eolivelli commented on a change in pull request #889: BP-14 Implementation of
WriteFlag.DEFERRED_SYNC on Journal
URL: https://github.com/apache/bookkeeper/pull/889#discussion_r157955300
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File path:
bookkeeper-server/src/main/java/org/apache/bookkeeper/bookie/Journal.java
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@@ -960,6 +976,16 @@ public void run() {
}
journalFlushWatcher.reset().start();
bc.flush(false);
+
Review comment:
No, maybe the name 'persisted' is confusing or it is not clear the meaning.
We have two kind of QueueEntry for writes:
- normal (ackBeforeForce=false): the *run* method is called once, only when
we are sure that data has been *persisted* (FileChanned forced) and after
calling the callback we can "recycle" the object
- deferred_sync (ackBeforeForce=true): the callback is called once, but the
*run* method is called twice, the first time is after the *flush(false)*, the
second time is after the entry as been *persisted* (FileChanned forced) and we
can recycle the object after the second call.
We are setting *persisted* = true when we are sure that the entry as really
been persisted (forced/fsync'd), the QueueEntry object will never be used for
this 'entry' and so we can put it in the recycler.
Every entry always starts with *persisted* = false and then the journal sets
persisted = true after the force().
*ackBeforeForce* is a "constant" for the entry and it marks the type of
write (deferred_sync vs normal)
We have to call the *run* method twice for deferred_sync entries because at
the first time we are firing the callback (acknowledging the write to the
client), and at the second time we will update the cursor on the Bookie side
which tracks which entries have been persisted durably (this is not present in
this patch).
If I remove the entry (or set it to *null*) from the toFlush, I call the
callback and then recycle the entry I will never have a way to track that the
entry has been really persisted (forced/fsync'd)
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