[kwlug-disc] Partimage on a mounted partition.

Joe Wennechuk youcanreachmehere at hotmail.com
Tue Apr 24 14:13:55 EDT 2012



> It looks like partimage works on the device, rather than on a mounted
> filesystem. The potential problem would be you're reading bits from
> the disk as other things are being overwritten.
>  
> Lets say you have bigfile.zip, and you start the backup. partimage
> sees bigfile.zip takes up 40MB, and starts reading it. While it is
> being read, bigfile.zip is updated, and is now 45MB. What happens is
> undefined:
> 
> 1. If your zip software built a new bigfile.zip, then renamed it over
> the original, then you'd possibly get the old copy of bigfile.zip
> 2. If your zip software actually mucked about with bigfile.zip
> in-place, you might get a corrupted file in your backup
> 
> Even in case #1, once the original bigfile.zip file is "gone", it's
> space could be overwritten (ex: by a transaction log indicating the
> new bigfile.zip contents), thus again causing corrupted data.
> Wouldn't this happen also by my current method using dd ?
> Generally, when you're backing up data at the device level, you want
> to guarantee nothing else is changing that data at the same time. It
> can be overwritten because it *might* be okay (/usr/local should not
> change during your backup window, for example), but that is really
> your call to make.
> 
> The easiest solution to this problem would be LVM. You can snapshot
> the logical volume, fsck the snapshot, cleanly back it up, and remove
> the snapshot. You can guarantee the backup is as the disk was at one
> point, rather than a moving target over time.
> I will look into this.. Is it hard to set up on a running system?
> As an aside, partimage doesn't appear to support ext4. Just FYI if
> you're building new systems.
> 
> --
> Chris Irwin
> <chris at chrisirwin.ca>
> 
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