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From joshelser <...@git.apache.org>
Subject [GitHub] accumulo pull request #176: Wrote blog post about durability and performance
Date Mon, 31 Oct 2016 13:42:05 GMT
Github user joshelser commented on a diff in the pull request:

    https://github.com/apache/accumulo/pull/176#discussion_r85739576
  
    --- Diff: _posts/blog/2016-10-28-durability-performance.md ---
    @@ -0,0 +1,136 @@
    +---
    +title: "Durability Performance Implications"
    +date: 2016-10-28 17:00:00 +0000
    +author: Keith Turner
    +---
    +
    +## Overview
    +
    +Accumulo stores recently written data in a sorted in memory map.  Before data is
    +added to this map, it's written to an unsorted WAL (write ahead log).  In the
    +case when a Tablet Server dies, the recently written data is recovered from the
    +WAL.
    +
    +When data is written to Accumulo the following happens :
    +
    + * Client sends a batch of mutations to a tablet server
    + * Tablet server does the following :
    +   * Writes mutation to Tablet Servers WAL
    +   * Sync or flush WAL
    +   * Adds mutations to sorted in memory maps
    +   * Reports success back to client.
    +
    +The sync/flush step above moves data written to the WAL from memory to disk.
    +Write ahead logs are stored in HDFS. HDFS supports two ways of forcing data to
    +disk for an open file : `hsync` and `hflush`.  
    +
    +## HDFS Sync/Flush Details
    +
    +When `hflush` is called on a WAL, it does not ensure data is on disk.  It only
    +ensure that data is in OS buffers on each datanode and on its way to disk.  As a
    +result calls to `hflush` are very fast.  If a WAL is replicated to 3 data nodes
    +then data may be lost if all three machines reboot.  If the datanode process
    --- End diff --
    
    When I read "reboot", I thought of it as an "planned" occurrence, instead of a planned
_or_ unplanned failure (such as operator error, hardware failure, etc, kernel panic, etc).
Admittedly, it might be my colloquial use of the word "reboot" affecting my interpretation
:)


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