RE: Common Oracle RDBMS Misconceptions

On Tue, 26 Jun 2001, Jesse, Rich wrote:

> First, I believe it's a misconception that on a Unix system there can be no
> data lost in an Oracle DB from a system crash. This HAS to be a function of
> "syncer", doesn't it? And, therefore, until syncer decides any buffer
> writes actually go to disk, transactions can be toast. Granted, this is a
> very short time, but the possibility would still exist for a standalone
> Oracle DB, especially for one with a high transaction count. But I haven't
> seen any "official" info, whether true or false, from Oracle about this.

The question of whether a write results in data being written directly to disk or only to the UBC depends on the "oflag" that is used when a process open a file with the open() call.

Oracle's log writer (I believe) opens the online redologs with the O_DSYNC flag. That means that a given write() call will not return successfully (and thus a commit will not return as complete) until that data has been written down to disk. Syncer is only responsible for the data's write out to disk if a flag like O_RDWR was used to open the file.

Your concern may apply to a disk subsystem and controller that have their own cache. In the case of these systems, they return success to the operating system as soon as a write has been successfully added to the controller's cache. Such controllers have battery backups, so that in case power is lost to the unit, pending writes in the cache can complete before the unit powers off. That battery backup on caching disk controllers is pretty key to running Oracle successfully.

This is just the kind of caveat that I think would be good to include in a presentation on misconceptions.

> Second, I hope you're going to have explanations and/or qualifications (even
> brief ones!) about the misconceptions somewhere on your website? There's a
> few in your list that have me intrigued!

Of course I plan to explain them, but tell me what intrigues you. I'd love to hash the issues out here before I make a fool of myself in SF in December.

--
Jeremiah Wilton
http://www.speakeasy.net/~jwilton

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Author: Jeremiah Wilton
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