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From "stack (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HBASE-6774) Immediate assignment of regions that don't have entries in HLog
Date Wed, 21 Nov 2012 23:14:59 GMT

    [ https://issues.apache.org/jira/browse/HBASE-6774?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13502413#comment-13502413
] 

stack commented on HBASE-6774:
------------------------------

If multiwals, yeah, should dedicate one for .META.

Agree all suggestions are not incompatible: i.e. we should do Jimmy's suggestion (You may
have suggested similar a while back IIRC).

I like the issues you raise w/ the soln. I suggest.  While we could read the last WAL while
splitting, just reading metadata off the end of all WALs concerned would take say ... about
a second for each unless we did it in //... and if tens of WALs, thats tens of seconds before
we could open a region even when all is functioning without hiccups (add hiccups, bad DN and
it goes up significantly).

I do like not having to have another subsystem in the mix doing log splitting. I suppose we
already have an optional dependency on zk farming out the work.

Could the regionserver send the master the regions mentioned in a WAL and let it do accounting
or it could send sequenceids by flush to the master and let it figure out up to what entry
it can skip edits?  It could do the writing to zk instead of every regionserver doing it for
every WAL roll.  We are already sending over seqids on heartbeat so we can skip stale edits
on crash?  Could expand this functionality so a by region dimension?  (Haven't thought it
through... just making suggestion)


                
> Immediate assignment of regions that don't have entries in HLog
> ---------------------------------------------------------------
>
>                 Key: HBASE-6774
>                 URL: https://issues.apache.org/jira/browse/HBASE-6774
>             Project: HBase
>          Issue Type: Improvement
>          Components: master, regionserver
>    Affects Versions: 0.96.0
>            Reporter: nkeywal
>
> The algo is today, after a failure detection:
> - split the logs
> - when all the logs are split, assign the regions
> But some regions can have no entries at all in the HLog. There are many reasons for this:
> - kind of reference or historical tables. Bulk written sometimes then read only.
> - sequential rowkeys. In this case, most of the regions will be read only. But they can
be in a regionserver with a lot of writes.
> - tables flushed often for safety reasons. I'm thinking about meta here.
> For meta; we can imagine flushing very often. Hence, the recovery for meta, in many cases,
will be the failure detection time.
> There are different possible algos:
> Option 1)
>  A new task is added, in parallel of the split. This task reads all the HLog. If there
is no entry for a region, this region is assigned.
>  Pro: simple
>  Cons: We will need to read all the files. Add a read.
> Option 2)
>  The master writes in ZK the number of log files, per region.
>  When the regionserver starts the split, it reads the full block (64M) and decrease the
log file counter of the region. If it reaches 0, the assign start. At the end of its split,
the region server decreases the counter as well. This allow to start the assign even if not
all the HLog are finished. It would allow to make some regions available even if we have an
issue in one of the log file.
>  Pro: parallel
>  Cons: add something to do for the region server. Requites to read the whole file before
starting to write. 
> Option 3)
>  Add some metadata at the end of the log file. The last log file won't have meta data,
as if we are recovering, it's because the server crashed. But the others will. And last log
file should be smaller (half a block on average).  
> Option 4) Still some metadata, but in a different file. Cons: write are increased (but
not that much, we just need to write the region once). Pros: if we lose the HLog files (major
failure, no replica available) we can still continue with the regions that were not written
at this stage.
> I think it should be done, even if none of the algorithm above is totally convincing
yet. It's linked as well to locality and short circuit reads: with these two points reading
the file twice become much less of an issue for example. My current preference would be to
open the file twice in the region server, once for splitting as of today, once for a quick
read looking for unused regions. Who knows, may be it would even be faster this way, the quick
read thread would warm-up the different caches for the splitting thread.

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