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From "Keith Turner (JIRA)" <>
Subject [jira] [Commented] (ACCUMULO-1083) add concurrency to HDFS write-ahead log
Date Tue, 12 Mar 2013 20:03:12 GMT


Keith Turner commented on ACCUMULO-1083:

bq. Concurrent, as in, a sub-set of tablets use a log, or concurrent, as in, there are multiple
log files available to log to, as described in the Big Table paper?

We will need to decide which of those strategies to use.  One has implications of writes and
the other has implications for recovery.  If a subset of the tablets uses a log, then every
batch of mutations coming into a tablet server may need to be split and queued to multiple
walogs.  If a batch of mutations can be written to just an walog, then that walog will need
to be added to all tablets that batch has data for.  I think this has the potential for better
write performance, but it can cause tablets to have more walogs than they do now.  But I am
not sure if an individual tablet having more walogs matters.  One thing that matters for recovery
is the amount of data that needs to be sorted when a tserver fails and I am not sure this
really changes with the two approaches.
> add concurrency to HDFS write-ahead log
> ---------------------------------------
>                 Key: ACCUMULO-1083
>                 URL:
>             Project: Accumulo
>          Issue Type: Improvement
>          Components: tserver
>            Reporter: Adam Fuchs
>             Fix For: 1.6.0
>         Attachments: walog-performance.jpg, walog-replication-factor-performance.jpg
> When running tablet servers on beefy nodes (lots of disks), the write-ahead log can be
a serious bottleneck. Today we ran a continuous ingest test of 1.5-SNAPSHOT on an 8-node (plus
a master node) cluster in which the nodes had 32 cores and 15 drives each. Running with write-ahead
log off resulted in a >4x performance improvement sustained over a long period.
> I believe the culprit is that the WAL is only using one file at a time per tablet server,
which means HDFS is only appending to one drive (plus replicas). If we increase the number
of concurrent WAL files supported on a tablet server we could probably drastically improve
the performance on systems with many disks. As it stands, I believe Accumulo is significantly
more optimized for a larger number of smaller nodes (3-4 drives).

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