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From ctubbsii <>
Subject [GitHub] accumulo pull request #176: Wrote blog post about durability and performance
Date Sat, 29 Oct 2016 03:00:11 GMT
Github user ctubbsii commented on a diff in the pull request:
    --- Diff: _posts/blog/ ---
    @@ -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
    +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
    +dies, thats ok because it flushed to OS.  The machines have to reboot for data
    +loss to occur.
    +In order to avoid data loss in the event of reboot, `hsync` can be called.  This
    +will ensure data is written to disk on all datanodes before returning.  When
    +using `hsync` for the WAL, if Accumulo reports success to a user it means the
    +data is on disk.  However `hsync` is much slower than `hflush` and the way it's
    +implemented exacerbates the problem.  For example `hflush` make take 1ms and
    +`hsync` may take 50ms.  This difference will impact writes to Accumulo and can
    +be mitigated in some situations with larger buffers in Accumulo.
    +HDFS keeps checksum data internally by default.  Datanodes store checksum data
    +in a separate file in the local filesystem.  This means when `hsync` is called
    +on a WAL, two files must be synced on each datanode.  Syncing two files doubles
    +the time. To make matters even worse, when the two files are synced the local
    +filesystem metadata is also synced.  Depending on the local filesystem and its
    +configuration, syncing the metadata may or may not take time.  In the worst
    +case, we need to wait for four sync operations at the local filesystem level on
    +each datanode. One thing I am not sure about, is if these sync operations occur
    +in parallel on the replicas on different datanodes.  Lets hope they occur in
    +parallel.  The following pointers show where sync occurs in the datanode code.
    + * [BlockReceiver.flushOrSync()][fos] calls [ReplicaOutputStreams.syncDataOut()][ros1]
and [ReplicaOutputStreams.syncChecksumOut()][ros2] when `isSync` is true.
    + * The methods in ReplicaOutputStreams call [FileChannel.force(true)][fcf] which
    +   synchronously flushes data and filesystem metadata.
    +If files were preallocated (this would avoid syncing local filesystem metadata)
    +and checksums were stored in-line, then 1 sync could be done instead of 4.  
    +## Configuring WAL flush/sync in Accumulo 1.6
    +Accumulo 1.6.0 only supported `hsync` and this caused [performance
    +problems][160_RN_WAL].  In order to offer better performance, the option to
    +configure `hflush` was [added in 1.6.1][161_RN_WAL].  The
    +[tserver.wal.sync.method][16_UM_SM] configuration option was added to support
    +this feature.  This was a tablet server wide option that applied to everything
    +written to any table.   
    +## Group Commit
    +Each Accumulo Tablet Server has a single WAL.  When multiple clients send
    +mutations to a tablet server at around the same time, the tablet sever may group
    +all of this into a single WAL operation.  It will do this instead of writing and
    +syncing or flushing each client's mutations to the WAL separately.  Doing this
    +increase throughput and lowers average latency for clients.
    +## Configuring WAL flush/sync in Accumulo 1.7+
    +Accumulo 1.7.0 introduced [table.durability][17_UM_TD], a new per table property
    +for configuring durability.  It also stopped using the `tserver.wal.sync.method`
    +property.  The `table.durability` property has the following four legal values.
    + * **none** : Do not write to WAL            
    + * **log**  : Write to WAL, but do not sync  
    + * **flush** : Write to WAL and call `hflush` 
    + * **sync** : Write to WAL and call `hsync`  
    +If multiple writes arrive at around the same time with different durability
    +settings, then the group commit code will choose the most conservative
    --- End diff --
    What does "most conservative" mean here? Maybe "most durable" would be a better word choice?

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