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From "dhruba borthakur (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HADOOP-1942) Increase the concurrency of transaction logging to edits log
Date Mon, 01 Oct 2007 22:56:50 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-1942?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12531655
] 

dhruba borthakur commented on HADOOP-1942:
------------------------------------------

I like your idea. In fact, the current code has a bug that the modificationTime used for batching
is not stored per transaction. This fix shud give us lots of  concurrency.

I am thinking of extending your idea to remember the counter in logEdit(). It can be something
like a TransactionId. logEdit() will return the transactionId. Then this transactionId is
passed into logSync(). logSync() will wait till that particular transaction is synced to disk.
This allows threads that do multiple transactions to issue only one logSync().

> Increase the concurrency of transaction logging to edits log
> ------------------------------------------------------------
>
>                 Key: HADOOP-1942
>                 URL: https://issues.apache.org/jira/browse/HADOOP-1942
>             Project: Hadoop
>          Issue Type: Improvement
>          Components: dfs
>            Reporter: dhruba borthakur
>            Assignee: dhruba borthakur
>            Priority: Blocker
>             Fix For: 0.15.0
>
>         Attachments: transactionLogSync.patch, transactionLogSync2.patch
>
>
> For some typical workloads, the throughput of the namenode is bottlenecked by the rate
of transactions that are being logged into tghe edits log. In the current code, a batching
scheme implies that all transactions do not have to incur a sync of the edits log to disk.
However, the existing batch-ing scheme can be improved.
> One option is to keep two buffers associated with edits file. Threads write to the primary
buffer while holding the FSNamesystem lock. Then the thread release the FSNamesystem lock,
acquires a new lock called the syncLock, swaps buffers, and flushes the old buffer to the
persistent store. Since the buffers are swapped, new transactions continue to get logged into
the new buffer. (Of course, the new transactions cannot complete before this new buffer is
sync-ed).
> This approach does a better job of batching syncs to disk, thus improving performance.

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