Re: [PATCHES] O_DIRECT for WAL writes

On Fri, 2005-06-24 at 09:37 -0400, Tom Lane wrote:
> ITAGAKI Takahiro <itagaki(dot)takahiro(at)lab(dot)ntt(dot)co(dot)jp> writes:
> > ... So I'll post the new results:
>
> > checkpoint_ | writeback |
> > segments | cache | open_sync | fsync=false | O_DIRECT only | fsync_direct | open_direct
> > ------------+-----------+-----------+---------------+---------------+---------------+--------------
> > [3] 3 | off | 38.2 tps | 138.8(+263.5%)| 38.6(+ 1.2%) | 38.5(+ 0.9%) | 38.5(+ 0.9%)
>
> Yeah, this is about what I was afraid of: if you're actually fsyncing
> then you get at best one commit per disk revolution, and the negotiation
> with the OS is down in the noise.
>
> At this point I'm inclined to reject the patch on the grounds that it
> adds complexity and portability issues, without actually buying any
> useful performance improvement. The write-cache-on numbers are not
> going to be interesting to any serious user :-(

You mean not interesting to people without a UPS. Personally, I'd like
to realize a 50% boost in tps, which is what O_DIRECT buys according to
ITAGAKI Takahiro's posted results.

The batteries on a caching RAID controller can run for days at a
stretch. It's not as dangerous as people make it sound. And anyone
running PG on software RAID is crazy.

-jwb