stack created HBASE-16689:
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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
Advertising
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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