[elephant-devel] Re: Live backup

[Alex Mizrahi]

 IE>   If you do it at the application level within a transaction (even if
 IE> it did fit in memory) you would either implicitly halt all activity or
 IE> constantly be aborted by any write operation anywhere in the db.

ah, indeed, this way bdb works.. PostgreSQL uses MVCC by default, so reading 
activity doesn't block writting.

documentation says you can also get MVCC in bdb via "snapshot isolation" --  
it would implement copy-on-write on database level.
see here:
http://www.oracle.com/technology/documentation/berkeley-db/db/ref/transapp/read.html
http://www.oracle.com/technology/documentation/berkeley-db/db/api_cxx/txn_begin.html
DB_TXN_SNAPSHOT
This transaction will execute with snapshot isolation. For databases with 
the DB_MULTIVERSION flag set, data values will be read as they are when the 
transaction begins, without taking read locks. Silently ignored for 
operations on databases with DB_MULTIVERSION not set on the underlying 
database (read locks are acquired).

or that's what you meant when said "Implement a  copy-on-write policy in 
your transactions"?

 IE> Berkeley DB also supports low-level hot backups if you know that your
 IE> OS reads file pages atomically.
 IE> (http://www.oracle.com/technology/documentation/berkeley-db/db/ref/tran
 IE> sapp/archival.html )  For an example see the source code for
 IE> db_hotbackup in the BDB  distribution.

that's probably preffered way indeed.