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Bryan Duxbury commented on HADOOP-2731:
---------------------------------------
I've been taking a look at the RegionServer side of things to try and
understand why a split wouldn't occur. Some code:
{code}
if (e.getRegion().compactIfNeeded()) {
splitter.splitRequested(e);
}
{code}
We only queue a region to be split if it's just been compacted. I assume the
rationale here is that unless a compaction occurred, there'd be no reason to
split in the first place. I'm not convinced that's true, however. A store will
only compact if it has more mapfiles than the compaction threshold, which in
the case of some of my regions, wasn't the case - the individual mapfiles were
1.5GiB, but there were only 2. As a result, compaction and thus splitting was
skipped. Shouldn't we be testing to see if the overall size of the mapfiles
make splitting necessary, rather than letting the compaction determine whether
we do anything?
Perhaps we should add an optional compaction. Instead of testing
HStore.needsCompaction, which only checks if it is above the compaction
threshold, maybe we should also have a isCompactable, which just checks if
there is more than one mapfile. The optional compacts could happen behind
mandatory, threshold-based compacts. Then, we could always put an HStore on the
compact queue whenever there is an event that would cause a change to the
number of mapfiles, with the constraint that if the store is already on the
compact queue, we don't re-add it.
If we did all of that, then it would probably put us in the right state to keep
the split thread doing exactly what it is doing right now, but splits will also
happen in downtime.
> [hbase] Under extreme load, regions become extremely large and eventually
> cause region servers to become unresponsive
> ---------------------------------------------------------------------------------------------------------------------
>
> Key: HADOOP-2731
> URL: https://issues.apache.org/jira/browse/HADOOP-2731
> Project: Hadoop Core
> Issue Type: Bug
> Components: contrib/hbase
> Reporter: Bryan Duxbury
>
> When attempting to write to HBase as fast as possible, HBase accepts puts at
> a reasonably high rate for a while, and then the rate begins to drop off,
> ultimately culminating in exceptions reaching client code. In my testing, I
> was able to write about 370 10KB records a second to HBase until I reach
> around 1 million rows written. At that point, a moderate to large number of
> exceptions - NotServingRegionException, WrongRegionException, region offline,
> etc - begin reaching the client code. This appears to be because the
> retry-and-wait logic in HTable runs out of retries and fails.
> Looking at mapfiles for the regions from the command line shows that some of
> the mapfiles are between 1 and 2 GB in size, much more than the stated file
> size limit. Talking with Stack, one possible explanation for this is that the
> RegionServer is not choosing to compact files often enough, leading to many
> small mapfiles, which in turn leads to a few overlarge mapfiles. Then, when
> the time comes to do a split or "major" compaction, it takes an unexpectedly
> long time to complete these operations. This translates into errors for the
> client application.
> If I back off the import process and give the cluster some quiet time, some
> splits and compactions clearly do take place, because the number of regions
> go up and the number of mapfiles/region goes down. I can then begin writing
> again in earnest for a short period of time until the problem begins again.
> Both Marc Harris and myself have seen this behavior.
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