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From "Yi Liu (JIRA)" <j...@apache.org>
Subject [jira] [Created] (HDFS-6782) Improve FS editlog logSync
Date Wed, 30 Jul 2014 10:01:53 GMT
Yi Liu created HDFS-6782:
----------------------------

             Summary: Improve FS editlog logSync
                 Key: HDFS-6782
                 URL: https://issues.apache.org/jira/browse/HDFS-6782
             Project: Hadoop HDFS
          Issue Type: Improvement
          Components: namenode
    Affects Versions: 3.0.0
            Reporter: Yi Liu
            Assignee: Yi Liu


In NN, it uses a double buffer (bufCurrent, bufReady) for log sync, bufCurrent it to buffer
new coming edit ops and bufReady is for flushing. This's efficient. When flush is ongoing,
and bufCurrent is full, NN goes to force log sync, and all new Ops are blocked (since force
log sync is protected by FSNameSystem write lock). After the flush finished, the new Ops are
still blocked, but actually at this time, bufCurrent is free and Ops can go ahead and write
to the buffer. The following diagram shows the detail. This JIRA is for this improvement.
 Thanks [~umamaheswararao] for confirming this issue.

{code}
edit1(txid1) ------ write to bufCurrent -------- logSync --------- (swap buffer)flushing -------
edit2(txid2) ------ write to bufCurrent -------- logSync --------- waiting -------
edit3(txid3) ------ write to bufCurrent -------- logSync --------- waiting -------
edit4(txid4) ------ write to bufCurrent -------- logSync --------- waiting -------
edit5(txid5) ------ write to bufCurrent --full-- force sync --------- waiting -------
edit6(txid6) ------ blocked
...
editn(txidn) ------ blocked
{code}

After the flush, it becomes
{code}
edit1(txid1) ------ write to bufCurrent -------- logSync --------- finished --------
edit2(txid2) ------ write to bufCurrent -------- logSync --------- flushing -------
edit3(txid3) ------ write to bufCurrent -------- logSync --------- waiting -------
edit4(txid4) ------ write to bufCurrent -------- logSync --------- waiting -------
edit5(txid5) ------ write to bufCurrent --full-- force sync --------- waiting -------
edit6(txid6) ------ blocked
...
editn(txidn) ------ blocked
{code}

After edit1 finished, bufCurrent is free, and the thread which flushes txid2 will also flushes
txid3-5, so we should return from the force sync of edit5 and FSNamesystem write lock will
be freed (Don't worry that edit5 Op will return, since there will be a normal logSync after
the force logSync and there will wait for sync finished). This is the idea of this JIRA. 



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