is it necessary to verify image before restoring.


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Mintmag
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The verify image is an impotent feature but I think I maybe be using too much when I could be saving a lot of time. Obviously you should verify after creating the image but what does verify before restore do and what is the reason for it being there? 
jphughan
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Suppose you have a disk that you want to restore from a backup to roll it back to an earlier state, but the CURRENT state is still somewhat usable, as opposed to a completely dead system.  If you choose not to verify the backup prior to restoring, and then Reflect encounters a data problem partway through the restore, the restore will fail with an error and your destination will be in an unusable state -- leaving you worse off than you were.  Verifying the backup beforehand tells Reflect to make sure that all data to be restored is in fact readable before it starts making changes to the destination.

Edited 31 October 2019 2:27 PM by jphughan
Mintmag
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jphughan - 31 October 2019 1:14 PM
Suppose you have a disk that you want to restore from a backup to roll it back to an earlier state, but the CURRENT state is still somewhat usable, as opposed to a completely dead system.  If you choose not to restore the backup prior to restoring, and then Reflect encounters a data problem partway through the restore, the restore will fail with an error and your destination will be in an unusable state -- leaving you worse off than you were.  Verifying the backup beforehand tells Reflect to make sure that all data to be restored is in fact readable before it starts making changes to the destination.

"If you choose not to restore the backup prior to restoring" Is this correct. Sorry this confuses me a bit.
L. W. "Dan" Danz
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Mintmag - 31 October 2019 1:09 PM
The verify image is an impotent feature but I think I maybe be using too much when I could be saving a lot of time. Obviously you should verify after creating the image but what does verify before restore do and what is the reason for it being there? 

I'd point out that verify-after-backup gives you a chance to correct a failure while you still have intact source data.  However, silent failures that this catches are fairly rare.  What's more likely is target disk failure during backup and/or verification, and it is very visible. IIRC, this verification is that the data can be read from the target disk and validated against a checksum recorded with the data.  It does NOT compare the  data on the target disk to the source disk data..

Verify-before-restore will catch corruption caused by decay of recorded data and prevents partial restores that destroy existing data, then can't be completely restored, as noted by JP.


  L. W. "Dan" Danz, Overland Park KS  

jphughan
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Mintmag - 31 October 2019 2:01 PM
jphughan - 31 October 2019 1:14 PM
Suppose you have a disk that you want to restore from a backup to roll it back to an earlier state, but the CURRENT state is still somewhat usable, as opposed to a completely dead system.  If you choose not to restore the backup prior to restoring, and then Reflect encounters a data problem partway through the restore, the restore will fail with an error and your destination will be in an unusable state -- leaving you worse off than you were.  Verifying the backup beforehand tells Reflect to make sure that all data to be restored is in fact readable before it starts making changes to the destination.

"If you choose not to restore the backup prior to restoring" Is this correct. Sorry this confuses me a bit.

Sorry!  That should have said, "If you choose not to verify the backup prior to restoring..."  I've corrected the original post.
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Imaging programs, when restoring a full image will destroy the existing partition on the target disk before reloading the data from the image file. If reading the image file encounters an error before the restore is complete the process stops and your disk now is totally useless because the partition was destroyed. By verifying the image before partition destruction you confirm that you will not have any problems reading the image file data. It only takes one bad checksum out of the many included with the image file to cause the restore to fail.

Other things such as another hardware issue can also cause a failure but by verifying the image you also give the whole system a good workout which would also give some assurance that all is working OK.
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Hey guys I wish to follow up further on this. It seems that verify is a good idea if you're not sure about integrity of an image. But if you've already verified it and know the image is in good standing, is there any reason to do it again? For example if you've verified an image after creating it should do it again before the restore?

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What I would do in a case like this mentioned in this thread is before doing the actual image Restore, do a manual Verify of the image you want to restore.  That way, 1.) you don't run the risk of the restore deleting a somewhat usable partition and the image being bad and failing to restore, leaving you with no partition at all, like JP said, and 2.) if you do you manually Verify, you don't really need to choose to do it again before Restore.  It would be highly unlikely, although, of course, still possible, that the image file would go corrupt between the time you manually Verified it versus when you restored it.

jphughan
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The fact that an image verified successfully at some point in the past is absolutely no guarantee that it is still good today -- which is one of the reasons I don't put much stock in performing a verification right after creating a backup.  If your system produces backups that can't verify properly even immediately after they're created, then you've got larger problems.  And even performing point-in-time verifications on some sort of regular basis STILL doesn't guarantee that the backup will still be intact whenever you want to restore from it.

In terms of whether to perform a verify just before restoring, Reflect does offer that option in the restore wizard, but it's disabled by default.  The benefit to doing it would is this.  Suppose you want to restore your system, but it's currently in a mostly usable state, even if not the exact state that you wanted.  If you do NOT verify before the backup, then the restore would begin, and if Reflect encountered a problem with the image in the middle of the restore, the job would fail and you would be left with the destination disk in an unusable state.  By comparison, if you chose to verify the image upfront, then Reflect would notice the problem before it touches your destination, and thus the job would fail while leaving your destination disk intact.  But even in that case, even though your destination remains usable, it's still not in the desired state, which is why you tried to restore in the first place.  And regardless of whether you verified before the restore, if the backup you WANTED to restore from isn't usable, then you'll have to restore from some other backup instead.  But you could have done that anyway even if the first restore job failed partway through.  So here again, the benefit of verifying before starting the restore seems a bit dubious to me.  It seems like a big time penalty to check for an issue that probably won't exist, and even if it does exist, the adjustment you'll have to make could have been made even if you hadn't chosen to verify upfront.

Edited 27 July 2020 8:57 PM by jphughan
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I can speak from experience, I had an image failure when restoring although it was using a different product than Reflect but that wasn't the issue. I had verified the images on creation but the USB disk they were on developed a problem, likely some platter surface damage. I had to go back about 3 images before I found one that was OK and restored successfully.
This supports jphughan's view but I will temper it with "at least I knew the image was good when it was created" rather than wondering if it was always bad.
Because of the way I work, I only do Full images when I feel like it, I do not image my data files which are the important ones, I use a different method.So Verifying the OS and apps partition when the image is created is not a time problem, I just fire it up when desired and walk away.
Another lesson to be learned from my experience is to never keep a real clean backup disk. If I only had a couple of older images on it I would have had nothing which also says have more than one backup disk and rotate.
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