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RE: Cluster file systems versus raw devices in Oracle RAC

From: Kevin Closson <kevinc_at_polyserve.com>
Date: 2005-12-27 03:05:01
Message-id: B9782AD410794F4687F2B5B4A6FF350103288C97@ex1.ms.polyserve.com

 

In the context of RAC, filesystems cannot be "so powerful" (as was stated in the original post), in fact they MUST be 100% passive in theat they must offer direct IO.

With direct IO, the best a filesystem can do is get out of the way. Direct IO is only sufficient when the code path is as close to the raw IO code path as possible. Given that, where is there room to be "powerful" ?

A good CFS is, on the other hand, much easier to administer (thingies like ASM not withstanding). But there is really no way to measure "improved managability"...that is, there are no TPCM benchmarks that measure Administrative Tasks per Month :-)

All of the above, however, is in reference to the database component files, however. There is much more to an Oracle setup than the datafiles. There is Oracle Home, imp/exp,SQL Loader, external tables, UTIL_FILE, BFILE, logging, trace, archived logging, ETL, etc, etc.

So folks that don't use a CFS with RAC get to spend their valuable time thinking about how to adminster the ancilliary files I mention a lot more than those who implement on a CFS environment.

Remember, the technology like GPFS has not been around forever. In fact, GPFS was NOT supported on less than 3 nodes until not that long ago, due to their split-brain avoidance mechanisms (note to readers, get your facts before you bother suggesting this point is not true).

So, the way it has gone, generally, is people have had to leave behind the normal, reasonable approach to administration when they went to OPS/RAC. They have been forced to leave filesystems behind.

The other problem is that most CFS historically (all of them except PolyServe and TruClusters CFS in fact) have been implemented with dedicated servers for lock management and metadata management. Sort of sounds like a single point of failure and a bottleneck, right ? Well, that architecture is in fact SPOF and bottlenecked. Taking something as available as RAC and stuffing it into an SPOF fileystem makes very little sense.

I know, I know, some of you will point out that the latest-latest release of this-or-that CFS have just introduced a distributed lock manager thus eliminating the SPOF...well, I guess better late than never huh ? :-) And nothing like cramming the right architecture into a product well after it is in the field, huh ?

OK, flame suit on.... let's have it :-)

>>>-----Original Message-----
>>>From: oracle-l-bounce_at_freelists.org
>>>[mailto:oracle-l-bounce_at_freelists.org] On Behalf Of Antonio Belloni
>>>Sent: Monday, December 26, 2005 10:03 AM
>>>To: oracle-l_at_freelists.org
>>>Subject: Cluster file systems versus raw devices in Oracle RAC
>>>
>>>Hi,
>>>
>>>Does anyone using cluster file systems in a RAC 10gR2
>>>installation, specifically IBM's GPFS?
>>>
>>>I've visited a company that is running RAC 10gR2 in AIX over
>>>raw devices. Why someone would choose to use raw devices ,
>>>with all the problems to administer , when all the modern
>>>file systems are so powerful? Is there any issues when using
>>>cluster file systems + RAC? Is there considerable
>>>performance benefits when using raw devices with RAC ?
>>>
>>>I┤ve always used Oracle stand alone instances over file
>>>systems (since version 7) , and performance was always very
>>>good. I┤ve tested raw devices almost 10 years ago , and even
>>>in that time (the hardware today is much better - SAN , 15K
>>>rpm disks , huge caches - and the file systems software
>>>today is much better) the cost to administer it does not
>>>compensate the benefits (only 5% more faster than file
>>>systems in Oracle 7).
>>>
>>>So , as I didn't see a line in the Oracle RAC documentation
>>>saying that RAC only works over raw devices and besides any
>>>limitations imposed by RAC , why use raw devices nowadays ?
>>>
>>>Regards,
>>>Antonio Belloni
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>_______________________________________________________
>>>Yahoo! doce lar. Faša do Yahoo! sua homepage.
>>>http://br.yahoo.com/homepageset.html
>>>
>>>--
>>>http://www.freelists.org/webpage/oracle-l
>>>
>>>
>>>

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Received on Tue Dec 27 2005 - 03:05:01 CST

Original text of this message

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