Old debate: Raw Device Vs. File System

[Filipe_1]

Hi Fellow forumers,

This debate, I know, is sort of old fashioned but...

We are provisioning an Oracle OFA anvironment for one of our customers.

We work here as (I think) the vast majority Oracle environments does: using filesystems for a number of reasons.

But the client stands that the use of raw device should achieve better performance anyway.

The problem is showing an "official" doc/paper/whatever wich supports filesystems.

I Know there is some good information on this issue by Oracle itself, and we already got something, but we need further information from HP/Veritas.

Does anyone knows any document like that? Wich arguments would one use to support filesystems instead of raw devices?

Here: Oracle 9.2, HP-UX 11.i v1. Online JFS.

Thanks in advance,

Filipe.

[Mark Greene_1]

You don't need a white paper, just explain the spec's of the hardware. What type of disks, what's their transfer rate, and what's the transfer rate on the controllers? Are you going to use mirroring or RAID 5? How many CPUs and how much memory?


mark

[Geoff Wild · Accepted solution]

Yes - a lot of threads on this...

http://forums1.itrc.hp.com/service/forums/questionanswer.do?threadId=29674

http://forums1.itrc.hp.com/service/forums/questionanswer.do?threadId=21382

Good doc on disk planning:

http://docs.hp.com/cgi-bin/fsearch/framedisplay?top=/hpux/onlinedocs/B2353-90698/B2355-90698_top.html&con=/hpux/onlinedocs/B2355-90698/00/00/32-con.html&toc=/hpux/onlinedocs/B2355-90698/00/00/32-toc.html&searchterms=Device%7cVs.%7cFile%7cSystem%7cRaw&queryid=20040315-065509

In the old days (8 years ago), the following was true:

3. What are the benefits of raw devices?

There can be a performance benefit from using raw devices, since a write
to a raw device bypasses the UNIX buffer cache; the data is transferred
direct from the Oracle buffer cache to the disk. This is not guaranteed,
though; if there is no I/O bottleneck, raw devices will not help. The
performance benefit if there is a bottleneck can vary between a few
percent to something like 40%. Note that the overall amount of I/O is
not reduced; it is just done more efficiently.

Another, lesser, benefit of raw devices is that no filesystem overhead
is incurred, in terms of inode allocation and maintenance, or free block
allocation and maintenance.

But no longer:

You could also mount file systems that you don't want buffer cache by:
-o mincache=direct
That will bypass the buffer cache all together - good option for Oracle on LVM.

Using raw device partitions introduces a level of complexity in configuration planning, administration, and the movement of databases.

Rgds...Geoff

[Jean-Luc Oudart]

Filipe,

"But the client stands that the use of raw device should achieve better performance anyway. "

If you have OnlineJFS i understand you can achieve the same with the extra manageability of fs over raw device

see this thread :
http://forums1.itrc.hp.com/service/forums/questionanswer.do?threadId=193104

Would you have the chance to benchmark it ?
I'd like to see the results and differences (if any) ?

Then, in regards to "better performance" I think database design and application tuning may give you more than tuning the IO subsystem.
My opinion.

Regards,
Jean-Luc

[The Real MD]

I'm going to answer this with the another question i'm afraid. Does the extra 10-15% extra performance benefit of using raw devices make it worth while for the lack of device management. Raw partitions are unmanageable without third party products, so in the event you need more disk space, suddenly its not quite as simple as extending a logical volume. Its also probably worth a look on the veritas website for official documentation.

Regards

Martin.

[Alzhy]

For VERY VERY LARGE Datawarehouse or DSS systems, I will actually recommend going RAW with HP-UX11i + Oracle9/10i. The perceived diferences of 10%-15% DOES matter if you run a very large system -- how large is LARGE? -- I'd say those above 10TB and systems with no less than 64 GB of real memory.

Management of RAW device based Oracle instances need not be complicated anymore. In fact transitioning from Cooked to RAW is relatively easy ... as A. Clay points out.. the use of links instead ot specifying the actual raw LVOL makes things a whole lot easier..

[Filipe_1]

More information:

Yes, the whole environment is on EMC.

The performance gain is easily bypassed by the three clear things:

1 - Ease of admin
2 - Veritas mount parameters wich disables or mimimizes the unix buffer cache.
3 - The VXFS robustness.

Jean, I din´t any benchmark, but these guys did. Very interesting:

http://www.jlcomp.demon.co.uk/raworfs_i.html

Its VERY interesting.

Also, there is this OTN link (requires registeration)


http://download-west.oracle.com/docs/html/A97297_01/ch2_tun.htm#i21051

Regards, and still needing more information on this issue,

Filipe

[Steven E. Protter]

A. Clay Stephenson actually took the time to run performance tests on this issue.

Raw versus filesystem.

On 10.20 there was a significant improvement going raw

On 11.00 there was an improvement though I believe he said it was debatable the improvement.

On 11.11 there was a negligible improvement raw versus filesystem.

Don't beleive me, believe the words themselves:

http://forums1.itrc.hp.com/service/forums/questionanswer.do?threadId=234347

Yeah, i kinda sorta already asked this question.

:-)

SEP

[Michael Schulte zur Sur]

Filipe,

there is another small argument for file system based oracle.
You can shutdown the instance and do a cold backup.
Try that with raw devices!

greetings,

Michael

[Todd McDaniel_1]

I have a large system... 72GB memory. 2TB of DB space on one box... and another one with 32GB memory with 4TB of DB space...

We went with mounted b/c it is MUCH easier to manage all that disk space while mounted... and with Loadbalancing tools like PowerPath along with our latest Hardware of a Superdome.

Also, we use the caching mount options along with striping for better read times.


nodatainlog,mincache=direct,convosync=direct


My argument as an SA for mounted is:

1) much easier to manage hundreds of LUNS.
2) Powerpath or similar solves many of the problems with throughput
3) did I mention easier to manage?
4) DBAs can manage their data more efficently


My DBAs were the ones who actually recommended it...


Here are a few docs online...

http://www.oracle.com/ip/std_infrastructure/blueprints/emailonhp/blueprint_email/Design/Configuration_Options/Files_Systems_versus_Raw.htm

http://www.ixora.com.au/q+a/0010/18224940.htm