On Dec 11, 2007, at 10:41 AM, Alex Mizrahi wrote:
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"?
I was thinking about something pretty similar, but at the application
level. Perhaps my original response was too off-the-cuff. I think
the only problem that may come up in the MVCC model is whether you run
out of storage for keeping the write versions of committing
transactions that start after the read operation snapshot was taken.
It can take a long time to read the entire DB within a single
transaction!
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.
Indeed. This puts the future transaction storage burden in the log
files instead of a shared memory region which is much more scalable.
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