[HBASE-16689] Durability == ASYNC_WAL means no SYNC

• Type: Bug
• Status: Open
• Priority: Critical
• Resolution: Unresolved
• Affects Version/s: 1.0.3, 1.1.6, 1.2.3
• Component/s: wal

Environment

At least get the above doc into the refguide.

• Assignee: Michael Stack
• Reporter: Michael Stack
• Created: 23/Sep/16 04:19
• Updated: 12/Apr/18 23:02

Description

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.
• Doc what ASYNC_WAL means (and other durability options)

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

Issue Links

• links to: WAL Write Performance