Re: An example of bugs for Hot Standby

>Following question may be redundant. Just a confirmation.
>
>Deadlock example is catstrophic while it's rather a rare event.
>On the other hand, LockBufferForCleanup() can cause another 
>problem.
>
> * One idle pin-holder backend can freeze startup process().
>
>This problem is not catstrophic, but it seems a similar problem
>which StandbyAcquireAccessExclusiveLock() tries to avoid.
>
>...Is this the problem you call "general problem" above ?
>


Here is a typical scenario in which startup process freezes until the end of 
a certain transaction.
1. Consider a table A, which has pages with HOT chain tuples old enough to be vacuumed.2. Xact 1 in the standby node
declaresa cursor for table A, fetches the page   which contains the HOT chain, and becomes idle for some reason.3. Xact
2in the active node reads the table A and calls heap_page_prune()   for HOT pruning, which create XLOG_HEAP2_CLEAN
record.4.Startup process tries to redo XLOG_HEAP2_CLEAN record, calls   LockBufferForCleanup() and freezes until the
Xact1 ends.
 

Note that with HOT pruning, we do not need VACUUM command, and most tables,
which has long history of updation, can be table A.


-- Hiroyuki YAMADA Kokolink Corporation yamada@kokolink.net