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Update C comment about O_DIRECT and fsync().

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This commit is contained in:
Bruce Momjian 2011-03-11 06:46:44 -05:00
parent 06711de9d9
commit 7d23e0f803
1 changed files with 6 additions and 7 deletions
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13
src/include/access/xlogdefs.h
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@ -85,13 +85,12 @@ typedef uint32 TimeLineID;
/*
* Because O_DIRECT bypasses the kernel buffers, and because we never
* read those buffers except during crash recovery, it is a win to use
* it in all cases where we sync on each write(). We could allow O_DIRECT
* with fsync(), but because skipping the kernel buffer forces writes out
* quickly, it seems best just to use it for O_SYNC. It is hard to imagine
* how fsync() could be a win for O_DIRECT compared to O_SYNC and O_DIRECT.
* Also, O_DIRECT is never enough to force data to the drives, it merely
* tries to bypass the kernel cache, so we still need O_SYNC or fsync().
* read those buffers except during crash recovery or if wal_level != minimal,
* it is a win to use it in all cases where we sync on each write(). We could
* allow O_DIRECT with fsync(), but it is unclear if fsync() could process
* writes not buffered in the kernel. Also, O_DIRECT is never enough to force
* data to the drives, it merely tries to bypass the kernel cache, so we still
* need O_SYNC/O_DSYNC.
*/
#ifdef O_DIRECT
#define PG_O_DIRECT O_DIRECT