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https://issues.apache.org/jira/browse/HBASE-2957?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12907314#action_12907314
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Prakash Khemani commented on HBASE-2957:
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Yes, delayed syncs should almost always work. The problem is that they don't
guarantee consistency. What I am arguing is that we can get consistency and the
delayed syncs like performance at the same time.
I also discussed this offline with Jonathan a little bit. Let me try one more
time.
Today, by default, HBase operates in "writers wait for sync" mode. This is good
because it guarantees both durability and consistency. It is bad because it can
be slow.
Deferred syncs neither guarantee durability nor consistency.
By consistency I mean the following - if A is the cause of B, and if B is
present in the logs then A must also be present in the logs.
If we can have HBase operate in "readers wait for sync" mode then we don't
guarantee durability but we still guarantee consistency. And the performance
should be similar to that of deferred syncs.
> 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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