Answer to the second one..
when the alter tablespace <name> begin backup is executed:

The tablespace is checkpointed, the checkpoint SCN marker in the datafile headers cease to increment with checkpoints, and full images of changed DB blocks are written to the redologs.

for the full story see the article by Jeremiah Wilton titled "Misconceptions"

-----Original Message-----
From: j [mailto:oracledba-ezmlmshield-x52946469.[Email address protected]
Sent: Tuesday, September 14, 2004 2:27 PM
To: LazyDBA Discussion
Subject: archivelog and hot back


Dear All,

I have two specific queries:

1) While changing from noarchivelog mode to archivelog mode , why it is
necessary to do "alter database open archivelog" even if log_archive_start
=true is set in the init.ora file.

2) During hot back when we give "alter tablespace <TSname> begin backup",
the header of datafiles are freezed, then where are transactions
associated with these datafiles are written?

Jay
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