Oracle-L: RE: Compressing Export Dumps / WinZip

From: Cary Millsap <cary.millsap_at_hotsos.com>
Date: Thu, 16 May 2002 08:48:35 -0800
Message-ID: <F001.004627C9.20020516084835@fatcity.com>

Using "compress=y" means only that the value of the "initial" storage parameter written to the "create" DDL statement in the .dmp file gets set to the value of "select sum(bytes) from dba_extents where owner=:v1 and segment_name=:v2".

"compress=y" is a wretched, awful thing for a number of reasons, not the least of which is that it causes imports to fail if you have a segment that's bigger than your OS-restricted maximum file size. The parameter is poorly named because it causes no space savings whatsoever. To the contrary, using "compress=y" preserves whatever wasted space has accumulated during the segment's history. Because of the parameter's misleading name, many people attribute temporary performance gains after using exp and then imp to the "compress=y" setting, but you'll actually get identical performance gains by using "compress=n".

DBAs should almost always use "compress=n" instead of allowing the default "compress=y" to change their extent sizes. Better yet, DBAs should use uniform extent sizes with their locally managed tablespaces and avoid the "compress=y|n" problem entirely.

Cary Millsap
Hotsos Enterprises, Ltd.
cary.millsap_at_hotsos.com
http://www.hotsos.com

-----Original Message-----
Shirley
Sent: Thursday, May 16, 2002 10:44 AM
To: Multiple recipients of list ORACLE-L

Doesn't it mean that all rows are compressed into 1 extent?

-----Original Message-----

Sent:	Thursday, May 16, 2002 12:08 PM
To:	Multiple recipients of list ORACLE-L
Subject:	RE: Compressing Export Dumps / WinZip

I don't know whether this is a tangent, but I notice that on the windows platform, "compressed" exports can still get 85% compression when zipping
them with WinZip.

Obviously Oracle "compressed=y" doesn't mean "compress the export file", it
just means that it places all the segments contiguously in the export file.

Right?

Regards,
Patrice Boivin
Systems Analyst (Oracle Certified DBA)

Systems Admin & Operations | Admin. et Exploit. des systèmes
Technology Services        | Services technologiques
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E-Mail: boivinp_at_mar.dfo-mpo.gc.ca
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Author: Boivin, Patrice J
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Author: Taylor, Shirley
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Author: Cary Millsap
INET: cary.millsap_at_hotsos.com

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