Master gets call. (M)
> 3. Master obtains a write lock on table so it can't be disabled from under
> us. The write lock will also disable splitting. This is one of the prereqs I
> think. Its HBASE-5494 (Or we could just do something simpler where we have a
> flag up in zk that splitRegion checks but thats less useful I think; OR we do
> the dynamic configs issue and set splits to off via a config. change).
> There'd be a timer for how long we wait on the table lock. (M -> ZK)
> 4. If we get the lock, write intent to merge a range up into zk. It also
> hoists into zk if its a pure merge or a merge that drops the region data (a
> deleteRegion call) (M)
> 5. Return to the client either our failed attempt at locking the table or an
> id of some sort used identifying this running operation; can use it querying
> status. (M -> C)
> 6. Turn off balancer. TODO/prereq: Do it in a way that is persisted. Balancer
> switch currently in memory only so if master crashes, new master will come up
> in balancing mode # (If we had dynamic config. could hoist up to zk a config.
> that disables the balancer rather than have a balancer-specific flag/znode OR
> if a write lock outstanding on a table, then the balancer does not balance
> regions in the locked table - this latter might be the easiest to do) (M)
> 7. Write into zk that just turned off the balancer (If it was on) (M -> ZK)
> 8. Get regions that are involved in the span (M)
> 9. Hoist the list up into zk. (M -> ZK)
> 10. Create region to span the range. (M)
> 11. Write that we did this up into zk. (M -> ZK)
> 12. Close regions in parallel. Confirm close in parallel. (M -> RS)
> 13. Write up into zk regions closed (This might not be necessary since can
> ask if region is open). (M -> ZK)
> 14. If a merge and not a delete region, move files under new region. Might
> multithread this (moves should go pretty fast). If a deleteregion, we skip
> this step. (M)
> 15. On completion mark zk (though may not be necessary since its easy to look
> in fs to see state of move). (M -> ZK)
> 16. Edit .META. (M)
> 17. Confirm edits went in. (M)
> 18. Move old regions to hbase trash folder TODO: There is no trash folder
> under /hbase currently. We should add one. (M)
> 19. Enable balancer (if it was off) (M)
> 20. Unlock table (M)
> {quote}
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