Roch Bourbonnais - Performance Engineering wrote:
Tao Chen writes:
> On 5/12/06, Roch Bourbonnais - Performance Engineering
> <[EMAIL PROTECTED]> wrote:
> >
> > From: Gregory Shaw <[EMAIL PROTECTED]>
> > Regarding directio and quickio, is there a way with ZFS to skip the
> > system buffer cache? I've seen big benefits for using directio when
> > the data files have been segregated from the log files.
> >
> >
> > Were the benefits coming from extra concurrency (no single writter lock)
>
> Does DIO bypass "writter lock" on Solaris?
Yep.
> Not on AIX, which uses CIO (concurrent I/O) to bypass managing locks
> at filesystem level:
>
http://oracle.ittoolbox.com/white-papers/improving-database-performance-with-aix-concurrent-io-2582
>
> > or avoiding the extra copy to page cache
>
> Certainly. Also to avoid VM overhead (DB does like raw devices).
OK, but again, is it to avoid badly configured readahead, or
get extra concurrency, or something else ? I have a hard
time that managing the page cache represents a cost when you
compare this to a 5ms I/O.
I think you're missing one other thing - handling the memory overload of
having orders of magnitude more accessed data than you have memory.
Think about how you can handle having a couple hundred GB of dirty data
being written by many threads (say either tablespace creates or temp
table creation for a large table join) - fsflush and writebehind et. al.
just can't keep up with it. Of course, I know ZFS is "better" but to be
useable in those situations it needs to be probably an order of
magnitude better or so, and I haven't seen any data on a decently big
rig with a proper storage config that shows that it is. I'm not saying
it's not, I'm just saying I haven't seen the data. :)
Like you said, Roch, I've been down this road before and don't want to
go down it again. ;)
- Pete
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