RMAN WRITING TO RAW DISK?
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Thread: RMAN WRITING TO RAW DISK?

  1. #1
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    Question

    I am about to implement RMAN for my backups but going through oracle8i backup guide I came across the statement ....because RMAN does not backup to raw disk,.....
    I assume 'raw disk' here means same as raw device, the question then is -Does RMAN backup to raw disk/device or not. Please advice.
    My appreciations.
    Tony





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  2. #2
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    You can backup RAW database partitions on UNIX using the dd command. You can also use RMAN, it works with raw devices too.




  3. #3
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    In some scenarios you *can* use raw devices as a destination of RMAN backup, while in some situations you simply *can't*. The problem is that with raw devices RMAN has to know in advance the exact size of the backup, so that it can allocate exact amount of raw device for the destination file.

    For example, RMAN has no problems performing cold database backup or datafile immage copy with raw as the destination, because it is known in advance how much space the backup set will require. On the other hand, there is no way it can know in adwance how much space will it need for online (hot) backup, not to mention incremental backups...

    So for any kind of hot backup you'll have to back up either to tape or to filesistem.....
    Jurij Modic
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  4. #4
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    Hi modic,
    do you mean to say that RMAN will know the size of the datafiles when performing a cold backup and not during a hot backup ?
    I thought RMAN could do a backup on raw devices irrespective of the type of backups.( Hot/cold).

    Thanx
    Vinit

  5. #5
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    Originally posted by vinit
    Hi modic,
    do you mean to say that RMAN will know the size of the datafiles when performing a cold backup and not during a hot backup ?
    I thought RMAN could do a backup on raw devices irrespective of the type of backups.( Hot/cold).
    No, the datafiles are not a problem, their size remains static (well, at least in most cases) during backup. But when RMAN performs a hot backup, the backup set consists of datafiles, controlfiles and archived redolog files. Controlfiles can grow during backup. But more importantly, when RMAN starts performing hot backup, it doesn't know how many archlogs will be generated during the backup time and what their exact size will be! Archlogs generated while performing the backup *must* be included in the backup set, otherwise the backup set will not be complete.

    So there is no way for RMAN to calculate in advance what the size of the backup set will be.
    Jurij Modic
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  6. #6
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    .. with raw devices RMAN has to know in advance the exact size of the backup.
    This is not true.

    (Do you backup raw files with RMAN?)


  7. #7
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    Originally posted by julian
    .. with raw devices RMAN has to know in advance the exact size of the backup.
    This is not true.

    (Do you backup raw files with RMAN?)
    Ok, can you then explain why you can't perform hot backup with RMAN with raw volume *as a destination of the backup*?
    Or for that matter (and this has nothing to do with RMAN, but the reason is the same), why you can't have raw device as the archive logs destination?

    I'm not sure I understand your last question. Nobody said you can't backup raw files with RMAN. The whole isue in this thread is raw device as a *destination* of the backup!

    I'm sure you know that with raw you have to specify the exact size of the file *at creation time*, in contrast with file system, where you can create file without specifying its size, start writing to it, apending new blocks to it until you are done, and thus creating a file of whatever size it needs to be to hold all information.

    I never tried to made a hot backup to raw volume as a destination - I belive the Oracle documentation on this one and I use some plain logic to belive it can't be done! Have you been sucsessfull in persuading RMAN to write hot backup to raw device as the destination, perhaps???

    [Edited by jmodic on 10-05-2001 at 06:00 PM]
    Jurij Modic
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  8. #8
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    I back up with RMAN my raw partitions to TAPE. I have hot-backed up with DD my raw-partitions too. Here is the biggest problem you might face when you use raw devices. With OPS, you have to (almost always) use raw files. However, Oracle requests that archivelogs are cooked. Imagine Modic, if you have to make with Volume Manager a raw partition for every archive log :-) You cannot often control their amount. If your redologs are say 10M, how many raw partitions of 11M (why 11 :-)) do you have to make? The Oracle solution is cooked files for archive logs. The answer to your first question is clear to you now, probably. So, is the second answer, I believe. The 3rd one: NO :-)




  9. #9
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    Ha, ha, you realy are a funny guy, julian.

    In your first answer to the original question you wrote smomething that was not the topic of this therad at all. Read the title of this thread again, please. Noone ever questioned if RMAN can backup raw files, yet you needed to point out the obvious.

    At that point I explained to ndisang what he was asking for - in which type of backups he can use raw as the destination for RMAN backup and in which cases this is imposible. Yet again you had to quote my totaly correct statement, claiming that this is not true. Of course you don't bother to explain why you think so.

    So don't bother to explain me things that I pointed out to you and that are more than obvious to me.

    In short: you didn't addressed the isue of the original question, I did. With RMAN you can't perform hot backup to raw device, the reasons for that are explained few post below this. And if you think something is not correct, please do explain what is wrong.
    Jurij Modic
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    24 hours in a day .... 24 beer in a case .... coincidence?

  10. #10
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    Once again: Your claim that "with raw devices RMAN has to know in advance the exact size of the backup" is not true.

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