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From "Eric Newton (Commented) (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HBASE-5504) Online Merge
Date Thu, 19 Apr 2012 14:54:47 GMT

    [ https://issues.apache.org/jira/browse/HBASE-5504?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13257523#comment-13257523
] 

Eric Newton commented on HBASE-5504:
------------------------------------

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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