how can program verify on the fly back up ?

Discussion in 'Acronis True Image Product Line' started by redmike, Apr 8, 2005.

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  1. redmike

    redmike Registered Member

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    I don't understand how the program can verify a backup that's done under windows ?

    By necessity something will have changed.

    Could you please explain this briefly or at length :rolleyes:

    thanks

    Mike
     
  2. DonKid

    DonKid Registered Member

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    Hi Redmike,

    TI can´t check images on the fly, at the moment.
    I really want this feature too.
    You have to check it manually.
    Click in tools, and then check image.
    Choose where your image is stored and choose the last image to start.

    Best Regards,

    DonKid.
     
    Last edited: Apr 8, 2005
  3. MiniMax

    MiniMax Registered Member

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    Yep - but as long as you keep track of the changes (and the previous data) it is possible to check that everything on the backup matches what was on the disk.

    At length: http://www.findarticles.com/p/articles/mi_m0BRZ/is_1_23/ai_99811019

    Note: Written by an Acronis employee ;)
     
  4. redmike

    redmike Registered Member

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    thanks for the article it's a good one - are you the Max that wrote it ?

    So I make a back-up of my disc and check it the following day - what is the back up being compared to ? How the system was when the back up was made ? If it's compared to the system as it is the next day then obviously it wouldn't be correct.

    If it's being compared to how the system was - where is that file stored ?

    regards,

    Mike
     
  5. MiniMax

    MiniMax Registered Member

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    No, not me. But I am flattered that you thought so :D

    The image is not checked against the contents of the disk. Again, I do not really know the details, but this is what I guess is going on:
    1. As TI is reading data blocks of the disk and writing it to the TIB-file, it is also computing a running checksum of the data.
    2. At the end of the imaging process (or when TI reaches the max size of a TIB file) the computed checksum is appended to the TIB file.
    3. When you ask TI to check the image, it runs though the same motions, e.g. it reads data blocks from the TIB file, and again computes a running checksum.
    4. If the computed checksum matches the checksum written at the end of image (or at the end of each TIB file), the image file is declared "good".
    5. If there is mismatch, the TIB file is declated "corrupt".
     
  6. Menorcaman

    Menorcaman Retired Moderator

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    Hi MiniMax,

    Agreed. Your deduction aligns with my reply in this <previous thread>.

    Regards
     
  7. Howard Kaikow

    Howard Kaikow Registered Member

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    The ONLY thing a backup can do is compare what it has just recorded to the content on the disk at the time of backup.

    Without such a check one cannot have much faith in the integrity of a backup.

    For example, at least one other well known backup program does a compare AFTER it has backed up a volume. If it finds any discrepancoes, those are reported. In almost all cases, the discrepancies are due to a modification, either by the user, or by the OS, or another app, AFTER the backup copy has been made.

    The user can then judge whether the backup is reliable. In most cases, such discrepanicues can be ignored.

    In my case, I always have discrepancies because certain software is almost constantky modifying particular files. The silliest is my scanner software, which updates a .ini file once per minute EVEN when I have the scanner unplugged, now that's bad software design!

    I have not yet read the rest of the thread, so maybe my concern is addressed in this thread.
     
  8. Howard Kaikow

    Howard Kaikow Registered Member

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    Since nobody with Acronis has responded to this thread, I am forced to ASSuME that TI does NOT compare the backed up image with the stuff on the drive.
     
  9. Menorcaman

    Menorcaman Retired Moderator

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    Hello Howard,

    You assume correctly! TI doesn't check the data in the image against the files resident on the source drive at the time of verification.

    In very simple terms, as TI creates an image it calculates checkums and stores these (along with the data from the in-use sectors) in the image during each disk write operation. During a "Check Image" or "Restore" TI uses these saved checksums to verify the data as its being read from the image.

    Regards
     
  10. Howard Kaikow

    Howard Kaikow Registered Member

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    Where is the algorithm used for the checksum documented?
     
  11. Menorcaman

    Menorcaman Retired Moderator

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    Not in the public domain as far as I'm aware. You could try asking support@acronis.com but I'd be very surprised if they divulged this sort of info to third parties.

    Regards
     
  12. jamesv

    jamesv Registered Member

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    This is a very safe assumption. It could be done with a lot of work, but wouldn't really work right unless done at the filesystem level - unless MS added snapshot functionality to NTFS.

    The problem is that TI allows you to work and make changes to a drive as it is being backed up. Those changes have to go somewhere. When the filesystem has snapshot capability free disk space can be used. But when the filesystem can't do snapshots then programs like TI have to either emulate snapshot functionality via very large change buffer files, or, simpler and overall more reliable, just avoid the issue altogether and checksum the image. TI already has to do a lot of work to mimic snapshots and this just doesn't add enough value to be worth even more hackery.
     
  13. Howard Kaikow

    Howard Kaikow Registered Member

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    The techniques for such verification require, in effect, a read after write strategy just after writing the sector to the backup. It's got nothing to do with file systems.
    Of course, sectors won't match is a use, or an app or the OS, is doing things while the backup is in progress. So what! As wit REtrospect, TI would report such discrepancies in te log file and the user could judge whether the discrepanicies mattered.

    Rarely will such discrepancies matter.
    Almost always, I run NOTHING, as user when doing a backup, but the installed apps and OS will almost always change something while the backup is running.
    In the case of Retrospect, the verify is done for each drive AFTER the drive has been backed up, so there is ALWAYS a discrepancy reported due to programs such as norton auntie-virus (even when autoprotect is disabled) and certain logging stuff in the OS and my scanner.

    All a backup can do is copy the stuff on a drive at a point in time, I just want furher assurance that there are no unexpected/unacceptable discrepancies.
     
  14. NielN

    NielN Registered Member

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    Even though I'm here looking for a solution to a different problem that Acronis has not responded to, I think I can help with your question.

    When TI is making a backup under Windows, it starts by making a status map of the drive, which will indicate whether or not a sector has been modified since the moment of the snapshot, and whether it has been backed up yet. At the this moment, NO sectors have been modified, and nothing has been backed up. As the backup task starts backing the sectors up, it marks them in the map as completed. TI is also intercepting ALL writes to the disk as Windows continues to run. If a write is to a sector that hasn't been backed up yet, TI copies the original sector to a holding area before writing the modification. The map gets updated to tell the backup task to get that sector from the holding area instead of the normal drive data area. When the backup is complete, it will truly be a snapshot of the drive at the moment the process started.

    Now I don't know exactly what is going on in the verification process, but since you can do it long after the imaging process, I suspect that it is doing a read of the image and checking that the checksums and CRCs embeded within the image verify. I don't think it is doing a byte-by byte compare of the original drive data, because to be able to do this it would have to be continuing to intercept all writes from Windows forever.

    Provided that TI correctly processed the image when it made the image (and with the release of TI9 and its plethora of bugs, this is sometimes in question), then the simple integrity check of the image data should be fine.

    I had to go back to TI8 to get a program that worked, and I backup only using the rescue disk, because having TI installed under Windows made my systems less stable. I have to use the TI8 boot disk, because there still isn't a version of TI9 that will boot on my systems.

    NielN
     
  15. Howard Kaikow

    Howard Kaikow Registered Member

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    It is only necessary to verify AT THE TIME THE SECTOR IS WRITTEN, any later verification is pointless.

    At worst, they could verify the sectors right after writing the entire drive, i.e., lock the drive during the verification, and report discrepancies. Few discrepancies will ordinarily matter, but the user should be informed and allowed to make their own decision as to what to do.

    I believe the difficulty is that reporting that sector xxx has changed is not very useful, rather it would be necessary to figure out which file was using sector xxx and reprt the file name. This might be time consuming and should likely not be the default verification option.

    Allow users to choose the better verification option.

    Indeed, users are often asking that backup programs have the capability of comparing a backup to the CURRENT content of a drive, so a side-effect would permit users to do this as well.

    Personally, I'd rather just do another full backup to make sure I've got the latest backup, but that brings us full circle to verifying that actual disk contents were recorded without error.
     
  16. NielN

    NielN Registered Member

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    Not true that verifying after the fact is useless. What if your backup is a tape, or any other media that might degrade with time? Would verifying this at at later time be useless? Pretty much all backup media, including hard drives are subject to failure at a later time.

    Locking the drive during backup and verification would defeat the major feature of True Image, where it allows you to continue working in Windows while the backup is occuring. Marking sectors as changed and moving them to a temporary holding area during imaging is done to allow you to continue working in Windows during the imaging process. I'm not sure that you are understanding the point on this.

    If you want the drive locked and otherwise not usable, use the boot disk which does the backup under Linux. Your computer will be not usable for anything else during this period. (This is what I do, because I want to install TI for use in Windows because it seems to make Windows less stable)
     
  17. Howard Kaikow

    Howard Kaikow Registered Member

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    I separate out the functions of validating the media and verifying where stuff on the media marches what is on the drive. The former is always useful, the latter only at the time of backup.

    I understand that, but to me a backup is done to save the stuff at a point in time. I never run anything while a backup is being run, other than the usual behind the scences apps and OS stuff.

    I have no need to do that.
     
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