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Commit1b2bb33

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Add a comment documenting the question of whether PrefetchBuffer should
try to protect an already-existing buffer from being evicted. This wasleft as an open issue when the posix_fadvise patch was committed. I'mnot sure there's any evidence to justify more work in this area, but weshould have some record about it in the source code.
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‎src/backend/storage/buffer/bufmgr.c‎

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@@ -8,7 +8,7 @@
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*
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*
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* IDENTIFICATION
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* $PostgreSQL: pgsql/src/backend/storage/buffer/bufmgr.c,v 1.250 2009/03/31 22:12:48 tgl Exp $
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* $PostgreSQL: pgsql/src/backend/storage/buffer/bufmgr.c,v 1.251 2009/04/03 18:17:43 tgl Exp $
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*
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*-------------------------------------------------------------------------
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*/
@@ -153,6 +153,18 @@ PrefetchBuffer(Relation reln, ForkNumber forkNum, BlockNumber blockNum)
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/* If not in buffers, initiate prefetch */
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if (buf_id<0)
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smgrprefetch(reln->rd_smgr,forkNum,blockNum);
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/*
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* If the block *is* in buffers, we do nothing. This is not really
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* ideal: the block might be just about to be evicted, which would
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* be stupid since we know we are going to need it soon. But the
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* only easy answer is to bump the usage_count, which does not seem
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* like a great solution: when the caller does ultimately touch the
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* block, usage_count would get bumped again, resulting in too much
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* favoritism for blocks that are involved in a prefetch sequence.
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* A real fix would involve some additional per-buffer state, and
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* it's not clear that there's enough of a problem to justify that.
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*/
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}
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#endif/* USE_PREFETCH */
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}

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