RE: VMWARE Snapshots to backup Oracle
Date: Tue, 6 Jul 2010 22:28:24 -0400
Message-ID: <6AFC12B9BFCDEA45B7274C534738067F55A23BF9_at_AAPQMAILBX02V.proque.st>
Hi Dave,
This is no different than any other hot backup. You put database into backup mode, and copy off the datafiles, exit backup mode. Then, you can use 'alter database backup controlfile....' to take a logical or physical backup of the controlfile. But, do *not* backup the online redo logs.
Suppose you take a hot backup (snapshot or otherwise) with tablespaces all in backup mode, at 12:00pm. Suppose you *did* take a backup of the online redo as part of that backup/snapshot. Now, at 2:00pm, your database crashes. So, what happens? You restore all files from the snapshot, *including* the online redo. Now, remember, that online redo is from 12:00pm. You want to recover to 2:00pm, when the crash occurred. So, you have your archivelog backups. So, you put database into recovery mode (recover database), and start applying logs. Now, suppose the last archivelog you apply is from 1:55pm. Where are the last remaining transactions from 1:55pm till 2:00pm? They're in the online redo! But, you've overwritten it, with the data from 12:00p, which is useless, and, by the way, was already archived off, shortly after 12:00pm, and has been applied to the database. So, now you're stuck. You lost the data from the current online redo when you did the restore, and your online redo is from 12:00pm (backup time), which is useless to you. So, your only option is to do an incomplete recovery, open resetlogs, and explain to your boss why you lost all the transactions from 1:55pm-2:00pm.
Now, you could argue that, in the above circumstance, you simply don't restore the online redo, and you'll be fine. That's true, (assuming that the snapshot technology you're using will allow for you to specify which files do and don't get restored), but, tell me, why backup the on-line redo in the first place? Show me a valid recovery scenario, from a hot backup, that requires a copy of the on-line redo to be in the backup set. I'm not aware of any such scenario. Can you think of one?
Cold backups are a bit different. I always argue against backing up online redo with cold backup either. People always argue that the online redo is required if the database was not consistent (i.e. cleanly shutdown) when backup was done. And, I'd argue that if that's how you're doing cold backups, you're doing it wrong. (Though, I think I'd first wonder why the heck you're doing cold backups in the first place!)
The only valid scenario that I can think of, where you *want* to include the online redo, is if you want a *restartable* snapshot, rather than a *recoverable* snapshot. Keep in mind, that restartable snapshot can only be used to restore the database to the point in time when the snapshot was taken. It's not recoverable, and cannot be rolled forward to a point in time.
Hope that helps,
-Mark
From: oracle-l-bounce_at_freelists.org [oracle-l-bounce_at_freelists.org] On Behalf Of David Roberts [big.dave.roberts_at_googlemail.com] Sent: Tuesday, July 06, 2010 21:36
To: oracle-l
Subject: Re: VMWARE Snapshots to backup Oracle
"Danger, Will Robinson, Danger"
Why would enabling backup mode, mean that backups of online redo logs are not required?
If you do a snapshot of the database while online, by implication the backup will only be useful after recovery, and the redo logs would be vital to guarantee the recovery?
Perhaps, I've missed something, please correct me if I'm wrong.
Dave
On Tue, Jul 6, 2010 at 5:24 PM, Bobak, Mark <Mark.Bobak_at_proquest.com> wrote:
> Right. If you do begin backup/end backup, you should *not* snapshot online
> redo, and you get a recoverable snapshot. If you do *not* do begin/end
> backup, you *should* snapshot the online redo logs, and you get restartable
> snapshot.
>
>
>
> Hope that helps,
>
>
>
> -Mark
>
>
>
> From: oracle-l-bounce_at_freelists.org [mailto:oracle-l-bounce_at_freelists.org]
> On Behalf Of Andrew Kerber
> Sent: Tuesday, July 06, 2010 12:19 PM
> To: post.ethan_at_gmail.com
> Cc: oracle-l
> Subject: Re: VMWARE Snapshots to backup Oracle
>
>
>
> I am not using it, but I understand that it works as long as you use the
> alter database begin backup command prior to taking the snapshot (and the
> alter database end backup command after taking it).
>
> On Tue, Jul 6, 2010 at 10:18 AM, Ethan Post <post.ethan_at_gmail.com> wrote:
>
> Anyone using snapshots to backup Oracle? Does it work? Anything I need to
> know?
>
>
> --
> Andrew W. Kerber
>
> 'If at first you dont succeed, dont take up skydiving.'
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