Hi Joni, In my opinion, you should always use ROLLFORWARD, unless you run into a performance or capacity limitation that prevents you from doing so.
If you use rollforward mode and you have a hardware crash or a software crash that causes you to have to restore your DB, TSM will restore the DB, roll forward from the logs, and you will be right back to where you started, with no loss of data. If you use normal mode and you have a hardware crash or a software crash that causes you to have to restore your DB, you can ONLY restore up to the time of your last DB backup. You WILL lose data. You may not care if you lose some backups, but if you also had a lot of tape activity since the last DB backup (migration, reclaim), you will spend MANY HOURS doing audits trying to get your tapes and DB in sync again. So rollforward DOES give you a lot of protection. The tradeoff is that when you have a very busy system, in rollforward mode the log gets much larger and it's harder to manage. There are some sites that don't have too many clients; they can control very precisely when things like migration and reclaim occur, and they feel that rollforward is unnecessary because all they would lose is one night's backups. On the other hand, if you are in an environment that runs HSM, you can't afford to lose ANY DB data, you MUST run in rollforward mode. If you have hundreds of clients and runs backups/reclaims 24 hours a day, which means you have a zillion DB changes going on every hour, you really NEED to run in rollforward mode. On the third hand (everything in the distributed world is always complex:>), I have one system that is so busy we kept crashing BECAUSE we were in rollfoward mode and kept filling up the logs. So I think the appropriate rule of thumb is to ALWAYS run in ROLLFORWARD mode, until your system is so busy you can no longer do so because of capacity/performance limitations. And the size of your logs is determined by the amount of activity (i.e., the number of CHANGES to the DB) between DB backups, not the size of the DB. SO you just have to keep an eye on the log %util and keep adding space until you have enough to get you from one DB backup to the next, based on YOUR installation activity. There isn't any way to calculate it ahead of time. (When you switch to rollforward mode, set the DBBACKUPTRIGGER to fire an INCREMENTAL DB backup when the DB gets to 70% full. That will give you an escape hatch if you guess wrong on sizing the logs...) ************************************************************************ Wanda Prather The Johns Hopkins Applied Physics Lab 443-778-8769 [EMAIL PROTECTED] "Intelligence has much less practical application than you'd think" - Scott Adams/Dilbert ************************************************************************ -----Original Message----- From: Joni Moyer [mailto:[EMAIL PROTECTED]] Sent: Tuesday, July 23, 2002 7:57 AM To: [EMAIL PROTECTED] Subject: rollforward vs. normal mode Hello everyone! I was just wondering if anyone has strong opinions about which mode to use for the log: rollforward or normal? And also, would anyone happen to know how much space I would have to add if I changed to rollforward mode? My log is 2324 MB and the database is 41832 MB. I was also wondering if it is a common practice to have the log be 10% the size of the database? Thanks in advance for any insight!!! Joni Moyer Associate Systems Programmer [EMAIL PROTECTED] (717)975-8338
