[HDFS-6782] Improve FS editlog logSync

• Type: Improvement
• Status: Patch Available
• Priority: Major
• Resolution: Unresolved
• Affects Version/s: 2.4.1
• Component/s: namenode
• Assignee: Yi Liu
• Reporter: Yi Liu
• Created: 30/Jul/14 10:00
• Updated: 06/Jan/17 22:15

Description

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.

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

After the flush, it becomes

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

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.