Re: O_DIRECT and performance

> Well, O_DIRECT has finally made it into the Linux kernel.  It lets you 
> open a file in such a way that reads and writes don't go to the buffer 
> cache but straight to the disk.  Accesses must be aligned on
> filesystem block boundaries.
> 
> Is there any case where PG would benefit from this?  I can see it
> reducing memory pressure and duplication of data between PG shared
> buffers and the block buffer cache.  OTOH, it does require that writes 
> be batched up for decent performance, since each write has an implicit 
> fsync() involved (just as with O_SYNC).
> 
> Anyone played with this on systems that already support it (Solaris?)

I have heard there are many cases there O_DIRECT on Solaris is slower
for databases than normal I/O.  I think bulk copy was faster but not
normal operation.  Probably not something we are going to get into soon.

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