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

[David Allan]

On Wed, 17 Feb 2010, Dean Anderson wrote:

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

Indeed, it does.  Whether it's more efficient to compress the whole thing 
or figure out which extents are mapped, I haven't looked into.  It's an 
interesting question.

Dave


> 		--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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