Re: os cache vs. db cache

In a database with tons of Full Table Scan you see the benefits!

--
LSC



On 12/19/07, Christo Kutrovsky <kutrovsky.oracle@xxxxxxxxx> wrote:
>
> Joining a bit late into this converation.
>
> Here's my opinion on OS vs Oracle cache vs PGA target. Strictly with
> linux in mind.
>
> I would *always* go for the Oracle cache. The reason for this is not
> so much the more efficient hot/cold management.
> - The Oracle cache is more efficient. Using the Oracle cache reduces
> your CPU usage (your *licensed* cpu) as well as is more concurent then
> the OS cache. The Oracle cache can be used on the spot, there is no
> extra work associated, just use it.
> - I can monitor the Oracle cache, i can see which objects are
> consuming it, how much dirty data and etc, all on a very granual level
> - I get to have *real* disk responce time statistics. As opposed to a
> blended rate due to OS cache
> - I have more control of what goes into cache and when. I have control
> how fast to write out dirty data to disk.
>
> Same goes for the PGA. I would always give more PGA then OS cache. Of
> course, there's the special case where each "work area" can be at most
> 4gb (even on 64 bit) but that can be worked arround using parallelism,
> where each parallel process can get it's 4gb.
> - I get to see which areas are taking up space, which areas spilled to
> disk, which queries are cached which are not.
> - Perhaps I can the the ones that are causing disk activity - you
> loose all this info with a blended OS cache.
> - It's more efficient, no extra CPU work required
> - It avoids double buffering - you will always have double buffering
> if you rely on OS cache.
> - The last thing you want on a CPU intensive task is more CPU to be used.
>
> Now mind you not, the default settings for PGA and sort/hash related
> writes are very unfriendly for the datawarehouse, but tunning these
> you can achieve some exceptional performance. Up to 2x on a going to
> TEMP simply by tunning the IO patterns. SAN cache or not.
>
> I just cant see the case where the OS cache would be more advantegous.
>
> Christo Kutrovsky
> DBA Team Lead
> The Pythian Group
>
>
> On 7/10/07, Robyn <robyn.sands@xxxxxxxxx> wrote:
> > Jonathan,
> >
> > Thank you very much for your reply.  One of the systems in question is a
> > warehouse and the issues you mention are very relevant.  This database
> has
> > been around since '97 or so, and the range of queries and reports is
> huge,
> > so of course, the range of plans and methods for data access are varied
> as
> > well. I have been working on tuning some of the key queries, and I am
> seeing
> > more queries use direct access reads, but it is not yet the primarily
> access
> > method.  However, I'm thinking the best approach for this database is to
> > continue tuning for more parallelization and more direct reads, and
> perhaps
> > increasing the os cache and decreasing the database cache will be
> effective
> > after we've made more progress on that front.  It's certainly something
> I
> > hope to see although I've got quite a bit of work to do first.
> >
> > However, I'm just getting starting on moving warehouse data into
> partitioned
> > tables and indexes (just got licensing for this option recently) so
> again we
> > don't currently have a lot partition index reads now but they should be
> > increasing soon.  If partitioned indexes continued to use the buffer
> cache,
> > I suppose this could change the mix as well.  Either way, I've got a lot
> of
> > work to do first ...
> >
> > Our temporary space is already on a cached files system although I
> expect
> > that was by accident, so I'm glad to hear that we may be benefiting from
> > this configuration.  I'll check into the PGA aggregate target
> suggestion;
> > hashing and sorting is a problem area for several of the queries I'm
> > concerned about this week and I'd noticed a decline in PGA cache ratio
> from
> > where it was about 3 months ago.  We have a year end closing coming up
> and
> > our revenue data will be extremely active in the next few weeks.
> >
> > Thank you again ...
> >
> > Robyn
> >
> >
> > On 7/10/07, Jonathan Lewis <jonathan@xxxxxxxxxxxxxxxxxx> wrote:
> > >
> > > Robyn,
> > >
> > > Since you're running a data warehouse, you may want to consider
> > > the effects of parallel tablescans and parallel index fast full scans,
> > > which do direct path reads and bypass the buffer cache.
> > >
> > > Of course, there are parallel execution paths that still USE
> > > the buffer cache (parallel nested loop joins with indexed access,
> > > access through partitioned indexes etc.) but if a very large
> > > percentage of the work you do is bypassing the Oracle cache,
> > > you may want to switch memory to the file system cache instead
> > > to try and keep some important objects "close to" the action.
> > >
> > > Alternatively, you might think about putting the temporary tablespace
> > > onto a filesystem that has caching enabled so that blocks written to
> the
> > > temp tablespace are in the cache (and not on disc) when you start to
> > > re-read them.
> > >
> > > Yet another option is to keep the caching low - depending on a SAN
> > > cache (for exanple) to help you with the TEMP space - and make as
> > > much memory as you can available for PGA activity so that you minimise
> > > the number of cases where sorts or hash joins spill to disc.
> > >
> > > It's not a clear cut choice, unfortunately - often it's a question of
> > observing
> > > the current activity and making the best guess about how moving memory
> > > around will benefit the critical processes.
> > >
> > >
> > >
> > >
> > > Regards
> > >
> > > Jonathan Lewis
> > > http://jonathanlewis.wordpress.com
> > >
> > > Author: Cost Based Oracle: Fundamentals
> > > http://www.jlcomp.demon.co.uk/cbo_book/ind_book.html
> > >
> > > The Co-operative Oracle Users' FAQ
> > > http://www.jlcomp.demon.co.uk/faq/ind_faq.html
> > >
> > >
> > > ----- Original Message -----
> > > > Date: Mon, 9 Jul 2007 14:03:42 -0400
> > > > From: Robyn <robyn.sands@xxxxxxxxx >
> > > > Subject: os cache vs. db cache
> > > >
> > > > Hello everyone,
> > > > Over the years, I've read differing opinions on balancing the os
> cache
> > and
> > > > the database cache.  Seems like I remember there were some advocates
> for
> > > > minimizing the os cache and maximizing the database cache to allow
> > Oracle to
> > > > better determine which blocks should truly be cached and avoid
> necessary
> > > > blocks being cached because they were in proximity to hot data.  I
> also
> > have
> > > > heard opposite opinions.
> > > >
> > > > I'm specifically focused on performance for a data warehouse.  Has
> > anyone
> > > > tested performance with a minimized OS cache and a maximized
> database
> > > > cache?  If so, what were the results?  Does anyone have a really
> good
> > paper
> > > > or book on the topic?  I'm trying to collect some information before
> I
> > start
> > > > discussions with the platforms guys.
> > > >
> > > > tia ... Robyn
> > > >
> > >
> > > --
> > > http://www.freelists.org/webpage/oracle-l
> > >
> > >
> > >
> >
> >
>
>
> --
> Christo Kutrovsky
> Senior Database/System Administrator
> The Pythian Group - www.pythian.com
> I blog at http://www.pythian.com/blogs/
> --
> http://www.freelists.org/webpage/oracle-l
>
>
>

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