[BBLISA] Backing up sparse files ... VM's and TrueCrypt ... etc

[Dean Anderson]

Doesn't compression do very well on a long string of zeros?

		--Dean

On Wed, 17 Feb 2010, David Allan wrote:

> 
> 
> On Tue, 16 Feb 2010, Edward Ned Harvey wrote:
> 
> > Does nobody backup sparse files?  I can?t believe there?s no good way to 
> > do it.  Of particular interest, I would like to backup:
> 
> Sparse files are sufficiently troublesome to program for that it's 
> possible that there's no good way in widely used tools.  I've spent most 
> of the last three weeks dealing with them, and they are a real PITA in a 
> lot of ways.  Well defined PITA and extremely useful, but there are a lot 
> of corner cases.
> 
> > I currently have Virtual Machines and TrueCrypt images excluded from the 
> > regular Time Machine and Acronis True Image backups of peoples? 
> > laptops.  But I?m not comfortable simply neglecting the VM?s and 
> > TrueCrypt volumes, as if they?re not important.
> 
> Can you run the backup from within the VMs?  That's my preferred 
> strategy after ignoring the disk files on the host.
> 
> > I?ve also tried rsync.  People all over the place say it should do well, 
> > but in practice, I found that doing a single incremental takes 2x longer 
> > than doing the whole image.  So again, IMHO, not useful.  Unless I am 
> > simply using it wrong.  But I put plenty of effort into making sure I 
> > was using it right, so I?m really pretty sure I didn?t get that wrong.
> 
> That sounds right, assuming that the size of the backup was the size of 
> the data, not the size of the sparse file including the unmapped blocks. 
> In order to determine if a particular block should be backed up, the block 
> has to be read, then every byte has to be examined to determine if it's 
> zero.  If the entire block is zero, then it's considered to be unmapped. 
> I can see that taking 2x the time.  Unfortunately, the good answer is 
> something like Linux's fiemap ioctl, but that's not supported even on all 
> Linux filesystems, let alone Windows and Mac.  They may have an 
> equivalent, but I don't know what it is, and whether any backup programs 
> use it.
> 
> Dave

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