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From "stack (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HBASE-2957) Release row lock when waiting for wal-sync
Date Mon, 06 Sep 2010 17:44:33 GMT

    [ https://issues.apache.org/jira/browse/HBASE-2957?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12906549#action_12906549

stack commented on HBASE-2957:

First, are you fellas running with hdfs-895?  (See HBASE-2467)  We have it deployed over here.
  It makes a big difference.

@Prakash So you are suggesting a patch that optionally allows returning to the client though
WAL has not been appended.  A flag in the Put would be checked somewhere around here -- http://hbase.apache.org/docs/r0.89.20100726/xref/org/apache/hadoop/hbase/regionserver/HRegion.html#1534
-- and if set, we'd skip the inline WAL append and in its place we'd just add the edit to
a queue for adding the WAL out of bound with the current update?  I'd guess you'd want the
flag on ICV too, somewhere around here -- http://hbase.apache.org/docs/r0.89.20100726/xref/org/apache/hadoop/hbase/regionserver/HRegion.html#3046?
 We'd change our current 'writeToWAL' flag so rather than being a binary, that it would allow
a new queue-the-edit-for-WAL-addition option?

> Release row lock when waiting for wal-sync
> ------------------------------------------
>                 Key: HBASE-2957
>                 URL: https://issues.apache.org/jira/browse/HBASE-2957
>             Project: HBase
>          Issue Type: Improvement
>          Components: regionserver, wal
>    Affects Versions: 0.20.0
>            Reporter: Prakash Khemani
> Is there a reason to hold on to the row-lock while waiting for the WAL-sync to be completed
by the logSyncer thread?
> I think data consistency will be guaranteed even if the following happens (a) the row
lock is held while the row is updated in memory (b) the row lock is released after queuing
the KV record for WAL-syncing (c) the log-sync system guarantees that the log records for
any given row are synced in order (d) the HBase client only receives a success notification
after the sync completes (no change from the current state)
> I think this should be a huge win. For my use case, and I am sure for others,  the handler
thread spends the bulk of its row-lock critical section  time waiting for sync to complete.
> Even if the log-sync system cannot guarantee the orderly completion of sync records,
the "Don't hold row lock while waiting for sync" option should be available to HBase clients
on a per request basis.

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