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https://issues.apache.org/jira/browse/HBASE-2957?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12906889#action_12906889
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Todd Lipcon commented on HBASE-2957:
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{quote}
A simple way to do that will be to attach a log-sync-number with every cell.
When a cell is updated it will keep the next log-sync-number within itself. A
get will not return until the current log-sync-number is at least as big as
log-sync-number stored in the cell.
An update can return immediately after queuing the sync. The "wait-for-sync" is
transferred from the writer to the reader. If the reader comes in sufficiently
late (which is likely) then there will be no wait-for-syncs in the system.
{quote}
We actually already do this! Rather than using the log-sync number, the
memstore has an internal timestamp for readability. When we scan a row, we
record this number atomically and only return cell versions older than this
timestamp. See the ReadWriteConsistencyControl class.
The only change we have to make is that we unlock the row before we call sync
and update the memstore TS, rather than after.
> 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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