Re: backup and recovery question

A copy of this was sent to Jeremiah Wilton <jeremiah_at_wolfenet.com> (if that email address didn't require changing) On Wed, 14 Oct 1998 17:00:17 -0700, you wrote:

>On Wed, 14 Oct 1998, Thomas Kyte wrote:
>>
>> A copy of this was sent to Tansel Ozkan <tansel_at_openix.com>
>> (if that email address didn't require changing)
>> On Wed, 14 Oct 1998 Tansel Ozkan <tansel_at_openix.com> wrote:
>> >
>> >I know that a backup of Oracle files for a open database
>> >in NOARCHIVELOG mode will not startup the database. However,
>> >I was not able to give a good explanation for the following
>> >question:
>> >
>> >What is the difference between a
>> >database started after a machine crash due to a power loss
>> >and a database started with all Oracle files copied at exactly
>> >same point in time for a database running in NOARCHIVELOG mode
>> >and when the database is open.
>>
>> one of the differences is that you cannot INSTANTANEOUSLY copy all files at
>> exactly one single point in time when the database is open.
>
>It is possible on some filesystems, such as Digital's AdvFS, to make a
>point-in-time snapshot of data. This snapshot is valid with respect to a
>given point in time, and blocks are copied to this snapshot on write.
>Also, many people break off a disk mirror and copy their database from
>that, leaving their tablespaces in backup mode for only a brief moment. In
>fact, it is conceivable that the brash among us could run in NOARCHIVELOG
>mode, take such a snapshot of all datafiles, the online redologs and
>controlfile, and call themselves done. Of course, they had better not
>want to recover that database beyond the newest online redolog in the
>snapshot, because there would certainly be no archived redologs for them
>to use. This would be like recovering from a machine crash at the time of
>the filesystem snapshot or raid mirror separation.
>

Yes, I am familar with both methods above. The AdvFS has the possibility of the "1555" error you might get from Oracle

01555, 00000, "snapshot too old: rollback segment number %s with name \"%s\" too small"

// *Cause: rollback records needed by a reader for consistent read are
//         overwritten by other writers
// *Action: Use larger rollback segments

If I have such a system and I have say a 50 gig database on it and I ask it to take that "snapshot" of the datafiles, then I better have *lots* of redundant storage to support the snapshot while the backup takes place. you can easily in an active database run out of space for this feature.

As for the mirrors, yes, that is an interesting approach as well (one that we actually support).

>> A crash happens instantly. A copy is measured in somewhat longer timeframes.
>> During the copy the files are getting horribly inconsistent with each other.
>> Also, during this period of time, redo is being generated perhaps and if it
>> wraps, we will reuse a redo file that perhaps one of the datafiles will need
>> upon recovery (but we won't have it).
>
>This is all true. In addition, block splits would result in inconsistent
>data blocks, and recovery wouldn't even know what point in time it should
>be recovering from, since a long copy process would read the file header
>at one point in time, then normal checkpoints would continue against that
>file as the copy progressed, leaving the user with datafiles with many
>baseline checkpoint SCNs, instead of one.
>
>To be fair, the checkpoint and block split issue appliers during normal
>hot backups as well, but the full-block-image-logging and begin-backup-scn
>header entry resolve these problems during media recovery.
>

right, we are expecting fuzzy files after a hot backup. We are not expecting them after a system crash tho.

>I don't even think a database in NOARCHIVELOG mode would ask for an
>archived redolog during crash recovery.

It will not, there are no archived redologs to ask for...  

Thomas Kyte
tkyte_at_us.oracle.com
Oracle Government
Herndon VA

--
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