Re: Log file I/O throughput

Dave Hau <davehau_nospam_123_at_nospam_netscape.net> wrote in message news:<3F341C8D.5030208_at_nospam_netscape.net>...
> Chuck wrote:
> > mccmx_at_hotmail.com (Matt) wrote in
> > news:cfee5bcf.0308080303.1ee2492c_at_posting.google.com:
[deletia]
> > Log File Sync means a session is waiting for a commit to complete. With
> > the volume of log's you are generating it sounds like you have a fairly
> > busy OLTP system.
> >
> > Here are a few suggestions to speed up log file i/o
> >
> > * Make sure your online logs are on otherwise quiet disks.
> > * Make sure logs are not on a choked controller. You sysadmin may have
> > looked at % disk busy, but not the % throughput that the controller can
> > handle.
> > * Never (!!!) use raid-5 for log disks. Raid 0, 1, or 0+1 are okay. Raid-
> > 5 can be very slow, even with caching enabled.
> > * SAN's introduce problems of their own. Forget what the SAN vendor tells
> > you about not having to worry about the layout of the storage inside
> > their box. It's hogwash. Make sure the logs are not on a busy physical
> > disk that may be busy because of activity not even coming from your
> > database server.
>
> What the SAN vendor said is correct for cache-centric devices, i.e.
> high-end devices typically with cache larger than 64GB, like the HDS
> 9900 series, EMC Symmetrix, IBM Shark.

     You still have to be mindful of how the SAN hardware operates internally. You can't merely take it for granted that it will operate optimally for your particular workload. You have to be careful that the IO that you generate doesn't trigger cache flushing activity that can induce bottlenecks. Also while 64G of cache or more is certainly nice, even that can be overwhelmed. Plus you have to worry about every thing else attached to that RAM that is remarkably slower.

>
> For storage-centric devices, or mid-range devices like EMC Clariion or
> HDS 9500 series, what you said is correct, that you have to look at
> back-end controller info in allocating volumes.
>
> Cheers,
> Dave
Received on Thu Aug 14 2003 - 10:23:08 CDT