Re: Raw devices and redo

I'm sure that each source is accompanied by some type of explanation? Surely neither would make such a bald sweeping statement without some substantiation?

I would agree with your summarization of Mr. Lewis's position. The type of I/O activity associated with online redo log files (i.e. high volumes of sequential write I/O) suggests an unbuffered type of file. Storing the data written by LGWR in any form of a file-system buffer-cache (which is characteristic of most file-systems) would be a waste of time and processing. Caching is associated with things which you want to write once and then read many, many times. This is not the case with redo data written down by LGWR. At best, it will be read only once more (by the ARCH process), and certainly not in a timely-enough fashion as to justify buffering it...

As the name suggests, "raw" devices (a.k.a "logical volumes") are just that -- an unmanaged, unbuffered, "raw" access to the underlying I/O subsystem. There are also file-systems with which the caching can be turned off (i.e. DEC/Compaq/HP AdvFS, etc), so like any simple sweeping statement, the real answer is more complicated, but in general Mr. Lewis's statements are certainly more correct than the other mystery document.

Please consider the possibility also that you may have misunderstood the message in the "couple years old document by Oracle", because the interpretation you stated is so far from correct...

• Original Message ----- To: "Multiple recipients of list ORACLE-L" <ORACLE-L_at_fatcity.com> Sent: Sunday, August 18, 2002 2:23 AM

> Hi,
> please give your opinion.
> I am reading Jonathan Lewis book Practical Oracle 8i.
> He says redo log definitly should be placed on raw devices.
> On the other hand I have a couple of years old document by Oracle
> stating redo logs definitly do not belong on raw devices.
> So who is right?
> Antje Sackwitz
> --
> Please see the official ORACLE-L FAQ: http://www.orafaq.com
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> Author: Antje Sackwitz
> INET: Stejskal_at_t-online.de
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