Commits on Jul 10, 2017

1. storage/engine: use SyncWAL instead of sync on commit …

   The previous implementation of batching for synced commits had the
   unfortunate property that commits that did not require syncing were
   required to wait for the WAL to be synced. When RocksDB writes a batch
   with sync==true it internally does:
   
     1. Add batch to WAL
     2. Sync WAL
     3. Add entries to mem table
   
   Switch to using SyncWAL to explicitly sync the WAL after writing a
   batch. This is slightly different semantics from the above:
   
     1. Add batch to WAL
     2. Add entries to mem table
     3. Sync WAL
   
   The advantage of this new approach is that non-synced batches do not
   have to wait for the WAL to sync. Prior to this change, it was observed
   that essentially every batch was waiting for a WAL sync. Approximately
   half of all batch commits are performed with sync==true (half of all
   batch commits are for Raft log entries or Raft state). Forcing the
   non-synced commits to wait for the WAL added significant time.
   
   Reworked the implementation of batch grouping. The sequence number
   mechanism was replaced by per-batch sync.WaitGroup embedded in the
   rocksDBBatch structure. Once a batch is committed (and synced if
   requested), the wait group is signalled ensuring that only the desired
   goroutine is woken instead of waking all of the goroutines in the
   previous implementation. Syncing of the WAL is performed by a dedicated
   thread add only the batches which specify sync==true wait for the
   syncing to occur.
   
   Added "rocksdb.min_wal_sync_interval" which adds specifies a minimum
   delay to wait between calls to SyncWAL. The default of 1ms was
   experimentally determined to reduce disk IOs by ~50% (vs 0ms) while leaving write
   throughput unchanged.
   
   For a write-only workload (`ycsb -workload F`), write throughput
   improved by 15%. Raft command commit latency (unsynced commits) dropped
   from 30-40ms to 6-10ms.

   

   petermattis committed Jul 10, 2017
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