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From "stack (JIRA)" <j...@apache.org>
Subject [jira] [Created] (HBASE-16689) Durability == ASYNC_WAL means no SYNC
Date Fri, 23 Sep 2016 04:20:20 GMT
stack created HBASE-16689:
-----------------------------

             Summary: Durability == ASYNC_WAL means no SYNC
                 Key: HBASE-16689
                 URL: https://issues.apache.org/jira/browse/HBASE-16689
             Project: HBase
          Issue Type: Bug
          Components: wal
    Affects Versions: 1.2.3, 1.1.6, 1.0.3
            Reporter: stack
            Assignee: stack
            Priority: Critical


Setting DURABILITY=ASYNC_WAL on a Table suspends all syncs for all table Table appends. If
all tables on a cluster have this setting, data is flushed from the RS to the DN at some arbitrary
time and a bunch may just hang out in DFSClient buffers on the RS-side indefinitely if writes
are sporadic, at least until there is a WAL roll -- a log roll sends a sync through the write
pipeline to flush out any outstanding appends -- or a region close which does similar....
or we crash and drop the data in buffers RS.

This is probably not what a user expects when they set ASYNC_WAL (We don't doc anywhere that
I could find clearly what ASYNC_WAL means). Worse, old-time users probably associate ASYNC_WAL
and DEFERRED_FLUSH, an old HTableDescriptor config that was deprecated and replaced by ASYNC_WAL.
DEFERRED_FLUSH ran a background thread -- LogSyncer -- that on a configurable interval, sent
a sync down the write pipeline so any outstanding appends since last last interval start get
pushed out to the DN.  ASYNC_WAL doesn't do this (see below for history on how we let go of
the LogSyncer feature).

Of note, we always sync meta edits. You can't turn this off. Also, given WALs are per regionserver,
if other regions on the RS are from tables that have sync set, these writes will push out
to the DN any appends done on tables that have DEFERRED/ASYNC_WAL set.

To fix, we could do a few things:

 * Simple and comprehensive would be always queuing a sync, even if ASYNC_WAL is set but we
let go of Handlers as soon as we write the memstore -- we don't wait on the sync to complete
as we do with the default setting of Durability=SYNC_WAL.
 * Be like a 'real' database and add in a sync after N bytes of data have been appended (configurable)
or after M milliseconds have passed, which ever threshold happens first. The size check would
be easy. The sync-ever-M-millis would mean another thread.

Let me take a look and report back. Will file a bit of history on how we got here in next
comment.



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