[jira] [Created] (ACCUMULO-4146) Per-durability write-ahead logs

["Christopher Tubbs (JIRA)" <ji...@apache.org>]

Christopher Tubbs created ACCUMULO-4146:
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             Summary: Per-durability write-ahead logs
                 Key: ACCUMULO-4146
                 URL: https://issues.apache.org/jira/browse/ACCUMULO-4146
             Project: Accumulo
          Issue Type: Improvement
            Reporter: Christopher Tubbs
             Fix For: 1.8.0


[~kturner] showed me a scenario with Continuous Ingest, where the system was configured to use "flush" for it's durability, but the Metadata table was configured to use "sync" (default for metadata). When the system grew large enough to have many tablets, lots of tablets were constantly writing to the metadata table on a relatively few number of tablet servers. This caused all ingest to drop to 0 very frequently as the tables waiting on a "flush" had to wait for the WAL to "sync".

This problem is primarily caused by the fact that we have only one WAL per tablet server, and so tablet writes waiting on a flush had to wait on a sync if any concurrent write to the tablet server required a sync.

We could possibly alleviate this problem if we permitted tablet servers to have multiple WALs open. One potentially good way to manage these multiple WALs is to group them by durability, so there'd be one WAL for sync, and another for flush. That way, writes requiring a flush would not wait on sync's.



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