Upper bound of outstanding WALs can be overrun; take 2 (take 1 was hbase-2053)
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Key: HBASE-2236
URL: https://issues.apache.org/jira/browse/HBASE-2236
Project: Hadoop HBase
Issue Type: Bug
Reporter: stack
Fix For: 0.20.4, 0.21.0
So hbase-2053 is not aggressive enough. WALs can still overwhelm the upper
limit on log count. While the code added by HBASE-2053, when done, will ensure
we let go of the oldest WAL, to do it, we might have to flush many regions.
E.g:
{code}
2010-02-15 14:20:29,351 INFO org.apache.hadoop.hbase.regionserver.HLog: Too
many hlogs: logs=45, maxlogs=32; forcing flush of 5 regions(s):
test1,193717,1266095474624, test1,194375,1266108228663,
test1,195690,1266095539377, test1,196348,1266095539377,
test1,197939,1266069173999
{code}
This takes time. If we are taking on edits a furious rate, we might have
rolled the log again, meantime, maybe more than once.
Also log rolls happen inline with a put/delete as soon as it hits the 64MB
(default) boundary whereas the necessary flushing is done in background by a
single thread and the memstore can overrun the (default) 64MB size. Flushes
needed to release logs will be mixed in with "natural" flushes as memstores
fill. Flushes may take longer than the writing of an HLog because they can be
larger.
So, on an RS that is struggling the tendency would seem to be for a slight rise
in WALs. Only if the RS gets a breather will the flusher catch up.
If HBASE-2087 happens, then the count of WALs get a boost.
Ideas to fix this for good would be :
+ Priority queue for queuing up flushes with those that are queued to free up
WALs having priority
+ Improve the HBASE-2053 code so that it will free more than just the last WAL,
maybe even queuing flushes so we clear all WALs such that we are back under the
maximum WALS threshold again.
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